Milling Head???

from what i know that bike is designed to run at mon 93 the minimum so it´s better to leave it alone...
no one will tell you how much you compression ratio will be since it depends on a number of things like: for what use is the bike, how fast you are, how long you load the engine, what kind of soil you ride, ignition advance, carburation etc......

if you still want to do it, i usualy put a thinner base gasket if the squish band is big enough...

Sent my friends 03 cylinder to Eric Gorr for porting, base and head machining. He said the bike rips now, never ran so good. Apparently the 03 is a bad year for the cr250. Well worth the $200.

P.S. I also believe he said that engine has too much compression stock and something about port timing being off.

to give you a example: i recently buy a 97´ cr 80 RB... the bike is very fast with a lot of power, but i´m a slow guy... most of the times i rode in low rpm...
i´m rebuilding all the bike since the previous owner don´t like maintenance or repairs, but it was very cheap... when i was measuring the top end i note that the engine has a american head (47mm instead of 46 for europe), so i make a new base gasket 0.3 mm thinner with gasket paper since the head has more volume and a greater squish clearance (1,0mm + 0.3mm from the deck height) to increase the compression ratio...

but more compression ratio is not always a good thing, if i ride fast with the throttle open a lot of time like desert actually i would decrease the CR:
1: because the bike wouldn´t need more low end
2: the increase of pressure will increase the internal temp. making it more prone to knocking
3: the increases in temp will increase wear
4: etc...

all of this is based in the principle that only the CR changes, nothing else...

the formulas you read about only calculates the actual CR, not the ideal... that is only found by testing, or experience

Trouble is a thinner base gasket changes your port timing which for a slower guy like yourself, isn't all that bad. I would like to keep the port timing stock and work with the head if I wanted more compression.


Anytime I make a head cut I make a corresponding cut into the squish to keep reliability. Or maybe a 20-40 cut, where 40 being the head and 20 being the squish. This all depends on how much wiggle room is in the stock configuration and the quality of the fuel being used.