Intermittent crankcase pressure issues

nm9stheham said
One you get a piston out, any short rings will be obvious. The ring side clearance (between ring and groove) is normally in the .004"-.006" range; you insert a feeler gauge between the groove and ring to measure this. With rings that are 1/64" short, the ring side clearance will be .019" or more; .010-015" side clearance is what you will find in a worn engine'e pistons/rings. With excess side clearance, the rings will twist up and down in the bore, round the ring faces, etc.

If this turns out to be the situation, then check the grooves in the pistons for being worn/damaged. You should check them with a new-thickness ring to see if the grooves have been worn/damaged too wide to meet new rings side clearance specs. If the grooves worn/damaged, then you will be back to a similar situation. Either replace the pistons, or see if there is a shop locally that can widen the piston grooves enough to use what is know as a ring spacer, or use wider rings if available. Re-grooving and using spacers an old-time method to salvage used pistons; not sure how well it would work with KB's, or if it is really cost-effective here.

IMHO, if there is no ridge in the cylinders, then you might get away with a fresh cylinder hone. But what appear to be shallow radial grooves around the bore in the above pix, is concerning... if they are real, and remain after a light hone, then that is not going to do new rings any good; ring wear will likely be more rapid than normal.

You nailed it.

The good news is that it seems to be REALLY close to 1/64" (.015625) off still. It's looking like I'm going to get away with re-ringing and rehoning. Have to check the other pistons obviously so wish me luck lol. Got one of the exhaust valves out as well and it seemed good. No marks on the valve and no noticeable play. I plan on building the 360 sitting in the garage in a year or 2 since 400 ft/lbs of torque isn't as scary as I thought it would be in this car so if the 5/64" rings end up a few thousands off it should be fine for it's short life.

Turns out not double checking tolerances is a bad idea when assembling an engine even when you're with people who have done it a few times. Who would have thunk it?