2004 CR250 Top end re-build

Question. I bought a new (OEM A) cylinder head from Mid West Action Cycles. The exhaust exit is a different shape than the one on the bike now. The new exit shape is a D shape, the old shape is round. I don't think it's been ported because it looks like a raw casting inside. Any cause for concern here??


Do you mean the bolt-on exhaust flange or the actual opening on the cylinder's exhaust port? The exhaust port is d-shaped, but the stock Honda exhaust flange is round - that's why the cool D-shaped Boyesen flange makes such a difference. Been through this with my bother's 2004. If your "original" cylinder had a round exhaust port - it wasn't the original part.

The actual opening. That may be true. I bought the bike through Craigslist so I don't know much about it. Everything else was identical except for that. Maybe the '02 or '03 had a different shape???? Possibly this bike had a different year on it?

Thanks for the info., good to know that that shape is correct for that year.

Sure, but now if you bolt on the flange, you'll see what the issue is with the D-shaped exhaust port and the stupid OEM ROUND flange. Close to 30% of the area is mis-matched - an uncharacteristically sloppy (for Honda) engineering oversight. Get the Boyesen D-shaped exhaust flange ASAP - I think you can still get them straight from Boyesen - http://boyesen.com/exhaust-flange-3.html - Trust me, it's worth every penny. You will notice more difference with this than with an aftermarket pipe.

Just wanted to give you an update. Turns out the bike is an '02. I feel pretty stupid not checking the VIN. I went off the guys word that it was an '04. That's one of those lessons that you only do once! $84 dollar lesson!!! So, that appeared to be the only difference in the cylinder and the bike ran fine. I just hope the cylinder isn't shorter or taller and changed the compression ratio? I'll need to do a bit more research.

Edited by cr250r_fun

no you should be fine the engine was the same from '02-'04 the only thing different about the '04 is it has a Throttle Position Sensor on the carb

That's good to here. Thanks for the info!

no you should be fine the engine was the same from '02-'04 the only thing different about the '04 is it has a Throttle Position Sensor on the carb

Just an FYI. The 04 bottom end caseses are different vs. the 02 & 03. The 02, 03 & 04 all have different porting specs on the cyl. & recomend useing the powervalves to that year cyl. to match the ports. The cyl. heads are the same. You can interchange the 02-04 cyl.'s on the 02-04 bottom ends.

Bummer, it's all installed and running. I could buy an '04 power valve off e-bay and a new top end gasket kit and put that in. I wonder how big of a deal it is running an '02 PV? I don't have a base line myself since I bought the bike as a non runner.

Thanks for the heads up.

Edited by cr250r_fun

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