sandflyz

Member
Dec 10, 2004
71
0
Is there anyone on here that has tried a cometic high compression gasket set on there stock 03 CR 250. If so, how much did it bump your compression from stock?
 

+30

Member
Aug 2, 2005
276
0
never actually heard of it, - how did they manage that? just use a thinner material? isnt it already very thin, I dont really see how it could make a difference. we would be talking about prolly less than .005 or so wouldnt we? and another thing on the cometic web site they have 2 different pn's for 05 - 06 cr 250. one is for a 66.5 bore size and one is for a 68.5 bore size. all the other years list the 68.5 only. would the 68.5 be for a 265cc? then why are all the previous years 68.5...hmmmm maybe ill pick up the phone and see whats afoot.
 
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sandflyz

Member
Dec 10, 2004
71
0
Geeze your more confused than me and it was my question. :rotfl: Cometic does make a high compression gasket. It's steel and its .010 " thick. The stock gasket is around .020" so it would have the same effect as removing .010" off my head. I ride at 4500' and plan on using race gas mixed with pump. If i like the effect i will measure my squish and see what i have to work with.
 

+30

Member
Aug 2, 2005
276
0
yeah, this ones got my interest, Id like to see how it works out , may do the same to my 03, with pump gas at these prices, it makes race gas more and more attractive. ;)
 

NO HAND

~SPONSOR~
Jun 21, 2000
1,198
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This is probably the last thing I would do to a 03 cr250. This bike has some serious detonation problems. Increasing the compression will make this already annoying problem even worse.
 

motometal

LIFETIME SPONSOR
Sep 3, 2001
2,682
3
well, that depends on what fuel you are running. at 4500 ft, even a partial mix of decent race fuel will take care of any detonation. And .010" is not a large amt to be removing from the head, the effect of the reduced squish clearance is probably at least as substantial as the increased compression. In some cases, decreasing the squish clearance can actually reduce detonation.

Use .060" solder on four corners and measure the clearance, all the way out at the edge of the bore. Report back to us.
 

NacNac250F

Member
Apr 15, 2003
154
0
if your only talking .010" then that is not much at all. Your only talkin gaining about 10lbs. more of compression and if your running half race half pump then you won't have any detonation issues. I'd say go for it because you can always switch back to stock thickness if you wanna run pump gas again if race gas is too much, you can't uncut a head so it's a convinience to run a high comp head gasket.
The different size head gaskets from cometic are for a larger bore. The 68.5" head gasket is has a bigger hole because if you bored your bike and ran a stock head gasket it would stick into the cylinder a little.

+30- a 68.5" head gasket was probably used on an older one because they had a bigger bore but a slightly shorter stroke. That must mean that the newer bikes have a slightly smaller piston diamater but a slgihtly longer stroke. They both are 250cc and have 250 cubic centimeters of space inside the cylinder just one has a longer stroke and the other a bigger bore.
 

mx547

Ortho doc's wet dream
LIFETIME SPONSOR
Nov 24, 2000
4,787
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the stock bore on a 02-03 cr 250 is 66.5mm.

i'm going to try the gasket myself.
 

sandflyz

Member
Dec 10, 2004
71
0
Actualy i think the stock bore is 66.4mm. I got the new high comp gasket in the mail today. It's 66.5mm. This shouldn't be an issue should it?
I might put it in tonight and see what its like
Here is the part # for anyone who wants to try it. Its steel and .010". This one is from Cometic
H1288SP1010S
 

sandflyz

Member
Dec 10, 2004
71
0
Well i put my high comp gasket in today, it only takes about half an hour. Then when i was tightening everything up before going for a ride today and i snapped a head stud. Of course they always snap off flush with the top of the cylinder so you have nothing to grab onto. Anyway i had to disasemble the whole top end and drill out the boken stud. Took about 4 hours of my time. I got it all back together but wasen't in the mood to ride anymore after all the grief. Hopefully i will get a chance sometime this week to take it for a rip. It did give me a chance to take a look at my piston and rings after about 6 hours of ride time with my Mo betta everywhere ported cylinder and head. Eric does great work and everything still looks like the day i installed it. I still would rather been riding than repairing though. This was the second time i snapped a head bolt on this bike in as many times as i have had the top end appart. I'm starting to get tired of that. Espcially when the studs are 5 dollars ea.
 

sandflyz

Member
Dec 10, 2004
71
0
Well i took it for a ride today. No difference. The bike did not detonate on the pump gas i ran. This winter i will measure my squish and send the head to Eric to remove a bit off the head. For anyone else who bought the gasket though it might have a bigger effect depending on your elevation. I would be intrested to hear of any success. For now I've done everything i can to get more bottom end. Its time to ride it.
 

+30

Member
Aug 2, 2005
276
0
I was searching for more bottom end on my 03cr- but I had to switch to an rm to get it. Still like the cr handling though. Did you do the kehin pwk conversion on your cr? theres alot of 03-04 cr owners who have traded in their mikunis for the kehin. I did it and didnt take enought time to jet the kehin i guess cause everyone else seemed to like the switch. also the s8 nozzle was mentioned a few times for the mikuni.
 

sandflyz

Member
Dec 10, 2004
71
0
What is the RM like to ride? I was thinking about buying one. Or a YZ. My jetting is great. I bought the JD kit and couldn't be happier. The motor still has no bottom end though and aside from boring it to a 265 i don't think theres any hope. I'm wondering if boring it to a 265 would even do much at this point?
If i could get some bottom end out of this thing i would have noissues with it whatsoever, its a great bike. Light and great handling, suspension is A1.
Just no bottom which is what i need for the type of riding i do. Lots of enduro.
 

tony91

~SPONSOR~
Jan 30, 2002
493
0
C-12 and a Rad valve did wonders for the bottom end of my CR. If you can get it consistently, a moto specific race gas will make you smile.
 

MensaTech

Member
May 17, 2006
3
0
Home made gasket

I recently purchased some gasket material from local car parts store that had about a .002 - .004 thickness. When it was time to replace my rings I made the gasket and used it along with some new boyseen carbon fiber reeds. It has made a night and day difference on my bike. I wouldn't go with the cometic gasket because it's better to go as thin as possible and still have a gasket. I even toyed with the idea of using Yama or hondabond instead of using a gasket at all, but figured this would be a more reliable alternative. I posted all of this under "2004 CR250 ITS ALIVE" if you want to read.
 

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