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Wiseco vs Yamaha Piston Ring End-gap Measurement
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Posted by: Imho---------------------
According to the manual for the 1992 YZ125 the piston ring end-gap is 0.022"-0.028" but the Wiseco piston I just received has a piece of paper which states that the ring end-gap should be 0.004" per inch of bore (56mm = approx. 0.009"). I have emailed Wiseco but have received no response. Is there a reason for the difference?
TIA.
Posted by: e.c.bert---------------------
Trust this fact; as long as the ring you use is manufactured for that piston don't give it a thought. If you're wanting a better seal just run a quality synthetic oil Like Motul at 32/1. No wories.
Posted by: NO HAND---------------------
A few years ago I've had the bad luck of ruining a perfectly new engine with ring end breaking. Since then, I think it is cheap insurance to check the ring gap. It hasn't been fine out of the box for me
. I measure and file to get a ring gap of 0.012. I thought it was 0.012 for 125s? With all those numbers it gets pretty confusing. I'm anxious to hear what others do.
Posted by: Imho---------------------
I did check the gap and it was 0.009" as per the Wiseco instructions.
Posted by: Tom Dixon---------------------
The 0.004 is right. You have to remember that your manual is in metric and Wiseco measures in american. I have put wiseco pistons in all different types and sizes of bikes and it works just fine. Make sure that if you have an exhuast web in your bore that you drill the piston for lubrication for the exhuast web, or you could score the piston. If you have an web in the exhuast port, that web has to be ground down a few thousandths of an inch to allow heat expansion; if the engine has been bored out. Hope I didn't confuse you.
Posted by: Imho---------------------
Thanks Tom. I checked the manual and it definitely says 0.022-0.028" end-gap. My only concern was 0.009" was so much smaller but I guess it must be ring or piston material that makes the difference as the bike is running fine after the break-in process. I had to relieve the exhaust valve due to it being 0.040" overbore but I'm not sure what you mean by exhaust web. Can you explain thanks.
Posted by: georgieboy---------------------
You have to relieve the exhaust bridge a 0.05mm for heat expansion. If you don't do it, big change the bridge will melt to the piston as there is no room left due to heat expansion. In a good world with a relieved exhaust bridge the bridge will never touch the rings or piston, so why drilling the holes?
Posted by: Tom Dixon---------------------
An exhuast web is the same as an exhaust bridge. You have to drill holes (2) in the piston to lubricate the bridge. (Wiseco tells you in the directions that came in the box with the piston) You need to lubricate the bridge so when it expands it doesn't scuff the piston and hurt it.:D
Posted by: Imho---------------------
Thanks Tom. The piston came from Wiseco with two holes drilled one above the other, about an inch apart, in line with the exhaust port. I think that is what you were talking about.
Posted by: Tom Dixon---------------------
Yes. Good thing the piston came with the holes so you don't have to mess with it. Just remember to read the directions and you will be fine.
Posted by: Imho---------------------
I received a reply to my email to Wiseco which states: 'The ring end gap measurements in the manual are metric dimensions..0.22-0.28 divided by 25.4 is ..009"-.011"'. I have a Clymer manual, which I double checked. I can only guess that it is a miss-print as it DEFINITELY says: 0.50-0.65mm (0.020-0.026"). All I can say is it pays to check.
Thanks to everyone for the help.
Posted by: Hootna---------------------
Dont follow the instructions on the WISECO piston paper, it is way off for a big bore. Well thats what I assume caused my 02 CR133 to eat the last piston. I have now gapped the new Piston Rings at .018 and the bike ran just fine. I also think I was running the jetting on the lean side but man this thing seized so quick that whatever heated did it real fast so I assume that this had to be the ring.WISECOs instructions states that the ring gap of .004 per inch of bore is not for modified motors. Do it at your motors risk....
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