1962. 390 cu.in piston rings queery.

[Kurt Wrebel]

Hi guys,
I'm just rebuilding the 390 engine that you helped me date as a 1962 and I need to check something regarding the piston compression rings. The top groove on the piston seems slightly wider than the middle groove. And the old top ring, which had another ring - like a shim in its groove as well,  is also slightly thicker than the middle one. But the thing is, the new rings I've had supplied, Hastings, part no. 139, which apparently cover all the early Cadillacs, 365's included, have both compression rings the same thickness and no shim included. Are these right?

The common sense thing seems to be to fit the new rings with the old shim on the top rings which seems to tighten it up nicely in its groove. Has anyone else come across this? Kurt.

[Dave Shepherd]

Kurt, there should be no shim, that is usually done to make up for a worn piston ring groove, measure the grooves see if they are in spec, then it's on to new pistons.

[Kurt Wrebel]

Thanks guys,

I can't see that the top groove could be that much worn to need a shim - because the pistons really are in excellent order, in fact, the entire engine looks like a low mileage one. That top groove is definately engineered that way. I'll check into cord piston rings and see what they're all about.

Kurt.

[Pat]

The top ring land usually takes the brunt of the wear ...so it was quite common in the "old days" to recut the top ring land to make it square and then shim the new ring. You could also get a waffle shaped expander to put behind your old rings to restore the tension of the ring. Of course,when you had the piston out you would have it knurled to reduce cylinder wall clearance.
Aah, the good old days when they squeezed every last mile out of a engine for little cost as possible.
Cheers,Pat

[Kurt Wrebel]

It's starting to make sense now Pat. That may be why the top groove is larger. The new set of rings I've got fit great in the middle groove but not so in the top one - definate slack there. Looks like a new set of pistons coming up.

[TJ Hopland]

So you had a universal assortment of ring shims?  And a lathe (if you were lucky)? And just done made er fit better?  Or was there some science behind it?

I did not live through those days but I do recall stories where you started out pumping gas then one day the boss said "go on and pull the heads of of Mr Smiths Buick and see if you can figure out how to grind valves,  and if the Borax down the carb didnt take care of the smoke pop those pistons out and slap some rings in there before lunch".    After that you were an ASE certified mechanic, as long as you worked at Al and Steve's Express that is.     Now people get mad when they need spark plugs every 100K.   

[Pat]

TJ,
From what I recall, you placed the piston in the vise and mounted a tool onto the piston and rotated it by hand until the ring land was true. Once that was done you placed a flat shim (which looked like a rail from an oil ring) in the groove that brought the land width up to spec. There was also a smaller tool you used to clean out the carbon in the ring land in a similar fashion.
Cheers,Pat

[TJ Hopland]

I would imagine that tool looked something like a tubing cutter?   Seems like I have seen a tool in an old shop that could have been that. It would seem like a logical design because it would fit several sizes and could have different cutters for different needs.