1998 YZ250 Big Bore Kit

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sledhead999

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Mar 4, 2005

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• Mar 4, 2005

• #1

I have been looking around for big bore kits, basically the biggest I have found for this year (92-98) is a 292cc. It runs a 74mm piston or +6mm over the stock 68mm. I was wondering if anyone has had one of these done? I am wondering if this is too big of an over bore and not leaving any water jacket material left. What about the inner O-Ring location?

My bike does light laps at the track and sees most of its time at the sand dunes. I would like to get more mid to WOT out of it any ideas.

The 265cc kits just dont seem to get my attention mainly because it doesnt seem like you get much out of 15cc's.

Thanks.

 

Last edited by a moderator: Mar 4, 2005

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Rcannon

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Nov 17, 2001

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• Mar 5, 2005

• #2

The problem with going bigger is it cuts away too much of the powervalve. I had a 317 cc kit for my 97 YZ. No cylinder issues, but the barely there powervalve fell apart.

 

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sledhead999

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Mar 4, 2005

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• Mar 5, 2005

• Thread starter
• #3

Rcannon,
Where did you find a 317 kit for your '97 YZ? Would you say I would be better off staying at a 292cc kit for the valve reason?

Thanks.

 

9

91kx

Member

Mar 6, 2004

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• Mar 5, 2005

• #4

Ohlins used to make a 360 kit with a completely different cylinder for 98 and older yz's.I'm pretty sure they're no longer available but with some luck you could maybe find a used one.

 

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Rcannon

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• Mar 5, 2005

• #5

I would stay with an Eric Gorr 265. The reason?? For the cost of the other kits, you could buy a spare CR 500. My kit was from RPM. It was made years ago using a special piston Weisco made for them from a blank intended for a 1985 KX 250 three wheeler.

Once weisco makes the piston for them, that specific company owns that size. They can not, or will not, make one for you and I. This means RPM or whomever is able to charge you as much as they would like for the replacement piston. The cost in my case was over 200.00.

Eric's pistons are about the same as a normal weisco. His kit is only 1/3 the cost up front. The 265 kit also leaves enough room for a strong powervalve.

Anything over some specific requires one of the pins to be ground away that hold the valve together. I would stay with a small enough size to keep this strong.

Besides, the bike has enough power. The porting is the weak part. With porting and 265 cc's it will be a whole new bike. Heck with porting and a standard piston it is a whole new bike.

I figure the porting guy on these bikes also designed the oil filler. Can you believe how much fun this bike is while refilling the tranny oil???

 

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sledhead999

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Mar 4, 2005

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• Mar 5, 2005

• Thread starter
• #6

OK, sounds like you have been down this road before. That is exactly what I was looking for.

I see the 265 kit costs $500. Do you happen to know what is included in the kit? Is the porting included?

I might know where a 96 YZ250 cylinder is that I can buy. That would keep me riding while the BB kit is done. Is there anything wrong with this year of cylinder vs. my 98 cylinder?

Thanks for all the help.

 

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Rcannon

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• #7

The 96 cylinder has a different intake. Your 98 reed and manifold will not bolt in to the older cylinder. I believe the pipe is different as well. I am not sure what is included with Erics kit, but it does say porting is included.

I bought my old 317 kit on ****. I learned a lot about the kits and sold it for exactly 5 times more than I paid for it. I was very lucky. It also came with a stator and several new powervalves. **** is great in the winter!

 

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sledhead999

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Mar 4, 2005

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• Mar 5, 2005

• Thread starter
• #8

I am glad I asked about the 96 cylinder before it bought it. From the side the powervalve cover looks the same but I never thought about those other things.

Does Eric's shop sell ready to go cylinders?

Also do you know of any companies making aftermarket heads for this bike?

Thanks.

 

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sledhead999

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Mar 4, 2005

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• Mar 7, 2005

• Thread starter
• #9

Anybody?

 

Rich Rohrich

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• Mar 7, 2005

• #10

Eric keeps a lot of cylinders in stock, ported and ready to go but the stock changes from week to week so it's best to give him a call and verify availability of specific models.

 

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sledhead999

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Mar 4, 2005

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• Mar 13, 2005

• Thread starter
• #11

Does anybody happen to know what the stock squish band is for this year? How about compression ratio?

My bike mics out to about .065" squish. I think maybe the previous owner knocked .010" thou off but not positive.

Thanks.

 

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Rcannon

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• Mar 13, 2005

• #12

I would like this answer as well. I have a stock 98 head for the bike. The measurement is @ 1mm. My 97 head measured 1.52mm. I bought the bike used. I believe the 97 head may have been modified.