Rings and Bore Size

[CobraSix]

was looking over an engine in my garage and debating with some friends. have an engine, standard bore. Rehoned it and now it's about .002 bigger than stock. Now, for this engine there are available rings for .005 oversize (not true overbore) with stock pistons.

Question is this. Can stock rings be used, or .005 rings on the .002 hone, or hone the block some more to .005 and use .005 rings?

I guess the last answer is the best, but any problem using stock rings, or .005 rings on the .002 overhone?

 

[THE FRENCHTOWN FLYER]

Don't hone the block out to +.005. The stock rings will work, but I would use the +.005 rings and check the ring end gaps in the tightest part of the bore. File the gaps to obtain the minimum specified clearance. And be sure to remove the ridge at the top of the cylinder or broken rings will result.

 

[CobraSix]

Thanks.

Yeah, the ridge at the top is gone. The bore is smooth the entire length. Just .002 over stock.

 

[MustangSix]

The other thing to look for is taper. is the bore the same diameter top to bottom? Typically the top will wear more so fitting a ring to that diameter could be too tight at the bottom of the bore.

 

[Anonymous]

Google a product called Gun Kote; company is KG Industries, formerly Kal-Gard. It is a spray-on, heat-cured (300F, 1 hour) moly disulfide coating. In the heyday of two-stroke motorcycle roadracing in the '70s, the bike magazines used to run big color ads from Kal-Gard, and their major product was Piston-Kote (same thing as Gun-Kote). You can build up the skirt diameter of your VERY WELL CLEANED pistons two or three thousandths. Running will quickly burnish off half-a-thou, thereafter the coating will last a long time. I have an old phongraph turntable to set the pistons on when I spray them with an airbrush. This also works well for building up worn throttle slides in old Amal carbs, and similar jobs.

Building up the piston diameter should make it rock less, helping the rings work.

 

[THE FRENCHTOWN FLYER]

I suppose there's always skirt knurling. Does anybody still do that?

 

[Anonymous]

My pal the auto machinist, who served as apprentice/slave-laborer to his father forty years ago, knurls the occasional skirt for me. I have sometimes combined this with the moly coating, when I was reasonably sure of the pistons.

Cobra, if you're a little oversize for the stock cast pistons, you might be pretty close to the right clearance for forged pistons, which are generally given extra clearance. See what rings they call for.

 

[THE FRENCHTOWN FLYER]

Cobra, if you're a little oversize for the stock cast pistons, you might be pretty close to the right clearance for forged pistons, which are generally given extra clearance. See what rings they call for.

Forged pistons are generally made a little smaller for an equivalent bore size so forged may not be a viable solution. For example a cast piston for a 4.000 bore might mike 3.998 and a forged might mike 3.994.

 

[xctasy]

Its awfully easy to screw up an 8 grand engine with forged pistons with the wrong ring sizes. My current engineer for XEC is the same one who told my mate Blair that just a few thou error with a set of forged pistons, and you can bin a thinwall engine block with piston rattle. Our 375 hp engine that gave great mpg and performance had forged piston slap at 1500 rpm, and we had to rebiuld the whole engine again.

Also happened to Ford many times during the 90's as plateau honing was perfected to suit the new piston technology. And it wasn't because Ford didn't do there homework.

Regards going to a tighter bore for forged engines, thats certainly true of the first 1979 TRW 2.3 Lima Turbo in the Mustang Turbo, the forged piston 5.0 GT engine in the 80's and later 90's versions, and then every forged piston Ford since.


But 'In the begining' (1968 to when the Federal Emission tests 50 000 mile durablity run caused problems passing TRW pistoned small and big block v8s), the wildest versions of the 302 and 350 Chev and most aftermarket TRW pistons before that had really wide clearances, they were always piston rattlers untill Ford practically commericalised forged pistons when the Pinto 2000 Sierra and 2300 Merkur/SVO Turbo came out.They were still cold engine rattlers, but that was becasue they were rough 4 cyl engines.

The 1992 5.0 liter with TRW's was one of the smoothest forged piston engines around, to this day people attempt to copy that combination.

I saw the results of running stock 4.030 bore and stock Ford factory clearances for cast pistons, then at the last minute, we got 351 4V Boss forged pistons, and scored the cyinders of my mates NASCAR block back in 1988. The rule is, find your combo, then do everything the piston maker says regards rings and bore RA. The first engine forged piston engine scored the bore, and had to be reuiolt with a whole new block.

 

[Anonymous]

Forged pistons are generally made a little smaller for an equivalent bore size so forged may not be a viable solution. For example a cast piston for a 4.000 bore might mike 3.998 and a forged might mike 3.994.

Makes sense, that would be how they use the same rings . . . same bore size.

My very limited experience with forged pistons (outside of outboard racing) was years ago, with the cheapest forged TRW pistons, the ones that had four valve head recesses so they could be used on either bank of a V-8. The blocks were going +.030", the clearance called for by TRW was .007" skirt, IIRC. I went with slightly less than that, figuring they were not racemotors. They rattled a bit anyway until they warmed up.

So as usual, what I thought I knew is decades out of date.