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The famed Mopar slant six: Durable workhorse, 1959-1991


Chrysler needed a new six-cylinder engine long before engineers started working on it; the old flat-head six, while very durable, was not quite as powerful or efficient as it could have been. That was no reflection on its designers; a great deal of time had passed. The situation came to a head when corporate leaders ordered the creation of a new compact car, the future Valiant, to be on the market by model-year 1960.

1960 Slant Six

Originally, the Valiant was to have a four-cylinder, but as the specifications were developed, it became clear that would not provide enough power; a small six would be needed, but it would have to fit into a small space. The people who had just created the company’s first V8s designed for mass production (rather than adapted to it) were put to work on the new engine, and they used quite a lot of what they had learned on the B/RB series V8s.

Creating a new engine

Robert S. Rarey had led the B-series V8 project, which created brand new, highly competitive, and inexpensive-to-make engines extremely rapidly. Despite the fast engineering turnaround, these turned out to be extremely reliable, and capable of great power.

Willem Weertman described the creation of the slant six in his book Chrysler Engines. Robert S. Rarey and Willem Weertman gave the Valiant team the choice of various forms of engines, including four-cylinders and V6 designs, with both aluminum and iron blocks. A typical in-line six, with the water pump in front, would not fit; but then Fred Rose, the lead designer, and Ray Latham, the engine design supervisor, thought of leaning the engine over by 30 degrees, which let them move the water pump to the side of the engine.

1977 Slant Six

They leaned it to the right rather than the left to give the fan just the right spot, near the center of the radiator; the engine was moved a little to the right of the car’s centerline to make more space for the driver’s foot.

Today, engine programs can take years; but the slant six size and type was finalized in April 1958, and it was in production for model-year 1959 cars.

The Valiant team specified a 170 cubic inch in-line six, with a 3.4 inch bore, 3⅛ inch stroke, and the 30° tilt; fortunately, the proposal landed on Plymouth chief Harry Chesebrough’s desk. That same day, while approving it, he also asked for a more powerful version to completely replace the flat-head six, then used in trucks as well as cars, in sizes up to 251 cubic inches.

There was no way to increase the cylinder bores, especially since Valiant program chief Bob Sinclair refused to give even a quarter of an inch more length. Within five days, though, engine designers Bob Rarey and Willem Weertman had figured out how to add one more inch of stroke (to 4 ⅛ inches) instead; they did need to create a different crankshaft, a new block with greater deck height, and longer connecting rods and pushrods; but everything else would be the same. The 225 cubic inch version joined the development program around a week after the 170, in April 1958.

1977 Slant Six

At that time, engines were given a single-letter code name (A for small-block V8s, B for big-block V8s, RB for raised-deck big-block V8s); the new six was coded G, but with this change, with complete disregard for consistency, they changed it to RG for the regular stroke and LG for the long stroke.

The next step was for Paul Ackerman, head of Engineering, to weigh three proposals for the Valiant and for replacing the flat-head six. Two of those required the company to create new four-cylinder and new six cylinder engines, and one called for the one slant six in two sizes. It was probably a no-brainer in 1958; on the same day, April 16, Ackerman gave the go-ahead to the slant six, without any four-cylinder.

The engineers wanted to use aluminum for the engine blocks to cut weight and use fewer machine tools; but they also created an iron version in case they couldn't make the aluminum version work. Bob Kring, an engineer from Kokomo Casting, worked with the engine design people to speed up the creation of the new block and heads; his actions as a go-between were invaluable, since the casting people had never done engines and the engine people had never done aluminum. Like current Chrysler engines, it used bore liners (iron, in that case) to prevent cylinder wear—a simple, cheap, effective method.

John Hurst, an engine designer at Chrysler, created the idea of the slant six’s unique and efficient “bunch of bananas” intake manifold; John Platner and Don Moore developed and fine-tuned it. The intake even had a supercharging effect, something Chrysler was to push to extremes in its big V8s; but the main goal was to get the same fuel/air mixture to all cylinders. Since the intake and exhaust ports were on the same side of the head, exhaust gases warmed the floor of the intake, making warm-up faster.  Other companies’ straight-six manifolds had some cylinders share tubes or had sharp bends—or larger differences in the tube lengths.

The first slant six engine was dyno-tested in the final days of November 1958, less than six months after the green light had been given. Drawing by hand, working with both iron and aluminum versions, the engineers had created a working iron-block 170 cubic inch engine. Two weeks later, the long-stroke aluminum 225 took its turn on the dyno. The plant in Kokomo (Indiana) started casting aluminum 170 cubic inch blocks in March 1959, less than one year after the green light had been given.

Ironically, the first slant sixes ended up under the hoods of the 1960 Dodge Darts, not the Valiant. For 1960-62, the Dodge Dart was a midsized car, “compact for a Dodge,” rather than a long-wheelbase Valiant, which would be the case in 1963. Dodge’s version of the Valiant was the Lancer, a name dropped after a brief couple of years until it was picked up years later (in front drive form) and then adopted globally by Mitsubishi. (The Dart, incidentally, also got an optional alternator; the Valiant was the first car in the world with a standard alternator.)

Trenton Engine prepares to make slant six engines
Preparing Trenton Engine for the new slant six assembly line, 1959; they would also be made in Ontario, Canada. Chrysler photo via Dave Van Buren

Many of the aluminum engines are still around, treasured by some current slant six owners; but production was much faster using the iron design, and the slant six was needed for a huge number of cars and trucks. For a while, Chrysler made both engines, trying to increase the aluminum production speed; but the aluminum version was eventually dropped. The executives might have regretted that, a decade later.

Less than two years after executives approved the plans, the slant six was being installed in Chrysler Corporation cars; it would stay there through 1983 (longer, outside the US) and in trucks through 1987. The marine version stayed in production all the way into 1991. Production was in Trenton, Michigan for most of its run, moving to Toluca, Mexico when volume fell.

Slant six engineering and design

Pistons were a tin-plated aluminum alloy; steel struts kept the dimensions correct despite heat buildup. The top ring was chrome-plated and the second ring was tin-plated; the oil control ring had chrome-plated steel rails.

1960 Slant Six

According to Pete Hagenbuch, early sixes had oil leak issues; his team looked at the pistons and cylinders, and decided that the 170 was satisfactory but the 225 needed something different—Chrysler’s first-ever steel rail oil ring, the chrome-plated Sealed Power SS 50. For the 1961 model year, this chromium plated top ring was complemented by a reverse twist second ring (with the chamfer on the bottom of the ring instead of the top); and the supplier had developed a way to abrade the chrome plating, as would happen during a long engine break-in, which helped the engines get off to a good start.

spark plug tube Spark plug tube from 1974 slant six

The oversquare design helped with torque, as did the long stroke on the 225, but the small bore size limited valve size. Other than working around the tight cylinder spacing, the heads were similar to those of the big B V8s, with similar designs for overhead valves, wedge chambers, and the tappet chamber to lubricate without drilling holes in the block or heads. Rocker arms were steel stampings. A great deal of ingenuity went into using the same types of parts as in the B series V8 engines—lessons learned from the parts proliferation of having three very different Hemi V8 series earlier.

The slant six was a bit unique for much of its run for using aluminum tubes to separate the spark plugs from the pushrods, given the lack of space for cast walls. It cut weight, though it also created opportunities for leaks, and after 1974 the tubes disappeared.

Electrical gear was on one side of the engine; service parts were mostly easy to reach, though the fuel pump and distributor were far down in the engine bay.

The slant six was not straightened, even when used in trucks that had plenty of space; though that might have eased maintenance, especially in terms of spark plug, starter, and distributor access, it would have added to the cost and eliminated the clever intake.


While it was normally used only in trucks and economy cars, the one time Chrysler put some effort into performance, it paid off quite well. The 1960 HyperPak, installed by dealers, included a four-barrel carburetor, hot cam, and other parts to bring a rating of 196 gross horsepower, beating many V8s of the day; some believe that number was lower than reality. Modern racers have used turbochargers and different carburetion (or fuel injection) and manifolding to produce far more than that now-quaint figure, and the slant six holds up well under the load.

Even with a one-barrel carb, it edged out Ford and Chevrolet motors in power and economy, even though Chrysler had a reputation for being less optimistic in horsepower and torque ratings than Ford. Consumer Reports consistently praised the engine, into the 1970s, for beating GM and Ford’s straight sixes in speed and economy. (Much later, one former engineer said they had actually under-rated the 170 and over-rated the 225 somewhat).

slant six power table

In Australia, where six cylinder cars were taken more seriously, local engineers created a two-barrel version rated at 160 gross bhp; and in South America, various two-barrel versions (using Holley and Carter carburetors) were rated at 155 gross horsepower.

Back in North America, the 1977-and-later “Super Six” package used a modified two-barrel Carter carburetor to increase responsiveness without loss of economy, but didn’t add much to the official ratings. The 1976 Feather Duster package, which went for economy rather than speed, brought a 30 mpg EPA highway rating.

Pete Hagenbuch told Allpar that the Super Six was developed in 1975, almost immediately before production, by three people—a valve engineer and two road-test mechanics. Most of the benefits came simply mounting a more appropriate carburetor; even small Japanese engines had two-barrels, while Americans had stuck with one-barrels. The engineer and mechanics also tested numerous spark advance schedules before selecting one, and added a low-restriction air cleaner. The cheap single-exhaust held output back, but performance was still far more sprightly without hurting economy.

1977 Super Six (slant)

Gene Yetter told Allpar about a hotter Mopar camshaft, part number 3512639 (244° duration, 26° overlap, 0.406” lift on intake, and 0.414” lift on exhaust; 0.010” valve lash on intake; 0.020 on exhaust), recommended for engines from 1971 to 1977. The similar Dutra/Erson RV10-RDP cam is can be reground from stock solid lifter cams (grind #2106), to replicate the Mopar part’s timing.

Production numbers

Trenton Engine was one of the plants making V8 engines; they were the first to produce slant sixes and were the main source of the leaning tower of power. The following numbers are for Trenton Engine’s production (Trenton was not the only plant to make slant sixes, but it was the primary one):

All in the slant six engine family

As they had with the flat-head engines, Chrysler engineers quickly went to work on special versions for industrial, commercial, and marine use; these had special equipment such as two-row timing chains, chrome-plated upper piston rings, valve rotators, Stellite-coated exhaust valves, high-volume oil pumps, and polyacrylic valve stem seals (depending on the use and the year); ordinary car slant six owners can retrofit some of these parts, such as the double roller chains (NAPA parts 9168, S338, and S339, 1962-75 only).

The heads were redesigned for the 1963 model year, and as a result, owners had to install spark plugs without the little “crush washers”—until the 1975 model-year. Chrysler launched another revised head for 1967; it increased both economy and power, by eliminating dead spots, adding quench/squash areas, and keeping the fuel/air mix around the exhaust valve, with the spark plug igniting the center of the mix. Now, too, both the 170 and 225 used the camshaft that was designed for the 225.

With the 1970 model-year, the old regular stroke 170 was dropped; customers could instead get a long-stroke 198 cubic inch slant six. It was a sensible move, allowing for more standardization of parts and providing a bit more power for their base six-cylinder, especially given the added weight of their smallest cars and trucks. The Valiant, Dart, and commercial vans had all gained weight since 1959-60.

The 198 was essentially the same as the 225, except for a shorter crankshaft throw, longer connecting rods, and a shorter stroke.

CID Years C/R Horsepower
* = Net
170 1959-1963 8.2 to 1 101 @ 4400 155 @ 2400
170 1964-1966 8.5 to 1 101 @ 4400 155 @ 2400
170 1967-1969 8.5 to 1 115 @ 4400 155 @ 2400
198 1970-1971 8.4 to 1 125 @ 4400 180 @ 2000
198 1972 8.4 to 1 100 @ 4400* 160 @ 2400
198 1973 8.4 to 1 95 @ 4000* 150 @ 1600
198 1974 8.4 to 1 95 @ 4000* 145 @ 2000
225 1960-1962 8.5 to 1 145 @ 4000 215 @ 2800
225 1963 8.2 to 1 145 @ 4000 215 @ 2400
225 1964-1971 8.4 to 1 145 @ 4000 215 @ 2400
225 1972 8.4 to 1 110 @ 4000* 185 @ 2000
225 1973 8.4 to 1 105 @ 4000* 185 @ 1600
225 1974 8.4 to 1 105 @ 3600* 180 @ 1600
225 1975 8.4 to 1 95 @ 3600* 170 @ 1600
225 1976-78 8.4 to 1 100 @ 3600* 170 @ 1600
225-2V 1977-78 8.4 to 1 110* 180

* Federal numbers only; California differs in some years. Gross horsepower is around 35 hp lower than net due to measurement differences alone.

The easiest way to tell the 170 from the long-stroke 198 and 225 versions is the hose from the water pump into the head; the 170 has a hose that’s less than two inches long, while the 198 and 225 have a three-inch hose. As for telling the 198 from the 225, well, you might have to look at the emissions sticker.

Chrysler started installing hardened valve seats long before unleaded gasoline became mainstream, as a preparation for a future of catalytic converters and unleaded fuel, so some cars without catalytic converters could run on unleaded with no problems.

Slant six production equipment in July 1959, at Trenton Engine
(Chrysler photo via Dave Van Buren)

Electronic ignition was added in the 1973 cars; it was a good, reliable system over the long haul, other than ballast resistor failure causing starting problems (it can be shorted out in a pinch, but the ballast resistor continues to sell for under $5 and replacing it is easy, so keeping a spare is a better practice).

Another change for the 1973 cars was a new oil filter standpipe, which had required long-canister filters, such as the PureOne PL30001; Scott Sigethy wrote that the Ford FL-30001 (made by Purolator) worked well because its double seals prevent the engine from being starved for oil. Starting with the 1973 engines, the standpipe was changed, and owners should use shorter filters, since the tall filters don’t fill all the way on these engines, causing late delivery of oil on startup.

During the 1976 model year, Chrysler started using a cheaper nodular iron crankshaft instead of the forged steel version; both are strong enough even for hefty power gains, according to many tuners. Catalytic converters arrived around this time. Steel-backed aluminum-alloy main bearings replaced the original ones in 1978.

Chrysler Canada started working on hydraulic lifters for the slant six at the Windsor Engine Plant, and tested them in fleet service to rack up some serious miles before they went into full production; work began in 1976, the test took place in 1978, and the hydraulic lifters finally made it to slant sixes in the 1981 model-year. Instead of adding an oil channel, the engineers grooving the rear cam bearing to feed oil up to the rocker shafts; it went through the rocker arm bodies and hollow pushrods to reach the lifters. That meant they could keep the same block.

Numerous carburetors were used over the years, as one can imagine, starting with the Ball & Ball single-barrel (better known as the Carter BBS, one takeover later), which lasted all the way into 1971; from 1962 to 1973, they could also use a Holley 1920, and only in 1963, a Stromberg WA-3 carburetor ended up in some cars. The BBS idle system changed from 1967 to 1968, and the manual and automatic cars had different carburetors.

Clean air regulations caused many year to year changes. For 1969, for example, ignition timing was changed, eliminating the need for vacuum advance control valves on cars with manual transmissions; the choke was shut off more quickly; idle bypasses were created (to remove drilled holes in the throttle valves) for idle air, to vaporize raw fuel better; and the thermostat went from 180° to 190°  on the 225, and from 190° to 200° on the 170. Holley 1920 carburetors now had a cap on the idle mixture screw to limit its range of motion, too. One last device for 1969, more for drivability, was an anti-icing device for the carburetor, which used air heated by the exhaust to prevent icing; this was added to the 225 slant six and 318 V8 only. Ironically, in the same year, engineers added a cold air intake to the Road Runner and GTX. (Cam timing for the 1969 slant six was 240-236-16 for both the 170 and 225.)

For 1970, Chrysler started using manifold-heated air in a different way, familiar to most classic car folk: an “exhaust stove,”  created by putting a sheet of stamped metal over the exhaust, was connected to the air horn. When temperatures in the air cleaner were below 100° (as measured by a thermostat mounted below), a flap moved to prevent ambient air from entering, and to allow air in from the “stove.”  When the temperature climbed, the flap moved and ambient air was taken in. It helped the cars warm up much faster, and was used in conjunction with another new gadget: a choke heater. The idea there was to have a little wire heat up the choke coil, so the choke would open earlier and the car would not run as rich, saving fuel and cutting emissions. These were common-sense moves that made the car more drivable as well.

The Holley 1920 gave way to a Holley 1945 starting with the 1974 cars. Much lambasted by those who missed the simpler 1920, the 1945 is actually quite similar and works well over the years, while supporting emissions systems. The 1975 slants had a new head, which eliminated the old spark plug tubes and required owners to use tapered-seat plugs—with the crush washers. At the same time, the design change required owners to remove the heads to replace the lifters.

From 1976 to 1981, the Super Six setup used a special Carter BBD two-barrel; in 1978 (model year), the Super Six started using a larger (14-inch) air cleaner, which was adopted by the single-barrel 225 in 1979. The larger air cleaner replaced the old 11-inch model, increasing airflow by capacity quite a bit.

The single-barrel carb linkage also changed for 1979 slants, to reduce pedal effort and make the throttle open more for the same amount of pedal travel; engineers also switched to a throttle-drive accelerator pump. The goal here was presumably to make the slant six feel more powerful in heavier vehicles, including B-body cars.

Chrysler started using electron beam welded aluminum intake manifolds for 1979; these are reportedly more troublesome than the usual version. Chrysler had an industry first with that method, which did cut weight by 14 pounds per car. In the same year, the company started using concealed idle adjustment screws on the Super Six—the screws were put into little tunnels and plugs were pushed in after them. This was an interesting move, given that people typically had to adjust carburetors every six months.

Another 1979 introduction was the electronic feedback carburetor, a clever idea that used an oxygen sensor (just like fuel injection) and adjusted the fuel/air mixture accordingly, striving for a 14.7:1 ratio (different ratios were used for warm-up and heavy acceleration). The Holley 6145-based system was fairly complicated and not particularly reliable, and was launched midyear as standard equipment on California cars (not trucks) with the 225 and automatic transmission; they became standard on federal-emissions cars in the 1981 model year. (Trucks followed a different schedule, as did industrial and marine applications.) Electronic feedback systems were used with dual catalysts, one for nitrides of oxygen and unburned hydrocarbons, and one for hydrocarbons and carbon monoxide. A light flashed on every 15,000 miles to remind the driver to maintain the system.

There were no slant sixes in cars in 1984; they were now exclusive to trucks. The only cars which had used it (M-body Diplomat, Gran Fury, and Fifth Avenue/Newport) came with a standard V8—just the single 318 cubic inch economy version, choked by unwise emissions equipment, with executives still refusing appeals from Engineering for fuel injection.

For 1984, buyers could still get the 225 slant six, oddly equipped with a single-barrel carburetor only, on the D100, D150, and even D250 pickups; and, with four wheel drive, on the W100 and W150 (the W150 and D250 were not sold in California). Canadian versions had different emissions systems, and slant six trucks were not available with tow packages.

The final slant sixes, sold in Mexican cars through the 1987 model-year, used a rejetted Carter YPF two-barrel carburetor and recurved distributor to slash emissions by 30%, while putting out 98 horsepower (net)—a figure almost identical to the 1972 single-barrel output. These were made in Toluca.

Oddball slants

Many alternatives were tested by engineers and killed by executives. These included a 1967 246 cubic inch version with hydraulic lifters, ideal for trucks but needing an extra oil gallery in the block; and turbocharged, fuel-injected, and (two) diesel versions.

Early on, in 1962, engineers tried out an overhead cam version of the 225; this was tested from 1962 to 1966. Slightly larger versions, at 180 and 246 cubic inches, were tested in 1964-65; the larger one may have helped for truck use. The hydraulic-tappet version tested in 1967-68 was based on the bigger of these.

A “slant four,” presumably for export markets, where Valiants were just about the only popular car Chrysler Corporation had, was tested in 1969. The idea was brought back in 1980 to 1983, this time based on a 1975-80 diesel slant six; one of these had a turbocharger, the other did not. From 1981 to 1983, the company tested another diesel conversion, this time turbocharged. Chrysler actually built a lightweight 225 from 1976 to 1980, but it only saved 12 pounds.

Jim Grundy, a Chrysler engineer in 1967-71, told Allpar that he had designed an overhead cam slant six at Chrysler, installing it into a Dart, but couldn’t find any interest in the project; he sold the Dart and bought it back in 1992.

The “Feather Duster” and Dart Lite used aluminum body panels, special gearing, and carburetor tuning to jump up to 30 mpg highway, by the very easy test procedures of the day. You can see the impact of weight between the light Valiant and the midsize Fury with the 14 vs 18 city mpg; highway mileage is more a factor of aerodynamics, so the two cars were very similar, but the V8 effectively clobbered gas mileage. (Extract from EPA gas mileage guide.)

That was not all. A third-valve pre-chamber, similar to the Mitsubishi MCA-Jet system, was tested in 1973-74, as Chrysler owned a large share of Mitsubishi; this would presumably have slashed emissions and increased economy. An aluminum fast-burn cylinder head was tried out (in the lab) from 1976 to 1979. Finally, the company tried turbocharging in 1977-78 and using multiple-port electronic fuel injection in 1979-80. Fuel injection, while not as sexy as turbocharging, would have made the cars far more drivable and desirable, especially when compared with primitive spark control computers; but executives likely blanched at the cost, $100 per injector. The turbo six was used in at least one Dodge Monaco, a full-sized car.

The 1976 Feather Duster and Dart Lite, which were designed to have a 30 mpg highway mileage rating, used an automatic idle speed diaphragm on the carburetor; in essence, it kept the hot idle speed the same as engine load changed.

A computer controlled slant six helped with emissions; this used Electronic Spark Control and then Lean Burn. Reliability was not helped by putting the computer on the air cleaner. Early versions had two pickups in the distributor; a later one was able to use just one pickup. Vacuum leaks and electrical problems can wreak havoc with these systems, and many owners replace them with plain carburetors and older control systems; some owners say to keep them in place until they develop insurmountable problems.

Finally: the first Bricklin concept car used a slant six engine, though their first production cars used AMC V8s pushing out 220 horsepower (net).

Tips for owners

Timing chains stretch over time, and can even jump a sprocket, causing confusing idle and power loss problems.

Each year, put two or three drops of light machine oil (e.g. engine or sewing machine oil) onto the felt pad under the rotor in the distributor. Clean or replace the crankcase inlet air filter every few years to avoid blue smoke. A low-quality distributor cap and rotor or wires can lead to stalling in wet weather; a distributor cap gasket can help.

Valve cover gaskets seem to work better as rubber-and-cork designs without sealers. If you do use gasket sealer, try applying it to the gasket and the metal, letting it dry a bit, and then putting them together.

The official method for changing spark advance on a 1971-76 engine, according to David Wordinger, is:

  1. Take the vacuum advance hose out of the distributor and plug it
  2. Set the idle speed to specifications
  3. Set the timing (because the mechanical advance will still be in place after the vacuum advance tube is blocked) by loosening the bolt at the base of the distributor (not too much) and gently rotating the distributor

This is a good time to lubricate the felt pad under the rotor.

Problems with stalling when cold are usually due to incorrect choke and fast-idle settings, but may be due to bad accelerator pumps within the carburetor, or a slipping vibration damper outer ring, which moves the timing mark. The Carter BBS has a better reputation than the Holley carburetors for this.

Cracking manifolds are a relatively common issue; POR15 manifold repair kits are a temporary fix for hairline fractures which can last for years.

The two-barrel intakes made in 1979-82 could be problematic, as they were a first use of electron beam welding two separate castings; the single-piece two-barrel intake from Mopar Performance has a better reputation. Porosity problems in the manifold can be fixed with powder-coating or several coats of epoxy paint. The two-barrel manifolds may warp at the head surface; thick gaskets can be a fix.


It took less than two years to fully develop the slant six from nothing to production. Yet, it ended up as a legend of durability, more efficient than GM and Ford sixes for two decades, with a national club (the Slant 6 Club of America) and a national racing series.

Slant sixes are still beloved, especially in Mexico and South America (and, after the Hemi Six, in Australia), where they were in more upscale cars. People still modify slant sixes to produce many times their original power. Some adaptations using parts from other cars; others use custom parts. Turbochargers are common, along with advanced fuel injection systems; you can even get newly machined high-performance heads and manifolds. Youtuber “Uncle Tony” even installed one in a Mazda Miata. It’s quite a tribute to Bob Rarey, Willem Weertman, and the host of engineers who created it practically overnight.

Appendix: intake manifolds

Daniel Stern, who with Doug Dutra are the leading slant six experts, pointed out that the exhaust manifolds are interchangeable from 1960 to 1986, despite various changes which added strength and changed the choke stove pocket. Early manifolds, for example, had no ribs, and could crack easily; ribs were added in 1963 and again in 1970. In 1980, they were changed to allow an oxygen sensor ot be added.

Dan noted that the manifold is designed to have some movement as it heats and cools; stamped steel gaskets aid this but are hard to seal. Special washers are part of the system, and care must be taken to get those right. Details at

Appendix: spark plugs

From 1959 to 1962, despite having spark plug tubes, owners are supposed to use the “crush washer” (the loose metal ring at the end of the spark plug) in their cars when changing plugs. The 1963 model year brought a slight redesign, adding a relief at the top of the spark plug hole; the spark plug tube itself is the seal, so for cars and trucks from the 1963 to 1974 model years, the washer must be removed before installing spark plugs. If the washer stays on, the plug will be too “cold,”  and heat transfer from the plug will be interrupted. Daniel Stern, incidentally, recommends the NGK ZFR5N spark plug for various reasons, including extra-long electrodes that move the spark point closer to the center of the chamber, to reduce misfires.

When changing plugs on these cars, it can help to change the spark plug tube gaskets, to avoid oil leaks later. Step by step instructions are at allpar.

A head redesign in 1975 finally eliminated the spark plug tubes, and mandated taper-seat plugs—with the washers. For theese, Daniel Stern recommended the NGK UR5GP.

Slant Six Club • Slant Six ForumsMaking slant sixes at Trenton Engine (at motales) 

Slant six specifications

Four aluminum-alloy-on-steel babbitt crankshaft main bearings had oil grooves in the upper half (and in the lower half of the #1 bearing). Connecting rod bearings were also aluminum on steel.

Closed-circuit crankcase ventilation started in the 1960s, with crankcase vapors routed to the cylinders through a PCV valve so they could be burned. This did not hurt power or economy.

Oil pressure ranged from 35-65 p.s.i. at 2.000 rpm.  Crankcase oil capacity was five quarts including the filter.

1968 Engines 170 + 225
Bore spacing 3.98 (1-2, 3-4, 5-6);
4.0 (2-3, 4-5)
Firing Order 1-5-3-6-2-4
Head and block Cast iron
Mounting points Two front, one rear
Piston weight 16.4 oz
Skirt top clearance .0005 - .0015
Skirt bottom clearance -.0005 - +.0005
Ring groove depth .179 #1/#2; .181 #3
Piston Pins  
Length 2.965
Diameter .9008
Type Press-fit in rod
Clearance (In piston) .00045 to .00075
Clearance (In rod) .0007 to .00014 interference
Offset in piston Right .06
Connecting Rods  
Bearing Lead-based babbitt on steel
Bearing overall length .985
Bearing clearance .0002 to .0022
Bearing end play .006 - .012
Idle air-fuel mixture 14.0 – 14.4

Where the engines were different (for 1968, the 170 used Ball & Ball BBS carburetors and the 225 used Holleys):

1968 170 225
Bore and Stroke 3.4 x 3.125 3.4 x 4.125
Compression 8.5 8.4
Gross horsepower 115@4400 145@4000
Torque (lb-ft) 155@2400 215@2400
Connecting Rods 170 225
Weight (oz.) 25.7 26.8
Length 5.707 6.699
Carburetors 170 225
Idle speed 750 650
4414 S (Manual)
4415 S (Auto)
R-3919 A (Manual)
R-3920 A (Auto)
Barrel Size 1.56 (Manual),
1.69 (Auto)
Degrees 5°  ATC manual
2.5° auto
Top Dead Center

Oil rings were different for 1968 (and other years). The 170 used cast iron rings with steel for expansion; the 225 used three-piece rings using stainless steel spacer-expanders and chrome-plaged segments.

1980 journals: 2.75”  x 0.877 (except journal #3, 2.75”  x 1.107”).

Making slant sixes at Trenton Engine (at motales) 

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