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Engine Mechanical TopicsDiscussion for motor builds, cams, head work, stripped bolts and other engine related issues. The good and the bad. If it goes round and around or up and down, post it here.
Heres some pics of pistons and cylinders, revolution performance 98 domed kit, cylinders new last year, pistons cleaned twice and reused, no oil in the intake but had a small head gasket leak so thats why I tore it down to rebuild and use some screamin eagle parts, new cylinders and forged pistons, only had 7000 on this build, so where does all the carbon come from? Bad oil ring and scratched the pistons so they dont burn it off? Im baffled here, heads were redone 18000 mile ago, small bit of red in the cylinder too, had this issue since the first build and the pistons got reused every time so maybe the pistons are just ****
Thats definitely possible, I gapped them
about 3 inches down in the cylinder, also has some scratches on the pistons from cleaning I tried to sand out so that didnt help either, its all being replaced but I dont want a repeat of this so where should rings be checked at? I went .017 and .019 on the gaps last time, so maybe go wider this time if they arent pre gapped, its a se kit this time so they may be gapped already and they run a tighter piston to wall clearance compared to the revolution kit
That’s definitely possible, I gapped them
about 3 inches down in the cylinder, also has some scratches on the pistons from cleaning I tried to sand out so that didn’t help either, it’s all being replaced but I don’t want a repeat of this so where should rings be checked at? I went .017 and .019 on the gaps last time, so maybe go wider this time if they aren’t pre gapped, it’s a se kit this time so they may be gapped already and they run a tighter piston to wall clearance compared to the revolution kit
Cylinders should be round and the same size. That's how they are when bored with good equipment with the cylinders properly clamped and torque plates.
The hone is just for cross hatching finish.
If cylinders are correct, should not matter. Measure them. I use inside micrometer. Now it's not calibrated. I use my outside micrometer to actually get a size from the inside micrometer.
If your checking a ring gap on used cylinders, check it at the bottom in the unworn area. Remember, any wear areas after finial hone sould not be over allowable wear tolerance.
And if you are near wear tolerance, engine life will be short. And noisy.
Usually, wear is greatest just a little way down from the top.
Remember, most pistons are cam turned and have a certain area to measure OD. On used, that area will show plainly.
You need measuring tools even if all you are doing is the finial assembly,
Last edited by Jackie Paper; Jan 12, 2025 at 05:55 AM.
i never trust ring gap from the box. i usually buy first over size and set gap at the tightest allowable, but the grinding must be done right.
pistons are not true round and sizing is done at the shirt level 90* from the pin.
ring are set gaps 120* apart on the gaps.
bore needs to be measured in several places going down.
pay attention to rings, bought a 1975 fxe for a song and a dance since the guy rebuilt it himself and installed the rings upside down and it smoked worst than before the top end.
rings will rotate in the pistons when running at a very slow rate.
did you check land clearance???
carbon can come from several sources and is normal even with unleaded fuel.
lack of lubrication on start and fuel wash.
many sources so you must back track, reason why i keep a log of all measurements and such.
also of note, some pistons are treated and reusing them can bring issues as the surface area is worn or altered.
Ok, I will definitely get an inside micrometer to measure when I check these rings, this new kit is a se kit from suburban speed which is a Harley shop that does the king of the baggers bikes, new cylinders with fresh bore and they say the rings are gapped and just need to be checked, when I did them last time I just took revolutions word on the 3.93 bore size, but didnt account for the nikasil coating in the bore, that coating breaks down fast as you can see in the cylinders, I gapped rings based on the size listed on the bore and may have been a little tight, thank you again
i never trust ring gap from the box. i usually buy first over size and set gap at the tightest allowable, but the grinding must be done right.
pistons are not true round and sizing is done at the shirt level 90* from the pin.
ring are set gaps 120* apart on the gaps.
bore needs to be measured in several places going down.
pay attention to rings, bought a 1975 fxe for a song and a dance since the guy rebuilt it himself and installed the rings upside down and it smoked worst than before the top end.
rings will rotate in the pistons when running at a very slow rate.
did you check land clearance???
carbon can come from several sources and is normal even with unleaded fuel.
lack of lubrication on start and fuel wash.
many sources so you must back track, reason why i keep a log of all measurements and such.
also of note, some pistons are treated and reusing them can bring issues as the surface area is worn or altered.
i will definitely check these at a few spots when I assemble it, and wash multiple times, the pistons may have also been coated to go with the nikasil cylinder coating and I damaged the coating cleaning them, they probably should have not been reused, being that I had this issue 3 times I went with a se kit this round, some of the crust is also connected to the revolution kit wanting marvel mystery oil put on the pistons and rings for assembly and possibly got burned to the tops of the pistons at break in when they had some scratches from cleaning and the coating being gone, land clearance is good as we didnt change the stroke and height but revolution does have a slightly different wrist pin height, we also went with a thicker head gasket this time because compression was too high with the last kit and was hard to get 93 octane gas on road trips
So much going on with the OP and the original build and his plan going forward it is useless to try and figure out what went wrong with the first; so many things that could have caused the problem.
Going forward, new cylinders and .010" OS pistons and having cyliners bored, pistons fitted and rings gapped by a professional machine shop that has provided this service is the best way to be sure the top end will be correct. Send the cylinders, pistons and rings to a shop, like Hillside Cycles, BigBoyz, VeeTwin Performance, etc. At least have the valve guides and seals inspected and replaced if necessary. Get the new top end to a competent tuner for break in and tune. This will leave "operator error" as the most likely reason for another screwed up top end. i.e. cleanliness and proper assembly
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Pistons and cylinders
