Posts
1254
Joined
4/9/2013
Location
Mayes County, OK
US
Edited Date/Time
12/3/2015 5:07am
I've always run the stock cast pistons in my YZ250, I've raced, well mostly rode, the bike at a B class level in motocross the entire time. I run 14/49 gearing and like to short shift the bike, so it doesn't really get revved super hard. have had zero problems with replacing the piston around every 40-50 hours, its a 300+ hour bike.
Anyways I recently bought an 06 yz125(this one gets the piss revved out of it) and I read everywhere that the top-end should be replaced every 15ish hours, this kind of bothers me because a piston, ring and pin run close to 100$; throw in gaskets and a wrist bearing and we are in 150$ territory. I'm wondering if I could get a longer life out of a forged piston and just re-ring every 15 hours? Also is it possible to re-use any gaskets?
Anyways I recently bought an 06 yz125(this one gets the piss revved out of it) and I read everywhere that the top-end should be replaced every 15ish hours, this kind of bothers me because a piston, ring and pin run close to 100$; throw in gaskets and a wrist bearing and we are in 150$ territory. I'm wondering if I could get a longer life out of a forged piston and just re-ring every 15 hours? Also is it possible to re-use any gaskets?
I always used OEM pistons and gaskets, logged every hour that I rode or raced it. When I raced it in D36 Cross Country the hours went by pretty quick some races were 1.5- 2.0 hours long, so topends came quick.
Once I got envolved with OTHG topend replacements slowed down as a race day only would put on less than an hour of engine running time. Of course compression checks help validate.
For what its worth, I never had a DNF due to failure, proper engine warm up, jetting , high quality oil, fuel and clean filters all attributes.
I always did a complete topend rebuild, never tossed on new rings....on a old piston, I was this far might was all go for broke with new parts.
I wouldn't advise reusing a base gasket, once its been crushed / torqued to specs then reused your just asking for problems.
I guess you could reuse the o rings on head, but why take a chance with a failure???
I compared prices buying each part separate or buying the OEM topend kit... I saved a few bucks by not getting the OEM kit, but just getting the OEM parts needed for the rebuild. Every other rebuild I would buy the complete kit and replace everything.....
Maintenance expense was higher, but so was the thrill.....
Not trying to come off like ass, but....it is what it is.....its part of the game...... That's the beauty of owning a late model YZ parts are really affordable.
I asked my local shop for a discount on OEM parts as I wanted to support them vise the big box shop....which they supported.
My topend ran me about $125 -130.
The Shop
cast. Yes you can squesse a few more hours out of a forged piston.. but just a few...not 15 or 20 more...more like 5 or 6.
II've only re-ringed a piston twice... but it did bring the compression back up to snuff... never re-ring the same piston twice... and after the first re-ring, expect to do the next top end rebuild in half the time you normally would due to the piston, small and bearing, wrist pin already having hours built up on them.
And disassemble, clean and inspect your power valve every topend you do...oven cleaner works perfect... spray on.. let sit an hour or so.. lightly scrub while rinsing... presto.. brand new looking parts again.. watch any rubber or plastic parts... they don't like oven cleaner.
Never use gaskets twice... never. Its a high end race bike... treat it like one.
I have a feeling that jetting plays a big part in top end deterioration. Probably get a lot more life with the next bigger main jet, its so crispy with the 430 but im sure a 440 would be kinder to the piston. Sea level around 45 degrees F.
Once they get a little loose, piston rocking sets in. This is more of a concern . If it's rocking the piston to any degree, skirt breakage becomes a real possibility. That's when the bill starts really adding up.
Clearances can be compensated for to some extent, by using C or D pistons to accommodate for bore wear, provided the plating is still in good shape.
One thing with getting the OEM piston kit versus individual parts, if you get the kit you cannot spec A-B-C-D piston. I think the kit by default comes with a B piston.
I would not try to stretch the hours on the 125 or 250 OEM pistons. The 250 OEM piston is known to have some cracking issues after 40 or so hrs. On both bikes I've noticed some out of roundness of the wrist pin holes and matching grooving of the wrist pin. Makes it a little tough to pull the pin out.
On our race bikes, I go by the engine builders schedule. Ours is built by Barr's. It is NASTY... He says 20 max on that....
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