125-200cc Milling cyl/head cr125

I got my bike torn down just to get the head powdercoated for cosmetics, and I was wondering if anyone knows offhand how much I could get away with milling from the cyl to get more compression?

I would not want to mill the head because the o-rings will make it tough. I am not a machinist but I do have acess to a mill so I could get it done easy enough.

Milling the cyl base will lower the intake/exh ports that about which will probably cause the peak rpm to lower right? So to fix that you would then raise the ports slightly?

I know I should just let a pro do it, but I am mechanically inclined and its sitting on the bench so give me some tips! It ran great previously with only problems idling in gear for more than 10 seconds (starts dieing down), but that was likely an adjustment to be made.

I would be inclined to look at improving the compression but leaving the ports alone - its a 125 so it should scream pretty hard.
You would need to get some squish band readings with the cylinder on the bike . From memory mine was around 1.25 mm and I reduced it to around 1.00 by just having the head shaved only . Taking material out of the head is when it gets more technical and is better to prevent detonation . Id send the head out to these guys http://www.rb-designs.com/ - with some solder test results

Thanks for the tip, I knkow a bit about the squish but milling the head will be quite hard (for me) as I will then have to cut the o-ring grooves down more right?

That sounds really like a pita!

We still have Ty's procircuit cr125 race bike in the shop. I wouldn't surface the head, if you wish more power, we have and can port the cr125. We usually run less compression and reshape the combustion chamber to burn fuel more efficiently.

does porting them lose some low end ?
If you start going all out its better value to get a spare 144 kit ? unless you are really into 125s or get a later model 5 port 125 cylinder
I didn't get the orings cut down but the orings did get stretched by additional heat

Depends on how we port it. But no, not necessarily. O-ring is on the cylinder so you can cut the head on a lathe. If you aren't racing and/or tied to 125cc, the 144 kit is the way to go, possibly 144/port/polish, etc.