Piston Ring End Gap

[Joe C.]

Can anyone explain how varying the end gap on pri and sec compression rings will affect engine performance and reliability. Is the gap for the oil scraper as critical as that of the compression rings? What are the implications at the extremes? What variations from spec range is acceptable? Assume that we are talking about cast iron rings.
Thanks in advance.

Joe

[Clem Z.]

Re: Piston Ring End Gap

going over the max spec will cause no big problem,just a little more blow by and no real HP loss but too small a gap and the ring ends will butt as they expand from the heat and cause all kinds of problems. the top ring has a larger end gap because it see more heat and the oil ring scraper will have .035 + end gap. just lightly "break" all the sharp edges on the ends of the rings with a fine honing stone after you set the end gap.

[Joe C.]

More

The above refers to a rebuild of a friend's 327/365, out of a '64. He is using forged pistons, with domes as original. Everything is back from the machine shop, and I am helping him assemble the engine.

I have measured the end gaps on the 1 and 2 compression rings, and they all fall between .018" and .021". Spec calls for .013 - .023. The oil scrapers measure between .033 - .036. Spec calls for .015 - .055. These are all basically in the middle of the acceptable range, but just wonder what the effects of bad gaps would do to the engine.

Block has a .030 overbore, and pistons are Federal Mogul Sealed Power L2166NF-30. The piston to wall clearance, at the top ring land should be between .036 and .046 as per shop manual. I measured these clearances @ .016, all around, for a total piston to bore diameter difference of .032. THIS IS THE ONE NUMBER THAT I AM NOT SURE OF. If the machine shop sold me 30 over pistons, with a 30 overbore, then the clearances should be AOK. The piston to wall clearances (at top) seems to be less than half spec. If I assemble it this way, what will be the results.

Thanks in advance.

Joe

[Clem Z.]

Re: More

just check the piston to wall where the piston manufacture says to check it.some are at the wrist pin heigth and some are lower down the skirt. the FM book list piston to wall spec of .0035 for that piston

[Craig S.]

Re: More

Joe - Federal Mogul lists a fit size on the box for the pistons if you still have it, as Clem says, check where the suggest. Also, I think the latest versions as I have going into my 66 427.425 with the coated skirts can throw you off in measuring.....they have a warning about it with the paper that comes in the box. The key is to have the machinist follow the box unless you have a guy like I had that builds specialty engines up all the time and has a pesonal method to blueprint each bore to piston for exact fit....Craig