View Full Version : Small engine bore

John R
07-15-2012, 01:04 PM
I'm building an engine with 7/8 inch bore. The piston fits nicely in the bore untill I put the rings on the piston. Then it was way too tight. I have opened the bore a few thou but it is still too tight. I have heard of the trick of the piston free falling through the cylinder untill you block the end with your thunb to block the air. Does that apply with the piston rings in place? If so I'll just keep reaming untill it slides. Or is there some other method? Is there a rule for determining how much the bore should be over the piston?
Any help will be greatly appreciated.
John R

07-15-2012, 01:44 PM
The free fall method only applies to cylinder fits where there is a lapped fit with no rings. If the rings are overly tight I would modify the rings (increase gap, check to make sure ID is not too small etc) rather than enlarge the bore. There are some great articles out there about fitting rings to model engines, google away.

07-15-2012, 01:53 PM
Have you taken an accurate reading of piston and and bore dimensions?
Sounds like the issue is with the rings not the piston.
Have you you tried inserting the rings into the cylinder by themselves using the piston to keep the rings square in the bore.

Wondering if perhaps if either the end gap is too tight or ring groove depth in the piston is insufficient.

edited to add: jkilroy types fast than I

07-15-2012, 02:25 PM
I'm no expert, but I'll add a +1 for checking the gap on the rings. You want some clearance between the piston skirt and the bore but you don't want too much. Too much and it can "slap" and damage the rings and bore.

Bob Fisher
07-15-2012, 05:27 PM
Piston ring machining that has worked for me. Cast iron machined to dia of bore, bored to greater than ring groove, part off to a thickness that is a good fit to the groove width, some sanding on a flat surface may be needed. Saw the ring with a slitting saw, I used .010, place a .125"spacer in the gap, stick to a magnet so that the area opposite of the gap is free of the magnet, heat the ring dull red and cool. The ring should now have a snug but moveable fit to the bore. Bob.

07-15-2012, 07:41 PM
The piston fits nicely in the bore untill I put the rings on the piston. I have opened the bore a few thou but it is still too tight.
John R
Sounds like you went too far already.

When designing piston rings, remember a piston ring is both a radial seal and a face seal.

07-15-2012, 09:05 PM
Take the rings off the piston and use the piston to push them one at a time in the bore. Then measure the gap between the ends of the ring. You should have a few thousandths there.

What is the piston to bore clearance?

07-16-2012, 01:04 AM
I had the same problem w/ the first steam model I built, piston and bore were within spec, but did not make the grooves in the piston deep enough. Worth a check!

07-16-2012, 01:20 AM
It's altogether too easy to make a ring go out-of-round when it's compressed. I read about one method, which was to compress the ring til the gap closed up, clamp it that way in a fixture, then machine it to the exact size of the bore. The ring was closed by drawing a wire around it, and a stub was turned to center it on, then it was pinched to the arbor by a washer so it was gripped all around.

I seem to recall an article by George Thrumble, or some such name- I know I got it wrong- his method was outlined years ago in a magazine called Strictly IC. That was apparently the preferred method of producing a ring at the time. Maybe it still is today.

John R
07-16-2012, 09:17 AM
Thanks to all. I'm checking per your advice.
John R