Oy tis not my year it seems. I have a 78 302 that I pulled from my truck after a year of enduring a mild cold knock (which got notably louder in the last two days the motor was ran). I tore it down expecting to find a spun rod bearing on the #1 cyl (whre the knock was loudest). Well I did find a very worn (but not spun) bearing on the #1 but also on the #4 with the wear focused on the piston side of the bearings. Both cylinders also had some notable scoring on exhaust side of the piston skirts, the other side was perfectly fine. All other cylinders turned up fine with no scoring or abnormal bearing wear. The engines actual history is unknown, the previous owner rebuilt it himself using a summit rebuild kit .020 over, etc. The full story is that I was only planing to do a minor revamp so that I could use this motor for a while. I purchased a Ford Racing 351W HD truck engine (397-M-6007-S58) and as it happens this engine has a wicked vibration in it that makes it unnerving to run so I have had the truck parked for the last six months (I have checked everything I can, flexplate balance, damper, T/C etc). According to Ford warrenty service I need to remove the engine and freight it back to them for repair/diag, which is better than nothing but still costly, and it leaves me with no engine for a while. Anyway I got off topic, I was mainly curious if anyone had any idea what could cause that kind of damage to two cylinders?

 

Hmm, where to start...

A "mild cold knock" can be piston slap, which goes away after the engine warms up and the pistons expand and tighten up a bit in the cylinder. With forged pistons which run a looser piston-to-cylinderwall clearence due to their higher rate of expansion, this is considered normal. I find it interesting that the same two cylinders that showed increased bearing wear also showed piston scoring.

I take it that this wear was on the piston skirts only and not on the cylinder walls?

I have to wonder about the skills of the engine builder here. .020 is a rather uncommon overbore, it usually takes .030 to clean up cylinder taper and scoring, ring ridge, etc. I'm specifically wondering if the builder simply honed the crap out of worn sloppy cylinders rather then going to the expense of having them bored and stuck in oversize pistons, and didn't quite get enough clearence on those two cylinders. Stranger things have happened...

As to the vibration of the 351W, you say you checked flexplate balance. Did the engine come with the flexplate, or was it one you procured elswhere or perhaps already had? You know the 351W has a 28oz. imbalance as do the pre'81 302s, I'm wondering if there's any chance a later 302/5.0 flexplate with 50oz. imbalance could have found its way onto this engine.