Oversized piston options for a 2001 yz250

[jjkaffun]

I am almost done with an engine rebuild - just need to figure out the top end. As you can probably tell I am dealing with a sleeved bore (not by choice - came with the bike). It has some minor scoring exhaust side on both the cylinder and piston. I am not exactly sure how much the cylinder could be honed out and still be in spec for a stock bore D piston (I have a feeling there is for sure some wear besides the light scoring being that its a sleeved bore). It had a Wiseco piston in it - so I believe that is between a b-c or c-d piston (cant remember). I am going to take it to one of the mechanics at my workplace and see if he can get some measurements for me - no tengo un bore gauge. If I could throw a D piston in there that would be absoF*&%inglutely awesome - I would get through this rebuild unscathed. 

However, after looking at what wiseco has to offer, oversized piston selection is limited for a 2001. The next and only step up from from 66.40 is 68.50. I would really like to avoid jumping up that high so quickly.  Of course the 2002-2017 yz250 has pistons available for every .5mm increase as do the earlier yz's. I am also thinking that there are some necessary mods in order to do things the right way if I ended up having to go with the 68.50mm piston (head mods, PV work, jetting, ect.) which I would rather not get into. And I am not entirely sure If the sleeve is able to be bored to that extent and remain reliable. This is a woods bike. 

But the bottom line is I want to go ride. asap

Has anyone had experience with this, particularly with an 01? Seems like 99-2001 got shafted on the piston selection. Is there another set of years or another bike that I could swap a piston with where I wouldn't have to bore it all the way up 2mm? If I got stuck with bumping it up 2mm what mods would be required? 

BTW the reason I tore down the motor was because my flywheel weight came loose and sheared the entire threaded portion and a bit of the taper off of the crank in doing so. So take this as a reminder to check your damn set screws on the flywheel weight periodically as the manufacturers suggest.

 

Edited March 29, 2017 by jjkaffun

[smez]

2 hours ago, jjkaffun said:

I am almost done with an engine rebuild - just need to figure out the top end. As you can probably tell I am dealing with a sleeved bore (not by choice - came with the bike). It has some minor scoring exhaust side on both the cylinder and piston. I am not exactly sure how much the cylinder could be honed out and still be in spec for a stock bore D piston (I have a feeling there is for sure some wear besides the light scoring being that its a sleeved bore). It had a Wiseco piston in it - so I believe that is between a b-c or c-d piston (cant remember). I am going to take it to one of the mechanics at my workplace and see if he can get some measurements for me - no tengo un bore gauge. If I could throw a D piston in there that would be absoF*&%inglutely awesome - I would get through this rebuild unscathed. 

However, after looking at what wiseco has to offer, oversized piston selection is limited for a 2001. The next and only step up from from 66.40 is 68.50. I would really like to avoid jumping up that high so quickly.  Of course the 2002-2017 yz250 has pistons available for every .5mm increase as do the earlier yz's. I am also thinking that there are some necessary mods in order to do things the right way if I ended up having to go with the 68.50mm piston (head mods, PV work, jetting, ect.) which I would rather not get into. And I am not entirely sure If the sleeve is able to be bored to that extent and remain reliable. This is a woods bike. 

But the bottom line is I want to go ride. asap

Has anyone had experience with this, particularly with an 01? Seems like 99-2001 got shafted on the piston selection. Is there another set of years or another bike that I could swap a piston with where I wouldn't have to bore it all the way up 2mm? If I got stuck with bumping it up 2mm what mods would be required? 

BTW the reason I tore down the motor was because my flywheel weight came loose and sheared the entire threaded portion and a bit of the taper off of the crank in doing so. So take this as a reminder to check your damn set screws on the flywheel weight periodically as the manufacturers suggest.

 

This may help? Search "yz250" on this page.

 

http://www.wossnerpistons.com/products/mode/filter/search/Yamaha?fitment-category-id=BBB40D59-0BEE-E011-963B-000C29296656&fitment-category-name=2+Off-road#&&/wEXAQUaY29uVG9wTGVmdF9jdGwwMV9idG5TZWFyY2gFOi9tb2RlL2ZpbHRlci9zZWFyY2gveWFtYWhhLXl6MjUwP3NvcnQ9bmFtZSZvcmRlcj1hc2NlbmRpbmek9y4stVaE2twRB+QIt2gk4JVZfw==

Edited March 29, 2017 by smez

[YZDOC]

Vertex pistons are for 99-2017. Size D will be hard to find in USA but Italy has them and ships.  

[EmpireofDerp]

So take this as a reminder to check your damn set screws on the flywheel weight periodically as the manufacturers suggest.
 

Can you not just loctite the set screws?

[YZDOC]

15 minutes ago, HeavyRotation said:

So take this as a reminder to check your damn set screws on the flywheel weight periodically as the manufacturers suggest.
 

Can you not just loctite the set screws?

I think you would still need to check them. If you wanted to set and forget. Buy longer set/grub screws, make witness mark on flywheel rim, drill small indent into it, and run longer set/grub screw into indent using green or lighter than blue locktite. Might still need to check them periodically. 

[jjkaffun]

I think I am going to try a 2002 yz250 wiseco piston the next size up. 804M06700 . It isn't listed on wiseco's website to fit the 2001, but I am going to try it. Im not sure why it wouldn't fit at the stock bore piston and 2mm piston are the same part number for the different years. (804MO6640 and 804M06850). I emailed wiseco but am going to order the piston anyway. Can anybody give me a reason why it wouldn't work?

Edited April 1, 2017 by jjkaffun

[jjkaffun]

Just saw on a few sellers on ebay list that 804M06700 will work for the 99-2001. im just gonna send it

[EmpireofDerp]

I think you would still need to check them. If you wanted to set and forget. Buy longer set/grub screws, make witness mark on flywheel rim, drill small indent into it, and run longer set/grub screw into indent using green or lighter than blue locktite. Might still need to check them periodically. 

Hmmm, I set them tight, rode it a couple hours, tightened them again, rode couple hours, then torqued the shit outta them after liberal application of red vibratite. Guess I'll have to just check em every oil change.

[jjkaffun]

well amazon will not let me ship it to my house in california. so much for my 120$ prime membership. shoutout california