Piston Ring Gap - Whats the perfect number?
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Intermediate
Joined: Jan 2019
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From: US
Piston Ring Gap - Whats the perfect number?
What tolerance do you follow for minimum gap? Strictly by the Harley manual or by the piston ring manufacturer? If they are different should I take an average of the two? (all parts are stock on the bike).
Another question I have is how close to the minimum gap should I flirt with? Will playing it safe by a hundredth . or even two be ok? or should I try to make rings as close to minimum gap for best performance?
Another question I have is how close to the minimum gap should I flirt with? Will playing it safe by a hundredth . or even two be ok? or should I try to make rings as close to minimum gap for best performance?
Last edited by dyna50; Apr 22, 2019 at 12:12 PM.
Grand HDF Member
Joined: Nov 2009
Posts: 4,315
Likes: 1,663
From: Tampa
rule of thumb is .004" per inch of bore. Having said that your pistons should have come with a spec requirement.
If you're fiddling around with stuff you already took out of a motor and replacing it with HD product, stick with what the FSM says.
That all being said, you could have 1/8" of ring end gap and the bike would still run great. Having the ends butt is where the issues lie.
If you're fiddling around with stuff you already took out of a motor and replacing it with HD product, stick with what the FSM says.
That all being said, you could have 1/8" of ring end gap and the bike would still run great. Having the ends butt is where the issues lie.
Seasoned HDF Member
Joined: Aug 2008
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mr. matt put out some good info.
to build on his base:
keep in mind piston construction and ring material, makes some diff.
what i usually will do is fit the piston and jug and take good measurements and buy the next ring size up. i will grind the ring to the lowest admissible clearance. usually when you buy piston/ring kits, they will err to the upper clearance.
they also have total seal or gapless rings. a little more to setup but will make a diff.
gap is only part of it, check land clearance also.
to build on his base:
keep in mind piston construction and ring material, makes some diff.
what i usually will do is fit the piston and jug and take good measurements and buy the next ring size up. i will grind the ring to the lowest admissible clearance. usually when you buy piston/ring kits, they will err to the upper clearance.
they also have total seal or gapless rings. a little more to setup but will make a diff.
gap is only part of it, check land clearance also.
Banned
Joined: Sep 2009
Posts: 3,569
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From: deland,florida
What tolerance do you follow for minimum gap? Strictly by the Harley manual or by the piston ring manufacturer? If they are different should I take an average of the two? (all parts are stock on the bike).
Another question I have is how close to the minimum gap should I flirt with? Will playing it safe by a hundredth . or even two be ok? or should I try to make rings as close to minimum gap for best performance?
Another question I have is how close to the minimum gap should I flirt with? Will playing it safe by a hundredth . or even two be ok? or should I try to make rings as close to minimum gap for best performance?
ring gap really depends on the build & application,on normal street stuff,follow the ring manufactures specs.setting gaps to the minimum is OK,just make sure their measured right.
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Intermediate
Joined: Jan 2019
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From: US
For this application it is just a stock engine street ride setup so all the replies do help me out! Thanks all. I ended up taking my top end apart and measured cylinders (within spec) and not much deviation between top, middle, and bottom measurements. So long story short, I honed and got new rings and valve guide seals... I also have another topic that I know from reading in this forum has different opinions, but I'll give it a shot.. What do you recommend for cylinder, piston and piston ring prep for assembly?? Iv'e read and talked to people that work for pretty well know companies and it seems some go with assembly lube on mentioned parts, and others are totally against it.. I was really thinking of going the route of using the break in oil I have on the cylinders, and lightly coating the rings(with break in oil) before install onto pistons, and then assembly lube for the pin... I'm still not 100% on which prep I am going to do on said parts.. I spoke to someone who did the complete opposite, in which he did "not much of anything on the rings" and light coat of assembly lube in the cylinders... and apparently it was on a pretty stout bike.. SOMEONE convince me, with logic of course.
Seasoned HDF Member
Joined: Aug 2008
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at one time, assy lube was the cat's meow, still have some on a shelf.
with today's machining capabilities seems that a plain ole heavy oil will suffice. now if it were long time storage, the assy lube might be a more viable option.
another thing is that oils have come a long way. i do not use the assy lube anymore (the lite grease type)
days of ole, wasn't too hard to pre-lube an engine, still can be done, just not as easy as it was.
with today's machining capabilities seems that a plain ole heavy oil will suffice. now if it were long time storage, the assy lube might be a more viable option.
another thing is that oils have come a long way. i do not use the assy lube anymore (the lite grease type)
days of ole, wasn't too hard to pre-lube an engine, still can be done, just not as easy as it was.
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Seasoned HDF Member
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From: poway
I bore and hone my own cylinders. I guess everyone has their own routine.
I scrub the cylinder in mineral spirits with a shop rag and dry. Then coat the cylinder by hand with Marvel Mystery Oil and wipe with a white paper towel. MMO is red. If the towel is gray, I repeat until new towel looks pink. Slop on a little more. Rings get a drop of MMO, Pin coat of Torco Assy lube (also red). After assembly, I dance around and shake a dead chicken while chanting.
I scrub the cylinder in mineral spirits with a shop rag and dry. Then coat the cylinder by hand with Marvel Mystery Oil and wipe with a white paper towel. MMO is red. If the towel is gray, I repeat until new towel looks pink. Slop on a little more. Rings get a drop of MMO, Pin coat of Torco Assy lube (also red). After assembly, I dance around and shake a dead chicken while chanting.
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Road Warrior
Joined: Aug 2007
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From: South Australia
I bore and hone my own cylinders. I guess everyone has their own routine.
I scrub the cylinder in mineral spirits with a shop rag and dry. Then coat the cylinder by hand with Marvel Mystery Oil and wipe with a white paper towel. MMO is red. If the towel is gray, I repeat until new towel looks pink. Slop on a little more. Rings get a drop of MMO, Pin coat of Torco Assy lube (also red). After assembly, I dance around and shake a dead chicken while chanting.
I scrub the cylinder in mineral spirits with a shop rag and dry. Then coat the cylinder by hand with Marvel Mystery Oil and wipe with a white paper towel. MMO is red. If the towel is gray, I repeat until new towel looks pink. Slop on a little more. Rings get a drop of MMO, Pin coat of Torco Assy lube (also red). After assembly, I dance around and shake a dead chicken while chanting.
Frozen OK in a pinch.