Thinking about bores, ring sizes and ring end gaps

[pbsch]

I'm thinking that with a hone on a set of well used cylinders, the bore may be a bit big on a set of std replacement rings. That got me to thinking if the end gap on a set of STD rings is too large, could a set of .010 over rings be used and just filed on the ends?


Based on the assumption that overbores are stated in increase in diameter, I came up with these numbers
CYL size bore mm circum mm
std 92.029 197.1215166
0.005 92.156 197.3935442 * I realize this isn't a real option, but the numbers are important
0.01 92.283 197.6655719
0.02 92.791 198.7536825
0.03 93.553 200.3858484
0.04 94.569 202.5620696

If this chart is correct, the .010 over ring is only a total of .022in (.56mm) longer than a standard.
Assume after 150K miles of wear and a hone, the bores are .005 oversize already.
The spec for piston ring end gap is .25-.50mm.
The difference in circumference between .005OS and .010OS is .027mm
I know that I would have to check the ring gap at numerous points in the cylinder due to uneven wear, targeting the minimum of .025 end gap at the point of least wear (probably somewhere between 3/4s & 9/10s of the way down the cyl)

Three questions:
Is my assumption about overbore correct?
Is it feasible to use the .010 over rings and file off the ends of the rings in the event they are tighter than the .025 minimum end gap spec?
Does the end gap have a significant impact on the leak-down performance of the cylinder?

Thanks,
Paul

[95Grand-Am]

I know what your saying but cylinders wear in a taper with most of the wear up top and little to no wear at the bottom. So at the bottom of the bore if you use oversize rings have a good chance of the rings butting and poping the tops off the pistons. I would use standard ring or overbore the engine if your looking for the best seal.

[Ed_Mc.]

I'm thinkin' with that much wear, you're gonna have piston slap issues if you don't bore the block out and used oversized pistons. I expect that if you were to check your block for wear and cylinder taper, you'd be very surprised. These run with quite snug tolerances so if you're gonna put a lot of effort and $$ in a rebuild, may as well build one that'll last.

While you're at it, add set of the early Z34 DOHC pistons, when used in an iron-headed RWD motor they give you a nice increase in compression ratio.

Check out the parts list on this '3400' iron-head roller-block build, they show use of Federal Mogul Z34 DOHC pistons, H684CP .020" oversize.

http://www.fierosound.com/34engine/3400details.htm

HTH..................ed

[60dgrzbelow0]

What you have in mind to do would work if you were trying to bend mild steel (like a coat hanger) in something a lot less demanding than brittle, hard cast compression rings that will end up trying to seal the cylinders for millions of cycles of motion. But this is more complicated by the fact that the castings have been made to involve the spread of that extra fill space of 0.027mm over the ENTIRE CIRCUMFERENCE OF THE RINGS. Ergo... if you make the mistake of trying to squeeze those rings into a tighter circle than they were designed to be squeezed down inside of... even with the additional filing of those rings down to an already dangerously close fit of only 0.25mm...(getting closer to 0.50mm would be a better idea) what you will wind up with are a set of rings that are already pre-stressed too much... being asked to compress even more tightly when the piston approaches BDC ...and then trying to expand and fill in the looser dimension as the piston approaches TDC where the cylinders on a used engine have long ago been wallowed out quite a bit. Rings put under this much "pre-stress" will more than likely crack and fracture very early on after even the most delicate of break-in strategies. Ed_Mc. has offered you some sage "Holmes on Homes" sort of advice... Pull that engine... tear it down...and save enough jingle to let a machine shop bore those old, tired cylinders 0.030" over... nice and straight and true and then build yourself an engine that won't leave you waking up in a cold sweat at 3 O'Clock in the Morning wondering when the damned thing is going to fail... and make you have to start ALL over again!