(Bill Vose, on tearing down his 1989 2.5 Turbo engine, found that letters were stamped on the rear pan rail—D C C C. He suspected it was a piston-to-cylinder fit marking. Dave wrote back:)
You are 100% correct about the markings being piston size (cylinder bore size). For decades, boring and honing cylinders could be controlled only so closely. The process has finally gotten so good in the last 15 years or so that there is now only one size of piston for each engine—at Trenton, starting with the 3.2/3.5 V6 iron blocks.
Oops: turns out the above illustration is wrong. F meant “face front of engine” in this case.
If I remember correctly, in the 2.2/2.5 Engine era, the bore/piston sizes ran A thru E—I recall around .0003 or .0004 inches larger with each new letter. Back in the Slant Six and Big Bore V-8 (350 - 440) days, the jump in size was bigger. (Yes, we made a B 350 engine before I started working there.) As I recall, the normal Slant Six and V-8 engines ran A thru G; 0.020-inch overbores ran P thru T. When any overbore was done, all the cylinders were overbored.
One of the first jobs I had on the V-8 assembly line was running the teletype machine. When a block was loaded on the line, I would put in the type of engine and the bores’ size in order. It would print out in the piston area. The piston/ wrist pin/ connecting rod would be assembled and be sent to the line in sequence. Yes, every once in a while they would foul it up. (There had to be at least 8 or so different types of pistons, so now multiply that times all the cylinder bore sizes—3.4” for slant sixes, two or three at a time for V8s.)
When I had the teletype job, they would tell me to pull off certain size cylinder bores because they didn’t have pistons in that size.
One little known fact: guys would go looking at the bore marking on big block V-8 engines on new cars. They were looking for an edge in drag racing. I was told that a factory overbore was legal at Detroit Dragway, and a 0.020 overbore adds up on a 440 Engine. On a new engine, you could read the painted bore on the side of the block (or sometimes on the edge of the oil pan rail.)
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