[TRADERMIKE 2012]

Originally Posted by Schurkey

What you're describing is not at all what steve25 is telling you to measure. What you're describing (cylinder wall wear) is a valid concern, though.

Steve25 wants you to cram a .002--.0025--.003 feeler gauge blade into the ring grooves of the pistons--in several places around the circumference--with the ring in the groove. There needs to be no more than .003 gap between the ring and the ring groove, or the ring groove is worn-out and the pistons are junk.

Fifty--sixty--seventy years ago, there were lathe tools to re-cut the ring grooves so they were "square" again, and steel spacers to take up the excess clearance in the machined-oversize grooves. That crap is DONE now, nobody is going to do that any more. They'd tell you to buy new pistons that don't have worn-out ring grooves.

As for what you're saying--that your cylinders have wear towards the top (typical bellmouth wear, common as dirt) there's a limit to what's acceptable. When I was in trade school, dinosaurs roamed the Earth, and some of them were driven to church and the grocery store by grandmotherly types who never went over 30 mph, ever. For those folks, I was told that cylinder bore wear of .007 taper was the absolute limit of acceptability.

Keep in mind that .007 taper leaves a ridge of .0035. It's REAL easy to find ridges bigger than that, and if your ridges broke the rings, you've almost certainly got more ridge than .0035. You are almost certainly going to bore that engine, and again you're needing new pistons.

The most-recent engine I put together has a "service limit" spec of .001 bore wear, but actually had .0015 taper. The ridge would be about .0008-ish. I put it together like that, and "so far, so good".

You can check your bore wear with feeler gauges and a non-broken ring, and a clean flat-top piston. Put the ring inside the cylinder, push it towards the bottom of the cylinder with the piston, so it's sure to be square in the bore. Measure the end-gap with the feeler blades. Bring the ring half-way up the bore, square it up with the piston, and measure the end-gap. It'll PROBABLY be the same as the measurement farther down in the bore. Then bring the ring up to the underside of the ridge. Measure the end-gap, which is probably going to be bigger than the two previous measurements. Subtract the small end gap measurement from the big end gap measurement, and divide the difference by 3.1416.

Anything over about .002 is grounds for rebore. Unless you're Grandma, going to church at 15 mph.

BEWARE of "stock replacement" or "rebuilder" pistons. They are almost always SABOTAGED with a low compression height. It used to be that .020 too-short was common, now I see many that are "only" screwed-up by .010. Low compression height affects piston-to-head clearance, which affects "squish" or "quench", which affects the speed of combustion as well as compression ratio. You do not want short pistons. Getting pistons that are properly-made may cost more than the "rebuilder" junk products.



Some guys do, if the ridge is big enough to cause difficulty removing the pistons. The point of ridge-reaming is to save the pistons from damage during removal.

"I" get a bigger hammer, and pitch the pistons in the scrap bin. I have seen too many blocks damaged by heavy-handed ridge-reaming.


Haul them to your favorite automotive machinist. Let an expert examine the crank and rods.

First Guess: Nothing wrong with the crank that a regrind won't fix. Nothing wrong with the rods that an inspection and MAYBE resizing won't cure.

Someone is sure to say that "by the time you replace the rod bolts, and have the rods resized, you could buy brand-new rods from XXXXXXXXX company and be better-off." Well, yes. How high are you going to spin this engine? Do you NEED new rod bolts? IF (big IF) the RPM is modest, and the rods are inspected and found to still be round...why resize them?

But maybe you plan to beat on this engine, in which case upgrading the rod bolts and resizing after close visual inspection is probably warranted. (If the cast-iron rods and cast-iron crank were cracked...they'd probably be broken already.) I don't trust bottom-feeder Communist Chinese rods or cranks. Not everyone feels that way. And--supposedly--there's some Communist castings/forgings that get proper machining and QA here in America. How fat is your wallet? What are your goals for this engine?


Have a professional, experienced eye examine the block; .030 is kind of routine for overbore, but if it can clean-up with less, bore the minimum amount. If it needs more, bore to what it needs. If one or two cylinders are especially bad, consider having them sleeved.

Mikes reply:

You said:

"Some guys do, if the ridge is big enough to cause difficulty removing the pistons. The point of ridge-reaming is to save the pistons from damage during removal".

Mikes reply:

The next time I take an Engine apart, perhaps I would remove the Crank first and pull the Pistons out the bottom by pushing the Pistons toward the top out of the way, sounds easy, but I suspect that is harder than it sounds like?