Wiesco Piston Help

I'm putting in a new piston while I have the beast in pieces. I've misplaced the installation info and while checking ring end-gaps, they are way out of spec (according to the manual) on the Wiesco rings. Is there a reason they would be out of spec? Does the fact that this is a forged piston have anything to do with ring gap? If you can help, thanks!!!


Thanks Mike, I remember reading that post, but at the time I wasn't working on the machine. So it was the rings eh? Sounds like Wiesco needs some help making rings. I'll get some Yamaha rings and take these back.


I found the directions, and it says that there should be about 3 thousandths for every inch in diameter (about). Sounds a bit large to me. Any thoughts?

I'm not sure where you got the ring gap specs, but my service manual says .008-.014 inch for new rings. I actually had to file the Wiseco rings to get them inside the specs. Maybe it's that old metric/english conversion snafu. Anyhow, I've had good luck with the Wiseco piston.


Well, I measured the gaps (that the rings fit into) on the old Yamaha piston and on the new Wiesco. The oil scraper ring on the Wiesco is much larger on my piston, so using Yamaha rings is out of the queston. I questioned Wiesco about thier gap and this is the response I got...

Wiseco's Recommended ring end gap is .008ths.plus or minus .004ths. so your ring is within tolerance. Ring end gaps do not differ between forged and cast pistons. The rule of thumb in most cases is for every inch of piston diameter you should have .004ths ring end gap.

So, I'm gonna run em... Next time I'll get Yamaha parts.


Hi Ron,

Thanks for the heads-up. I got the original spec from my 00/426 manual. Maybe there is a misprint, or maybe I just can't read, but I recall the manual saying the gap should be about .002" and out of spec at .005". Wiesco says otherwise. So I'll go with what Wiesco say's for their product and see how things go.


The ring end gap is going to depend on the ring material and its expansion rate. A ring end gap of only .002 on a piston that is 95mm sounds to me like a problem waiting to happen. Are you sure the ring end gap didnt say .2mm? This would be very different. The stock piston is 3.74 inches in diameter,mulitplied .004 per in inch will yeild .015. When measuring the ring endgaps, make damn good and sure your ring is square in the bore. Use the piston to push it flat in the bore fairly close to the middle of the stroke. That should give you the most accurate reading. Take your measurment 3 times and average them if they come out differently, they shouldnt, but sometimes they do. I have seen more than one small block chevy that was assmebled with moly rings that were file to fit and didnt get filed to fit. The rings butt, crack, frag, and goodbye cylinder liner. Ive also seen the 400 small block actually crack the cylider walls when the rings butted because it was set up tight (drag racing)and the motor overheated and the rings had no where go. Wiseco says .004 -.008 is standard, make sure that's not per inch in diameter of piston size, instead of total end gap. It looks to me like piston size, but I dont know what material they use for rings(ie, castiron, chromoly, stainless etc.)

Well, my stupidity has got me again. I was looking at the side clearance measurements (which are in ten-thousandths of an inch). Looks like 8 to 12 thousandths with a limit of 22 thousandths is the range Yamaha uses. DUH :) Thanks for all the info everyone has given. I get an education everytime I log on! When I get the rattle-trap back together with the new 2001 splined cb gear, I'll post to let everyone know how it worked.


I think there is a big difference between what the manufacturer recommends and what WISECO shows. I wouldnt try to run the end gap at .002-.005 unless you want to pay for a new cylinder. The .004 per inch of bore will work but that is for a stock motor!!

Hey Shawn, Thanks for your expertise. The gaps on the rings are about .012", so they are probably good to go. I did all the things you suggest for measuring the rings before I got the .012 measurement. It's entirely possible that my dyslexia returned when I was reading the manual and saw the MM measurement, but I remember looking it over twice for the inches, but who knows. I'll look again when I get a chance to get into the garage again.


By the way, Your'e only soposed to "lightly" tap the tip of the screwdriver between the gap on the ring when it is in the cylinder for the measurement right... right... :):D:D:D

Hootna, I've already spent way too much $$$ on this "new" bike. Your'e right I can't afford a cylinder too. I can't wait to ride it again, but it leaves me thinking that that CR500F that I almost ordered would have been a bit more reliable.


Dont worry if it is an end gap it wont destroy yhr cylinder at least it didnt when it stuck. My CR133 stuck the first time I went wide open and I was so mad. I just got it back from Forward Motion and just broke it in at low rpms and low load and then bam I was wide open 4th gear when there was dead silence well there was the sound of gravel as my locked rear wheel was digging a hole to china.

I ring gapped it at the .004 per inch of bore and it was like .009-.10, well it is now at .018 and runs great of course if I would have been told this in the first place it would have saved me a couple of weeks down time. Thank God for my 250F!!!

Ring gap is an art. Getting a leak down test to 2% requires nothing more that years and years of experience, or the access to it.

Good luck Glen, if youve got your end gap at .020, Im sure the thing will haul ass. If you go with the 13.5:1 remember your going to need good gas. Pump stuff wont cut it.

Thanks guys, I'm gonna check the gaps on my old rings just for the fun of it and compare them a bit. I'll put her together this weekend if my wife doesn't give birth and see how it goes.


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