Top end replacement

yz250flash

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#1
I will be replacing the top end in my 2000 YZ250 shortly. This will be the first time since I have owned the bike. My question is; when looking for a top end kit if, I choose O.E.M., I have the option of piston size (A,B,C, or D) based on the corrisponding cylinder size (A,B,C or D), correct? With an aftermarket I don't see this option. Are they one size fits all or am I missing something?

Is a wiseco piston any better than the O.E.M. one?

Any help is appreciated.

Stacy
 
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#2
I think there was a post on this in another forum and I will try to recolect as best as I can for you. Pistons on a new bike are size A. As the pistons are cast or forged at the factory they are measured and some will naturally be larger then others, so due to quality control the larger the piston the higher the letter. Because a new cylander has no wear it is mated with an A piston. Now that you have ridden your bike for some time it may be time to step up to a size B. Aftermarket companies on the other hand just come out and say whether it meets stock dimensions or how much bigger it is instead of using letters. The only way to tell for sure which size you need is to get it an inside diameter reading of the cylander and compare that to stock. Any competent shop should be able to tell you the size and what size piston you will need.

I'm pretty sure my info is correct but you may want to look around to find out for sure.
 
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#3
Aftermarket pistons give you more choice, same or better quality, and is usually much cheaper. I've never used a stock piston before when rebuilding
 

Yogurt

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#4
The problem with stock pistons is, I believe you have to order, the piston, the rings, circlips, writst pin, and bearing all seperate. Where as if you order an aftermarket kit, it comes with all the neccessary pieces. Usually, and in most cases getting an aftermarket kit is cheaper.
 

dirt bike dave

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#5
Mach 1 Motorsports is packaging top end kits with OEM parts for common Yamaha, Honda and KTM bikes.  The prices are substantially below list.  I recently ordered from them for my CR250.  For the '00 YZ250 the top end kit price is $77.71 and that should include piston, rings, circlips, wrist pin, wrist pin bearing, head and base gasket.  These are some of the best prices on OEM parts I've ever seen. 

Go to the accessories section of their homepage, www.mach1motorsports.com

 
 
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yz250flash

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#6
I went to that site earlier. It is a good price but it doesn't inlude the head gasket. Still a good price though.
 
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#7
Vertex offers piston kits in ABCD.

I've had great luck with Vertex (OEM) on my KTM's and will replace my '04 250 two stroke piston with a vertex when time comes.


Yamaha cylinders stamped with A, B, C, D on the back of the cylinder (facing carb).

Vertex sizes not quite the same, so look up your cylinder size in your Yam manual, then order the right sized aftermarket or OEM piston.
 
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#8
Oh, ya, your 2000 is waaaaaayyyy overdue. Shucks, why are you worried about the right sized replacement piston when you ran the old one so long!?!
 

yz250flash

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#9
The top end has been replaced often just not by me. I have not owned it since it was new. The current top end doesn't have that much use on it but I want to do it anyway just to keep it fresh.
 
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#11
i just stuck a weisco in mine, (2000 YZ 125) first time since buying it 6 months ago and it was really due for a top end, it had a virtex in it but it didnt look too good.... weisco is one of the originals when it comes to pistons they have been around a long time.... its crisp as a jiffy now but thats common foir any new freash to end....