I bought this very nice original (paint & interior) 1964 Cadillac Sedan DeVille with what was said to be a rod knock on an engine with a recent rebuild. Upon hearing it, I was skeptical that it was a rod, but really didn't have any other theories (it was a strage knock to say the least). Pulling the valve covers revealed what appeared to be a clean - and one would agree - rebuilt engine. Anyway, the car was cheap so the deal was done and I drove her home to start the diagnosis. Oil pressure was great (~30psi is nothing to argue about at idle), so I felt pretty certain I wasn't dealing with a lower end problem, but my stethoscope said otherwise. After a compression check revealed a poor #6, I decided to drop the pan and see if I could see anything amiss. First, there was this in the pan: Hmm... not good. Looking up, I saw this: Now, I'm not one to complain about a skirt being lost in most situations (but I guess there's and exception to every rule). Fast forward a day or so - pulled the head and yanked out the #6 piston: I noticed that there seemed to be a chunk missing from the top of the piston directly above where the top ring should be (left side of the above picture). Then I noticed #2 and #4 had the same thing (#2 on the left side, #4 on the right side): Having never seen a 429 piston, I thought maybe this was part of the design (and then I actually started thinking and realized that this engine was probably the victim of some sort of lean running condition and a poor rebuild). Piston #8 looks the way the others should: So, here's my theory (and I'd love to hear yours): The car must have been detonating and that severe lean running eventually led to the partial destruction of at least some of the pistons (I've only pulled the one head). The engine was rebuilt in backyard fashion where the broken bits were removed (including the entire top ring from the #6 piston), the heads were redone and the whole thing was just sort of limped back together. I've not decided what to do as of yet - cylinder walls are in good shape with almost no ridge, I just don't know if I can find a set of .010 over pistons.
My guess is the broken pistons are the result of severe detonation due to burning low octane unleaded gas since the 1970s. Your car came with 10.5:1 compression ratio and needs 100 octane gas. Hi test 92 octane barely good enough with the timing retarded. Detonation could have done the damage even on hi test, if they burned regular goodby engine. Suggest a rebuild with lower compression pistons, and possibly hardened exhaust valve seats. I'm sure the Cadillac experts will be along with more specific advice. 1971 up Cadillac had 8.5:1 pistons. Unfortunately they will not fit your car. If there is no ridge, cylinder taper less than .007 and cylinder walls not scored you do not need to rebore. Just hone the cylinders and install new pistons.
Weird... I'd expect more damage towards the dome from detonation. Not having the parts in front of me, I'd guess damage done to the pistons when it was ringed, or thyey had insufficient end-gap. Do the cylinder walls show any signs of scuffing? Bob
The Jalopy Journal
