milling the head

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Apr 11, 2003
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#1
ive been thinking about making the jump to race gas next summer, because im rolling in dough, and because so far my expieriences with race gas is that my bike is so much easier to jet when the temps change compared to the crap at the pumps. My brother said that if im going to switch to race gas, i might as well take the head to our local motosports shop and get the head milled to tighten the squish band. I also hear that this mod is supposed to give the low-mid a boost with sumtimes taking a little away from the top end. Anyways, my question is will this decrease reliability or possibly cause harm to anything else? BTW I have a 91'CR125 with a 96'complete top end. any advice about higher compression ratios would be great
 

Jman271

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Oct 18, 2001
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#2
Some of the newer CR 125's could benefit from a higher CR, but I'm not to sure about a 91 w/ a 96 cylinder and head though, it sounds like a weird combo, but hey, whatever works right? If you shave the head and run race gas, you will most likely feel a hit in the low to mid, but take off too much and your bike will run a tad flat on top. You can also get into an area of too much removal of material where the CR is so high that it actually works against the piston as it rises on the up stroke, it is called pumping loss. Make sure you take into consideration what your squish is before you mod it. I would try to get it around +/- 1mm and see what you get. Also the squish band and angle are important for some of these mods mentioned b/c the 91 had a domed piston set up and the 92 and up had a flat top piston so I hope that you went w/ the correct piston for your set up. Also, if memory serves me correct, the 96-97 spark plug lug needs to be turned down a bit, the plug doesn't thread in all the way on those yr heads. If I can remember more, I will post it, I hope I answered you question, see ya-
 

bclapham

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#3
i would try messing with thinner head or base gaskets first- with the head gasket you can change comp ratio- or with the base gasket you can also increase compression and port timing for a bit more bottom end.

milling the head is a can of worms you dont really need to open up on such an old bike, and especially not with a mix match cylinder setup
 

viking20

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#4
I wouldnt just mill the head on any bike.You need to be sure how much clearance you have to begin with.Maybe Im wrong,but I believe your cr had a domed top piston originally,do you have any idea what the newer cyl/top did to your comp ratio in the first place?
 
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#5
I am running the flat top piston even though when i bought it off the guy it had a dome top in it, but im just glad its running good again and with my new recoat job, the top end is really screamin compared to what it was before. By listening to your responses im pretty sure that this is a can of worms that probably isnt worth messing with. Im just gonna buy a bunch of jets and make sure its always jetted correctly so my new top-end lasts, and i continue to have good power.....until i finally save enough for the CRF450R......
thanx for the info or else i might have just went and had the head shaved down and been pissed off with the results