LS1 bore tolerance

[theplayer]

I recently had my engine worked on due to cold engine knock. When the mechanist first measured the bore he was getting 3.906” for all the cylinders. The spec is 3.898” to 3.897”. He said that his bore gauge might have been set up wrong so he rechecked it with his micrometer. After rechecking it all the cylinders were in spec. My question is if my cylinders were actually 3.906” what problems would I have been experiencing? Major blow by, black smoke, no power? Would it have been apparent or could that be just enough off tolerance to cause a little knocking?

 

[Hib Halverson]

IF you had .008-.009-in. piston-to-bore clearance, you'd experience not only cold piston knock but piston knock all the time. For a Gen 3 with stock pistons, .008-.009 would be huge piston-to-bore clearance and quite unlikely.

You'd also see higher oil consumption, depending on what kind of oil the engine uses.

If the rings were ok, you probably would not have major blow-by. You would not have black smoke.

 

[theplayer]

Hib:
My corvette was produced in June 02 according to the sticker inside the door. I know it has the new pistons with the polymer coating on the sleeve because I saw them out of my engine. I suppose the new LS6 piston is all black color according to the pictures I have seen. My LS1 piston was the metal color and only the sleeve was black with the polymer coating. The coating was slightly worn off on some of the pistons. The mechanic said that was normal. I also saw small black lines in the cylinder were the coating had worn off. The mechanic said that was normal and not an issue. Is it normal and not an issue? I didn’t look closely at the rings. They did tell me that they were going to reuse my rings because if they put new rings in they wouldn’t seat right and it would use oil. Is that correct? Also, does my car have the new piston rings with the Napier profile face on the second compression ring like the ones shown in your article? http://www.c5registry.com/2k2z06/page5.htm
It said they were going to be standard in the MY02 but I’m not sure because I didn’t look at them that close.

 

[Hib Halverson]

As far as I know, the piston skirts and not the bores are what is polymer-coated. You will see some coating residue on the bores because the purpose of the polymer is to keep the engine from sticking a piston during break-in. By 3000-5000 miles most of the polymer will be gone. So...some polymer residue on the bores is normal. That you see residue also indicates your piston clearance was improperly measured the first time and that the actual clearance is probably right-on. If you had the .008-.009 the guy measured the first time, there'd be no residue on the bores.

Keep in mind that for 02 and later, the reason the pistons are coated is GM changed the tolerance for piston-to-bore clearance as a response to complaints about cold piston knock. The change in tolerance reduces problems with that but it also slightly reduces performance because of increased friction. IMO, the whiners out there who cry about a little cold piston knock have made it worse for all of us hardcore s. I'd rather have a little cold piston knock and the motor a little looser.

Your engine has the revised rings with the napier-face second and the new oil ring expander. All 02 LS6es had them. After the first two weeks of MY02 engine production, all LS1s had them too.