taking 20 thousands off of cylinder base

[Lorin]

I can have this mod done locally, with little down time or expense, for my 2003 yz250. Just wondering if it is worth doing to gain a little more bottom end. Bike is currently running an fmf fatty pipe and silencer,and a V-force reed cage (rest is stock). I am told that doing this will give better bottom end. In realizing that everything usually comes at a cost (robbing peter to pay paul), what would I reasonably expect to gain and lose with this?

 

[2 strokes for life]

Not sure the gain but why not try to get them to make you a reed spacer instead? Costs less and you dont have to tear the bike apart.

 

[Lorin]

How does a reed spacer help out? As to tearing apart the bike, I am going to do a top-end kit, so no real extra work involved. I can get the cylinder milled for $30, once it is off the bike.

 

[whenfoxforks-ruled]

I do not know what that is going to do to the port timing, advance it of course, but the compression is going to be raised.

 

[2 strokes for life]

Lorin, the spacer helps out by crating some sort of different vacume creating a stronger bottom end power and the nice thing is if you dont like it you just take it out.

 

[D36-108A]

what would I reasonably expect to gain and lose with this?

changed port timing
more piston protrusion
less squish clearance
less combustion chamber volume
more compression

 

[wake_rider]

A riding buddy of mine has this done to his kx250 and he hasn't liked it. It seemed to make his bike quite a bit more jumpy. The powerband is much more instant and unpredictable. I've put a higher compression head on my bike and I've liked it, so I don't know how this differs....

 

[Lorin]

I understand that this will in effect lower the transfer port, and boost compression; both of which boost low end, right? Not trying to re-invent the wheel, just trying to give it a little more torque.

 

[whenfoxforks-ruled]

Then get a big bore.

 

[Studboy]

Some people like the base turned down. I would check and see what your current squish clearance is first. Most bikes come with a squish that is too big already from the factory. If you are trying to get more low end then this may be a cheap way to accomplish that.

Like others have said, it will raise the compression (and tighten the squish), and change the port timing, favoring theoretically low end.

If you really don't like it after you are done, you can always stack the base gaskets to get some height back.

Your YZ should already have a reed spacer unless somebody took it out. I like the way it runs with the spacer in (I also have the V-Force reeds on my 02). Turning down the base will make a much more noticeable difference than any reed spacer.

 

[hot125mod]

wouldn't cutting the base of the cylinder retard the port timing.

 

[Lorin]

Thanks for the comments everyone. I like the fact that with the cylinder base mod, I can stack gaskets if necessary to return to stock. As to boring, porting, etc., I have been down that road in the past (worked well then), but dont really look to spend that much time, effort, and money on this bike. Just trying to obtain a little more torque with minimal down time, and money.

 

[Studboy]

Just make sure that you check your squish clearance beforehand so that you will know if you need to mod the head as well.

There is a good measurement technique on this site: http://www.rb-designs.com/
Under Gas-Gas head mods.

0.050" is a good squish clearance for the YZ250. My guess is that your stock bike is running around 0.070" right now, so 0.020" off the base should put you in the ball park, but check just to make sure.

 

[Pete Payne]

hot125mod,
wouldn't cutting the base of the cylinder retard the port timing.
Yesterday 05:56 AM

You would be correct !

 

[stumanarama]

cant you just mill the head or the top of the cylinder and not affect the port timing, or are you trying to change it in conjunction with upping the compression?

 

[hot125mod]

Yes cutting the head will tighten it up but cutting the top of the cylinder will change the deck height.

 

[2-Strokes 4-ever]

We've done this mod to all of our MXers that we've set-up for woods riding. Back before o-ring head gaskets many manufacturers offered a choice of thicknesses for the head gaskets.......(mill the cyl base and retain stock compression with head gasket thickness) We like the increased torque/low-end of the cyl mod, but you may need to have the head modified if you want to run pump gas.

 

[Lorin]

Thank you everyone, good information all around. Think I will go ahead and do this mod. I will try and post results when all is done and said.

 

[Lorin]

Finally got the bike back together with the cylinder base milled 20 thousandths. Although my testing hasnt been extensive, it seems to have been worthwhile, although not life changing by any means. the difference is fairly subtle, but well worth the $20 invested thus far. Initial indications make me think that race gas wont be necessary as of yet. Between this minor mod and lacing up an 18 inch rim with trials tire, I am really enjoying the bike again. Thanks again to all that replied. Lorin.

 

[bclapham]

you probably killed two birds with one stone- the squish gap on the YZ is often too big and this isnt good for detonation....milling the base increased the comp and reduced the squish gap so i bet its good to go.....if it does detonate, you can always narrow the width of the squishbad too, by blending it into the bowl.