I built Sn # 003 (the Yellow one you see everywhere) and never had a problem. Has hundreds of hour's on it. I've seen some people have problems. First is the bore on spec. at 1.00" and smooth and shinny? Second did you make the piston groove to the spec. Most of the ones I've seen changed the specs to what they think is correct and they make it too tight. The groove needs to be 0.105" wide and 0.110" deep. The ring is NOT squashed in the groove, it kind of floats. Next try replacing the "O" ring with a new one with a little light oil on it. I've seen engines that sat for a few days loose compression. For some reason the ring shrank, (haven't figured that one out yet, but I'm working on it). Last are you sure you have good valve and spark plug sealing?Decided to try starting my Jerry Howell Farm Boy. I used the recommended vinyl o-ring in place of a cast iron ring. Almost no compression. Has anyone had success with these o-rings?
you are correct in assuming they would start "rolling" as I tried a 1/16" cross-section ring with those dimensions I used with the 3/32"(.100) and the engine never seemed to run right and seemed the O-ring never seemed to "break-in"(new O-rings have a little more resistance). The 1/16" O ring had been twisting in the groove so I abandoned that size O-ring. I'm thinkin' with the pressure against the cylinder and inner piston groove may distort the o-ring and close up some of the side clearance with the groove.I presume some of your sideways squish distorts the O-ring and removes some of your clearance in the ring groove? Do you think it can still "rock" at TDC and BDC as you state?
I was concerned that without a bit of side pressure in the groove the rings might "roll up" as it slides in the cylinder.
I always wondered about the required side clearance in the groove. I looked at quite a few O-ring suppliers websites and all of them spec'd quite a bit of clearance in the groove for sliding seals. I guess they know what they're doing. Nice to see your confirmation of that.
Your dimensions seem to be consistent with those from the calculations provided on the Parker Seals (and other) seal company websites for sliding seals. I would have thought the 20thou or so side clearance would lead to "rolling up" of the ring. But the squish in diameter probably removes some of the side clearance. Your experience confirms the theory. I like that approach. I'm a sort of "try it and see" sort of guy.I use a 1/16" cross section viton o-ring (which is actually 0.070"0. I cut the piston groove 0.094" wide, (which is the width of my cut off tool) and make it 0.058" deep. I have made over 10 i.c. engines this way and never had a problem with compression.---Brian Rupnow
Just a thought - the smaller bore isn't causing the o-ring to kink as it's fed into the bore, is it? (As @Engine maker pointed out the ring groove clearances on the Farm Boy drawing are relatively large (and don't adhere to any of the recommendations above), so there's the opportunity for the o-ring to buckle away from the cylinder wall if the bore is under-sized for the ring.)The cylinder of my Farm Boy is .980. Not exactly 1 inch
Brian, what was the bore ranges of the engines you have had success with the the 1/16"(.070") O-rings? Thanks. Later, rt.Your dimensions seem to be consistent with those from the calculations provided on the Parker Seals (and other) seal company websites for sliding seals. I would have thought the 20thou or so side clearance would lead to "rolling up" of the ring. But the squish in diameter probably removes some of the side clearance. Your experience confirms the theory. I like that approach. I'm a sort of "try it and see" sort of guy.