Anyone file fit piston rings?

[Dennis H.]

In my research for a upcoming rebuild on my 73 L82 engine I have been looking at piston and ring options. Seems people recommend that if replacing go with the Keith Black Performance Hypereutectic piston (KB156) and Sealed Power Moly rings (E-251-K).

Have read several places on internet sites that ring gap is critical when using Keith Black Hypereutectic pistons and rings must be checked and file fitted. Sealed Power list their moly rings as file fitting not required.

GM 73 L82 Piston
For a 4.00 bore the production top ring gap is .010 to .020 and service gap .030 max

KB K156 Hypereutectic Piston
For a 4.00 bore the top ring gap is .026

I can honestly say I have built a lot of SB Chevy engines over the years and never file fitted rings. Was I doing it wrong all those years or are KB Hypereutectic piston that critical?

Also ran across a post of one of the NCRS forum old timers who I value his opinion greatly that stated he had never file fitted a piston ring.

Dennis

[Patrick H.]

Re: Anyone file fit piston rings?

I used the Sealed Power hypereutectic pistons when I rebuilt the engine on my 71 as I didn't want to mess with filing and fitting the rings on KB's. You may want to decide how much you really want them.

I also changed to ones with two valve reliefs instead of 4, as I was going to pay attention to how the pistons were placed. The factory went for assembly speed, not precision.

[Dennis H.]

Re: Anyone file fit piston rings?

I have never used Keith Black hypereutectic pistons so maybe that is the difference.

[Philip A.]

Re: Anyone file fit piston rings?

To do it technically correct you should install the rings into the cylinder and measure the ring gap and adjust it accordingly if needed with a ring file and label those rings to that cylinder/piston number.

[Duke W.]

Re: Anyone file fit piston rings?

At least in the 327 era the cast pistons pin bores are offset .060" to the thrust side. Supposedly this reduced pistons slap noise. This practice went back a long time, but I think it's seldom used today, like left hand threads on the right hand wheel studs. The four notches allow the same piston to be used on both sides. Pistons without offset pins only need one set of valve relief notches, which is the case with OE forged pistons, and have no pin offset.

BTW I'm pretty sure the KB 156 pistons are flattops for 327s. 350 pistons have lower compression height due to the longer stroke. The KB 157 has a 0.5 cc net dome and is a good choice to replace the OE type 327 forged pistons, which are very expensive. The smaller dome allows the use of a thin head gasket without getting the CR too high.

Duke

[Timothy B.]

Re: Anyone file fit piston rings?

Aren't the KB pistons the ones with the top compression ring very close to the piston top? That explains the .026 ring gap due to heat and expansion, correct?

[Dennis H.]

Re: Anyone file fit piston rings?

Aren't the KB pistons the ones with the top compression ring very close to the piston top? That explains the .026 ring gap due to heat and expansion, correct?

Yes from what I have read that is the reason the rings have to be file fitted.

[Jim L.]

Re: Anyone file fit piston rings?

Aren't the KB pistons the ones with the top compression ring very close to the piston top? That explains the .026 ring gap due to heat and expansion, correct?

This is, indeed, the reason KBs require file-fit ring gaps.

To answer the OPs general question, yes, I file-fit all ring gaps on all engines I assemble. I also use a bolt stretch gauge when torquing rod bolts.

[Duke W.]

Re: Anyone file fit piston rings?

Aren't the KB pistons the ones with the top compression ring very close to the piston top? That explains the .026 ring gap due to heat and expansion, correct?

Correct. The high mounted top ring runs hotter than on traditional pistons with the top ring further below the crown so the gap needs to be wider. There are very specific instructions for top ring gap on the United Machining Web site.

A lot of guys are down on KB pistons because they experienced top ring butting that broke top ring land. That's not the piston's fault. Blame goes to he moron who assembled the engine and didn't follow the instructions on top ring gap.

Duke