250-500cc Another Piston Thread

Mike that should fit fine. Let me know when you get it I"'ll swing by and measure your clearance.
The Vertex appears to be a very high quality piston.

I was wondering the same thing about that and ones on ebay. There (I have not been able) is not any info stating which size cylinder it is made for, you could assume a "C" because a C is in the part number, but I could not find an "A" or "B" part number. I went with an OEM piston kit (cost more of course, but was delivered in less then 5 days from Hall's) and is also a 2 ring piston. Vertex was OEM for my 300.

Mike, that looks to be the 1 ring version for the CR motocross engine. I would call them. The piston I ordered is P/n 22601B. Got it on Amazon for $114

Thanks everybody.

I emailed RMATV. They responded that it's a 2 ring piston.

I looked at service manual and found tables for cylinder and piston sizes. Looks like that size vertex should fit.

For C size cylinder, cylinder is 66.395-66.405. Piston would be 66.345-66.355. Vertex is 66.35 mm.

.045" clearance should work perfect.

Does anyone have a cylinder that is no a "C" size? I seems like everyone who has posted was a C.

My 300 was an 'A' size cylinder

Lanky, if your past the "C", l reckon it would be time for a rebore? I ran the 360 to the last of the sizing (D type or overbore) and found the bore wore quite quickly compared to the A-C type pistons.

Thats interesting, personally i would get it re nikasiled, i have 360 02 thats on c piston and will be sending that off to get taken back to std A size.

Have 93 cylinder in the bike atm wich has a liner in from kustom kraft, will see how long she lasts.
93cylinder is on wossner D piston.

Right on the edge Juicy, what l found on mine when l ran the D size was that it wore quite quickly where there were those faint score marks around 4-5 o'clock locking at it from the PV end that l honed out earlier, probably got 80hrs out of it before the whole bore got out of tolerance (OE coating).

My cylinder came stamped C from new. Starting out with an A or D cylinder doesn't mean your plating is any thicker or thinner and will wear longer or not as long, the coating should be approximately the same it's just that the cylinder before coating was slightly larger or smaller. This is why you should dial bore check the cylinder in several places because the measurement you get in a non wearing area is going to be what the original size was and any out of round detected is wear. If your cylinder is out of round and an A size and you put a C piston in it to compensate it will be too tight in the less worn area and would be more likely to seize if it got hot.

Fair point buuut the difference from C to D is half mill now from A to C is 0.03 mm
So the cylinder will be rebored to make to to a D.

My 95 made to the D piston, the ring gap needed to be slightly filed but the old girl got to the D cup...not that l intentionally wanted to run the D cup but what the heck a rebore was going to be done anyway...made no diff really.

Yes it's not likely to make any difference on a cast piston with very low expansion but a forged piston in a bigger bore like on a 300 when it's hot in a bottleneck at a mud race it could be a bad deal. I'm not saying it can't be done, only that if you measure correctly then you will know what you are risking. There are older threads on the Cafe where some WR 300s have stuck pistons.

Mine suffered early last year so i got it honed out an smidge more like 0.01 more the woosner gap says 0.06mm clearance but i seized it.
So far did a very cold practice day in wales it was snowing and there was lots of dropping in water mud and going nowhere with engine working. No issues.
You have to make sure your engines warmed up thourghly with cast liner and forged pistons!
£££ if not

You do realize that all of the sizes of pistons use the same ring so filing would only help it to wear faster.

A cast liner definitely is a different deal, too bad it did not remain stock with a coated liner that would have been avoidable.

Got 2002 stock cylinder but that needs nikasil n new piston..
Still the 93 is a beast of a ride

So the piston kit is finally coming this week. I'll try to install it this weekend.

Being my 1st time doing a top end I want to make sure I break it in correctly. I've read opinions on doing it very controlled and letting it just rip. I tend toward the controlled method.

I couldn't find anything in the service manual on break in.

The closest Owners Manual I found was for a 2007 WR250. Does this look worth following?

Mix 32:1 for break in

RUNNING IN
To obtain the best settling of the engine moving elements, for driving your motorcycle to the best of your capability, run in the engine for several hours, following these procedures:
1. FROM STOP POSITION. Start the engine and run at idle, but open the throttle periodically and briefly until the engine is thoroughly warmed up. Within 3-4 minutes the coolant temperature will have reached approximately 60° C/140° F. (Do not ride the motorcycle).

2. Stop the engine, and let it cool down naturally until its temperature is equal to the ambient air temperature. This will allow the piston to align itself to any imperfections which might exist at the cylinder wall.

3. Repeat steps 1 and 2. (Do not ride the motorcycle).

4. Bring the engine up to normal running temperature. Ride the motorcycle approximately 10 minutes at moderate speeds. Then repeat cool down procedure. AVOID HARD ACCELERATIONS.

5. Bring engine up to normal temperature. Ride motorcycle approximately 15 minutes at moderate to high speeds. Again avoid hard accelerations.

6.Repeat cool down procedure.

7. Full throttle operation must be avoided until the engine has reached operating temperature, even after the break in process is completed.

When the above procedure is followed correctly, engine durability and performance will be greatly enhanced.

thats a pretty good break in plan..that and avoid holding a set speed. lots of varying is needed while avoiding harder loads.

Check this article out, what he says makes sense to me (just an average white guy). But I do still run one heat cycle first, just to make sure everything is running alright, then I follow his method. http://www.mototuneusa.com/break_in_secrets.htm