Piston Ring Gap

[Tom H]

Guys, I hate to be such a pest about little details, but I've never done piston work on a car before. I'm assembling my 1725, now 1820 bored and stroked. I have custom pistons from Venolia, 3.248 bore. The instructions with the rings call for 3 to 4 thousandths per inch of bore on the top ring. That's about 10 to 13 thou gap. The WSM says 24 to 32 thou. The gap is already 18 thou before I do anything. Any problem leaving them as they are at 18 thou? My assumption is that the people who made the block have more "authority" on the proper gap than the people who made the ring. But maybe a compromise right at about 18 or 20 is best.

Thanks,

Tom

 

[todd reid]

Tom,
I am not a piston ring expert, but I know that tollerances have been tighten up over the years as the Auto Industry has tried to "dry up" the combustion chamber to help lower emmissions. I would suggest you talk to the machine shop that bored your block and Venolia. I wouldn't blindly follow the workshop manual - it as old as our cars, and in this case probably a little out of date.

 

[Ken Ellis]

Any idea of differences in ring materials? I'd vote with the manufacturer of the pistons and rings, if it were me. When I think about it, ring gaps are basically a convoluted leakage path from the combustion chamber to the crankcase (somewhat sealed by oil), so smaller is better until temperature causes the ends of the rings touch, and then things jam up. (In my 30-second thought experiment.) Could it be that 60s era materials had a higher expansion rate than current materials used for rings? Or is the same steel still used? Hmmm...

As per usual, more questions than answers...
Ken

 

[tony perrett]

If in doubt, I would go for the larger gap. The compression loss would be hardly discernible, but if the ring gap is too small, you could experience serious damage.

 

[Bill Blue]

Tom, I'd go with the ring mfgr recommendation, but that really is a minimum. Larger is simply going to lead to a little more compression leak and blow by. But 18 thou is not going to screw up anything. I'd slap them in and find something else to worry about.

Bill

 

[husky drvr]

Tom,

If your second and oil rings maintain the WSM specs at .009 to .014 inches, I don't think I would be too concerned about your .018 top gap.

 

[Tom H]

Thanks guys. I'll put them in today.

Tom

 

[Nickodell]

Any idea of differences in ring materials?....Could it be that 60s era materials had a higher expansion rate than current materials used for rings? Or is the same steel still used?
Ken

IMHO they still use the same cast iron (not steel) as always.

 

[Lee DeRamus saoca0404]

This is a little off the subject, but you might want to save those old rings. A friend that belongs to a club that restores the old antique stationary "hit or miss" engines sucn as John Deere, showed me how to weld cast iron using these old rings and an oxy/acetelyne touch. Workes just like brazing rod, but better.
With the new modern products now days, I have no idea what the rings are made of. When I rebuilt mine, I used the wsm specs, and good luck for almost 30 years, minus a few years down time.