2003 Honda CR 250

They had problems with the airbox leaking and I think a lot of guys add an extra base gasket but I like the all red look good luck with it

Imo, the two best mods are
1) properly sealing the airbox to airboot junction https://www.mxbonz.com/cnc_airbox_fix.htm
2) fixing the terrible squish clearance from the factory. Cheapest way to do so is run a 05+ base gasket. That also lowers the cylinder a touch so it helps boost low-end power a smidge. An even better way is to have the head modified or have the base of the cylinder machined. Any of these options will increase power, make it easier to jet, and decrease the likelihood of detonation.

Matt Fisher wrote:

Imo, the two best mods are
1) properly sealing the airbox to airboot junction https://www.mxbonz.com/cnc_airbox_fix.htm
2) fixing the terrible squish clearance from the factory. Cheapest way to do so is run a 05+ base gasket. That also lowers the cylinder a touch so it helps boost low-end power a smidge. An even better way is to have the head modified or have the base of the cylinder machined. Any of these options will increase power, make it easier to jet, and decrease the likelihood of detonation.

Thank you very much for the info Matt. Don’t think MxBonz sealer kits are sold in UK but I will try to find something to fix it.
I’ll probably go with the 05+ base gasket initially then, and perhaps look into head work at a later date.

Got a small amount of time on the bike tonight. Not many more parts to go and it should be ready to start the clean up.

One problem I came across was a seized screw in the throttle housing which has also rounded off. I’ve tried WD 40 and all sizes of screwdrivers, but cannot get it out. Think I may have to resort to drilling it out..

id try an impact driver before trying to drill it out. the guy doing the TM build right now did a cr like yours a while back. I'm sure there is a ton of info you could use if you need help or ideas.

rollin64caddy wrote:

id try an impact driver before trying to drill it out. the guy doing the TM build right now did a cr like yours a while back. I'm sure there is a ton of info you could use if you need help or ideas.

I’ve read blackdiamonds thread before, best bike on here I’d say. Read basically every other cr 250 build as well, but on some of the early ones I cannot seem to load the pictures up??

Got the frame and swing arm cleaned up today. Used turtle wax wheel cleaner with green scour/scotch brite pad (just what is sold locally and cheap).

Before:




After:



Some pretty bad gouges on the swing arm. Not entirely sure what to do about them as they’re deep.

Off to a good start. Looks good.

gunters96lt1 wrote:

Off to a good start. Looks good.

Thank you!

Parts I have cleaned up so far. Used a blow torch to get the airbox back to black.


Almost finished removing the casting marks from the dog bone, took me over 3 hours. Never before have I appreciated the effort it takes for this process using sand paper!

Keep up the good work bud, it’s looking good. Recently built a 2002 CR250 so any questions you have, I’ll do my best to answer

deccr250 wrote:

Keep up the good work bud, it’s looking good. Recently built a 2002 CR250 so any questions you have, I’ll do my best to answer

Thanks, that would be great. I’ll drop you a message if I’m stuck on anything. It won’t be anywhere near as nice as yours, but just trying to do my best with minimal parts on a small budget

Took the cylinder head off and there’s quite a few marks where previous pistons must have detonated. I’ve had this on another used bike I bought and local dealer said to sand it down slightly.
Bike runs great, has a new piston, but would just like some advice as to what everyone else would do?

Got the dog bone cleaned up bit better as well.

brooklynevans wrote:

Took the cylinder head off and there’s quite a few marks where previous pistons must have detonated. I’ve had this on another used bike I bought and local dealer said to sand it down slightly.
Bike runs great, has a new piston, but would just like some advice as to what everyone else would do?

Got the dog bone cleaned up bit better as well.

How deep are the marks on the cylinder head?? When I rebuilt mine it had a few small marks on the cylinder head in the same place as yours. I spent a bit of time cleaning it up and it came up near new. If they aren’t too deep then I’d have a go at cleaning it up and see how you get on.
Have you got round to checking the power valve unit over yet? You must’ve had the power valve cover off as you’ve removed the head but I’m wondering if you’ve had a chance to look at it closer? The reason I ask is because when I started stripping my ‘02 CR build down, the power valve was rusted up and filthy and the cable was ridiculously slack which is a common thing for these bikes when they haven’t been maintained correctly

brooklynevans wrote:

Took the cylinder head off and there’s quite a few marks where previous pistons must have detonated. I’ve had this on another used bike I bought and local dealer said to sand it down slightly.
Bike runs great, has a new piston, but would just like some advice as to what everyone else would do?

Got the dog bone cleaned up bit better as well.

deccr250 wrote:

How deep are the marks on the cylinder head?? When I rebuilt mine it had a few small marks on the cylinder head in the same place as yours. I spent a bit of time cleaning it up and it came up near new. If they aren’t too deep then I’d have a go at cleaning it up and see how you get on.
Have you got round to checking the power valve unit over yet? You must’ve had the power valve cover off as you’ve removed the head but I’m wondering if you’ve had a chance to look at it closer? The reason I ask is because when I started stripping my ‘02 CR build down, the power valve was rusted up and filthy and the cable was ridiculously slack which is a common thing for these bikes when they haven’t been maintained correctly

Did you use some fine sandpaper to clean it up? Marks aren’t too deep, so I’ll give it a go without taking much material off.

Yes I’ve had a look at the powervalve mechanism. It looked pretty clean and the one cable looked new. I haven’t had a look at the butterfly valves in the cylinder yet, but will get round to cleaning it soon. Thanks for the help

brooklynevans wrote:

Did you use some fine sandpaper to clean it up? Marks aren’t too deep, so I’ll give it a go without taking much material off.

Yes I’ve had a look at the powervalve mechanism. It looked pretty clean and the one cable looked new. I haven’t had a look at the butterfly valves in the cylinder yet, but will get round to cleaning it soon. Thanks for the help

One cable on the power valves is always prone to snapping, which would explain why one looks new. Probably been replaced at some point!
With the marks on the cylinder head I just used a scotchbrite pad and WD40 to clean it up and then used very fine sandpaper to take the fine marks off.
Tip...get the fine sandpaper and rub it against itself to make it even finer as you only want to use a very fine sandpaper to rub it down

deccr250 wrote:

How deep are the marks on the cylinder head?? When I rebuilt mine it had a few small marks on the cylinder head in the same place as yours. I spent a bit of time cleaning it up and it came up near new. If they aren’t too deep then I’d have a go at cleaning it up and see how you get on.
Have you got round to checking the power valve unit over yet? You must’ve had the power valve cover off as you’ve removed the head but I’m wondering if you’ve had a chance to look at it closer? The reason I ask is because when I started stripping my ‘02 CR build down, the power valve was rusted up and filthy and the cable was ridiculously slack which is a common thing for these bikes when they haven’t been maintained correctly

brooklynevans wrote:

Did you use some fine sandpaper to clean it up? Marks aren’t too deep, so I’ll give it a go without taking much material off.

Yes I’ve had a look at the powervalve mechanism. It looked pretty clean and the one cable looked new. I haven’t had a look at the butterfly valves in the cylinder yet, but will get round to cleaning it soon. Thanks for the help

deccr250 wrote:

One cable on the power valves is always prone to snapping, which would explain why one looks new. Probably been replaced at some point!
With the marks on the cylinder head I just used a scotchbrite pad and WD40 to clean it up and then used very fine sandpaper to take the fine marks off.
Tip...get the fine sandpaper and rub it against itself to make it even finer as you only want to use a very fine sandpaper to rub it down

Thanks very much for the help. I’ll give it a go now

brooklynevans wrote:

Thanks very much for the help. I’ll give it a go now

No worries, let me know how you get on

deccr250 wrote:

One cable on the power valves is always prone to snapping, which would explain why one looks new. Probably been replaced at some point!
With the marks on the cylinder head I just used a scotchbrite pad and WD40 to clean it up and then used very fine sandpaper to take the fine marks off.
Tip...get the fine sandpaper and rub it against itself to make it even finer as you only want to use a very fine sandpaper to rub it down

brooklynevans wrote:

Thanks very much for the help. I’ll give it a go now

deccr250 wrote:

No worries, let me know how you get on

Cleaned the head up a bit, but the marks around the edge of the insert are quite deep. Tried a bit of sand paper but didn’t want to take too much material off.

Also wondering if you’d be able to tell me how you remove the pin which holds the power valve in place. Doesn’t want to come out. Haven’t yet resorted to pliers as don’t want to damage thread.

brooklynevans wrote:

Cleaned the head up a bit, but the marks around the edge of the insert are quite deep. Tried a bit of sand paper but didn’t want to take too much material off.

Also wondering if you’d be able to tell me how you remove the pin which holds the power valve in place. Doesn’t want to come out. Haven’t yet resorted to pliers as don’t want to damage thread.

The power valve rod you’re referring to will be stuck in place because of the carbon that’s built up around it. If you have all the power valve mechanism off, then refit the small nut that holds the butterfly on but put a small washer behind the nut so you won’t damage it..then put a pair of pliers behind the nut and pull! It will come, it’s just stuck in there because of the carbon build up. If you can get someone to hold the pliers whilst you pull the cylinder head itself (if that makes sense) then that should get it out quicker and easier. Let me know how it goes

Get ahold of Phil Denton Engineering. He's on board here and somewhere in GB. He might be able to fix and mod your head. If it can't be fixed obviously replace.
If you are cleaning up your cylinder you want to use a scotchbrite pad and your WD40, not sandpaper. It will work and is less aggressive than sand paper. Then use hot soapy water and wash and rinse until a paper towel comes out as clean as it goes in.

captmoto wrote:

Get ahold of Phil Denton Engineering. He's on board here and somewhere in GB. He might be able to fix and mod your head. If it can't be fixed obviously replace.
If you are cleaning up your cylinder you want to use a scotchbrite pad and your WD40, not sandpaper. It will work and is less aggressive than sand paper. Then use hot soapy water and wash and rinse until a paper towel comes out as clean as it goes in.

Agreed. No way I'd run that head without having it machined first. The problem with just sanding it down is you'll decrease the compression while making the squish problem worse. If the dents are too deep, then the head is either welded then machined or simply replaced.

Cheers for the advice. I’ll see if I can find someone to skim the head for a reasonable price. Found new ones in USA for $85, so may just enquire to see if they will post to UK.

Searched for some screws for the front master cylinder as the ones currently on the bike are clearly not made to be on there (not a countersink bolt). Found a pack of 20 ti countersink bolts for the same price as 2 stainless steel bolts. Thought this was quite interesting. Linked them below, they’re from US as well which is convenient for most vital members
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F173322474394

Cylinder and exhaust flange were covered in oil and dirt. Got them cleaned up a bit tonight, but will give it a better scrub soon.





Still failing to get the powervalve out. Cannot get the rod which holds the valves in place to free up at all.

Have you tried spraying PB blaster & letting it soak - penetrate overnight ? If it's really bad you might need to repeat the process the following day.

brooklynevans wrote:

Cylinder and exhaust flange were covered in oil and dirt. Got them cleaned up a bit tonight, but will give it a better scrub soon.





Still failing to get the powervalve out. Cannot get the rod which holds the valves in place to free up at all.

They can be really tough to get out mate, spray some sort of contact cleaner in there and let it soak for a few hours. Putting some grips on there and giving it a pull won’t harm it as long as you take extra care that you aren’t levering against the head itself excessively. Put a folded towel against the head to protect it if you’re levering against it. It will come

I’ve sprayed loads of WD40 in there, and tried levering a bit with the spanner behind a nut but didn’t want to damage cylinder. I’ll try some more WD40 and mole grips with a rag behind it. Thanks for the advice