Setting ring gap on an Harley Davidson engine build

Posted by By at 14 March, at 01 : 02 AM Print

Making sure the ring gap is right for your bore size is an essential part of any engine building, including when slapping together some Harley Davidson kit. Neglect it and it will come back to bite you…big time…

So why do we have to do this?

We have to make sure there is enough space in the ring gap to accommodate ring expansion as it heats up during normal engine operation. If not enough space exists to accommodate this expansion then the two ends of the ring will butt up as the ring swells (expands) and pressure will start to mount. Remember that those first and second ring are pretty though and any deflection has nowhere to go in that ring groove when that ring expands… so something bad is will happen.

Its not a difficult task though, it just takes some patience and a little skill…
What ring gap should I run?

There is a rule of thumb for ring gap where you set it to .0045 times the bore size in inches for a street build. There are circumstances where you might want to run a little more gap… check your ring/piston manufacturer’s spec for your particular ring set and circumstances… Naturally, the more heat, the more gap is required. Since our beloved Harleys have a tendency to get hot, I like to get closer to .005 times bore size for gap setting on the top ring.

So let’s say you are building a 103 upgrade to your late model TC… This would translate to:

Bore for a std 103 is 3.875 in…

3.875 X .0045 = .01743… So gap to .018 in …. Always round up, not down…

OR

3.875 X ,005 = .01937… So you can gap .019 or .020 here…

Personally, I gap them .019… for both 103 and 107, .018 is pretty much fine too however, Carillo’s customer support suggest using the .005 figure for Harley Davidson motors…

The second ring will generally be set a little larger .004 to .008 inch larger than the top ring. Be careful while filing the second ring though, the material is usually much softer than the top ring so it will file much faster so don’t go over, take it slow with the second ring.

Oil rings can generally run small gaps (like .0015) but are quite thin and should not be filed… they should be ok out of the box… but again, you must do what the manufacturer of the ring puts in the install instructions.

Ring gap setting

To check gap, simply square the ring in the bore and measure with a feller gauge… The blade of the gauge should be a little tight, that is, you should be able to slide it out of the gap with a slight resistance but without disturbing the ring…

You will get to the desired gap slowly as you file away material and re-test… Be careful here as you can always remove more material but you can’t put it back if you go too far. But there is no need to sweat, the important thing is that it is not too small, a little bigger will still run… Remember the old adage, loose runs, tight does not…

You will also need to fit the rings to the individual bores as each bores will have their own size. Notice on the picture, I measure the size of the bore and mark it with a sharpie on the jug… I will do the same with the assembled piston (which is measured and best fitted to the bore also…) and mark the piston to go in that jug… You can also mark them front and back… I usually put the smaller jug in front as the tighter the clearance, the better cooling it needs… but I am a little anal this way…

I can’t stress enough the importance of checking ring gap… even if your ring have been pre-gapped by me or another shop… always measure twice and don’t take a chance. I have see pre-gapped rings come in and not be gapped at all… if that engine was assembled as-is, it would have been a disaster… Never take a kit for granted…

So take your time and make it right, you will be glad you did…

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I run a small garage in Napierville, Quebec. We do mostly interesting things, old cars and motorcycles. This site is where I share videos of what we do hoping that it will help some one in their own car endeavors. We are currently expanding our operations to include parts and accessories... stay tuned.

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