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Decking/Machining Cylinder Base YZ 250 2T

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Any one done this ? Machined the cylinder base to lower the ports.

 

I am interested to know what was achieved by doing this.

 

Obviously the head would need machining to stop the piston hitting it.

 

Thanks

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It will pick up a little bottom and mid and trade some peak hp and over rev to get it.

If you drop it about .5mm and are running a stock head - and can run some race fuel or avgas - then don't bother cutting the head it will be fine.

I drop the base on our full 250 mod set ups but only to get more time for the exhaust port to do it's thing without having the exhaust port open so late we kill bottom.  Of course on those set ups everything gets worked on....

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It will pick up a little bottom and mid and trade some peak hp and over rev to get it.

If you drop it about .5mm and are running a stock head - and can run some race fuel or avgas - then don't bother cutting the head it will be fine.

I drop the base on our full 250 mod set ups but only to get more time for the exhaust port to do it's thing without having the exhaust port open so late we kill bottom.  Of course on those set ups everything gets worked on....

Thanks. I haven't got a YZ 250 at the moment, I will get another at one point. I really want to try it next time.  I want strong off idle torque, like a 250 2t enduro bike has. Maybe Porting work would be needed to achieve this.

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If you want to try it temporarily go gasketless.  Use Loctite 515 or a similar anaerobic gasket maker or flange sealant.

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I have searched everywhere on the net for people's experiences of doing this to a YZ 250, I can't get a decent answer anywhere! lol

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It's a ghetto way to raise compression and is usually negated by using a thicker than stock head gasket. The reason this is done is to make sure both the cylinder head surface and deck (top of the engine block) are true. If you're blowing head gaskets or have seriously over heated it's worth looking into. You can always throw a strait edge on it and see if you can get a feeler gauge under it. Check the manufacturers recommended tolerances for your machine.

Edited by zderekv

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Thanks. I haven't got a YZ 250 at the moment, I will get another at one point. I really want to try it next time.  I want strong off idle torque, like a 250 2t enduro bike has. Maybe Porting work would be needed to achieve this.

 

 

A different pipe would be a better approach.  While lower ports can achieve the power number you probably want, it's their effect on the resonance tuning of the pipe that is what you are really after...

 

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A different pipe would be a better approach. While lower ports can achieve the power number you probably want, it's their effect on the resonance tuning of the pipe that is what you are really after...

What he said. Play with your gearing as well. Edited by zderekv

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