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02 exhaust flange, what's the deal?

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I see in posts and the Boysen catalog about an after market exhaust flange that is supposed to provide better flow. Well i took mine off to clean it up and seal it up and it matches up with the exhaust port in my head perfectly. How could the aftermarket one be better?

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It's most likely that when people replace the old gummed up dirty one this aftermarket one will flow better. People who put it on a new engine without running the stock flange will just assume it has more power. Sometimes that is how the aftermarket works....but I don't really know for sure on the flange boyesen has.

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I see in posts and the Boysen catalog about an after market exhaust flange that is supposed to provide better flow. Well i took mine off to clean it up and seal it up and it matches up with the exhaust port in my head perfectly. How could the aftermarket one be better?

I broke my stock flange and looked at the Boyeson flange when I replaced mine. The stocker does line up just fine, and I didn't buy the stories about increased gain with the Boyeson part. There are damn sure better ways to spend your money (IMO).

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W/ certain years the port and flange aren't matched like on the 04 for example. When I changed mine to the boyesen it felt like I gained low end w/ the port and flange now matched up, but I guess the 02 is already matched up.

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W/ certain years the port and flange aren't matched like on the 04 for example. When I changed mine to the boyesen it felt like I gained low end w/ the port and flange now matched up, but I guess the 02 is already matched up.

You make a good point, they used a diffent part number flange for each year between '02-'04. The port match on my '02 with stock flange is pretty good, might not be the case on '03 and/or '04's.

Pro-Circuit makes a flange out of solid (not cast as the OEM part) that's not much more costly than the OEM part. I would have gone that route, but I needed to ride the next weekend and my dealer had the OEM flange in stock.

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its not only that it increases flow, the spacing is diffrent i belev. thus making power at diffrent RPM.

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If the '02 exhaust port is indeed round, the Boyesen flange probably won't make any difference. The '04 is definitely a different story (not sure about the '03...) My brother-in-law picked up a new-in-the-box 2004 CR250 last year - it's very similar, if not identical to the '03. Most of the magazine tests dissed the motor for soft low-end, but rated everything else top-notch. In reality, unless you're a pro or really fast amateur, it has more than enough stock MX-type power. But, since we can't ever leave well-enough alone ... after researching multiple back-issues and dozens of online impressions and experiences, we've settled on the following combination of bolt-ons: Add a tooth to the rear sprocket. Slightly lower gearing really helps coming out of corners. Install the JD jet kit - spooge and smoke has all but disappeared. Get one of the Boyesen exhaust flanges while you still can. If you don't think it will make a difference, take off the stock one and look at the shape of the exhaust port - the port is D-shaped (laying on its side) and the stock flange is round! This is an uncharacteristic engineering oversight or manufacturing shortcut (at least for Honda) - I'd estimate 30% of the area is mismatched. The Boyesen flange is a work of art and matches the port shape exactly. The Pro Circuit flange was cheaper and supposedly stronger, but still round like the stocker. The Pro Circuit Works pipe with the 304 shorty silencer generally gets the highest marks for this bike, but honestly, it's more for the bling factor. As many have mentioned, the stock pipe is actually pretty good. Finally, we added the Boyesen RAD valve with the Pro Series reeds. I have to say, this combo works. The bike is now an absolute beast, and feels nothing like the stock motor. It went from mellow and smooth but strong to a real eye-watering arm-stretcher. It pulls hard and clean right off the bottom and my brother-in-law's only complaint now is the difficulty he has keeping the front wheel on the ground. I've been doing this a long time, and I don't know if I've ever gotten such dramatic results from only bolt-ons.

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I've always been intrigued with Boyesen's products so went right to the source to get the facts on this subject. Check it out:

When the new ’02 CR250 engine was introduced, the initial feedback was “what happened to the throttle response and bottom end power”. On the dyno the servo operated exhaust valve, case reed engine was impressive, but on the track it was…dare I say disappointing. We spent lots of time looking at ways to improve the low end performance issues with a RAD Valve solution and while testing found the stock exhaust flange cracked and failed at the o-ring groove. Initially our thought was to offer a replacement solution for the failing stock flange. Of course Eyvind wouldn’t settle on that, and looked at some ideas to improve the design to help the stumbling bottom end performance.

On the 02-03 CR250’s, the exhaust flange was circular and did not match up smoothly to the “D” shaped exhaust port. Additionally the inside shape has been optimized to increase pressure pulse energy to improve the throttle response and bottom end power. The ’04 CR250 stock exhaust port and flange was changed for the better, but still had room for improvement. A similar design change was made on the ‘04 Boyesen flange to help pump up the bottom end.

To summarize, the Boyesen Exhaust flange’s billet design eliminates failures of the stocker while adding much wanted bottom end power and throttle response.

Available for: 02-03 Honda CR250 :: BEF-02, 2004 Honda CR250 :: BEF-02A, 2002 CR125 :: BEF-01, 98-08 KTM65SX :: BEF-40A.

MSRP :: $58.95 - $84.95

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Interesting statement " On the 02-03 CR250’s, the exhaust flange was circular and did not match up smoothly to the “D” shaped exhaust port. "

My 02 exhaust port is round and does match up with my exhaust flange. Perhaps it was changed in 03 or something. However my stock one did crack off at the o-ring groove and is now sealed with RTV.

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They recommend an aftermarket CNC machined exhaust flange on the CR250's because the stock flange has a common issue of cracking and allowing debris to go in o the engine blowing it up.  The aftermarket piece eliminates this possibility. Any possible performance gains are just bonuses to not blowing up your baby :) Just thought I'd post since I just learned this haha 

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Interesting statement " On the 02-03 CR250’s, the exhaust flange was circular and did not match up smoothly to the “D” shaped exhaust port. "

My 02 exhaust port is round and does match up with my exhaust flange. Perhaps it was changed in 03 or something. However my stock one did crack off at the o-ring groove and is now sealed with RTV.

the 03 exhaust port is round as well.  It is only the 04 that has the D shaped port from everything I have seen.  I replaced mine with the Pro circuit simply to prevent the cracking.

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the 03 exhaust port is round as well. It is only the 04 that has the D shaped port from everything I have seen. I replaced mine with the Pro circuit simply to prevent the cracking.

did you use the oem Honda rubber o-ring on the new pro circuit flange? Just asking cause I just bought the pro circuit flange

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anyone know about the 97?

If I recall correctly the 97 just has the 2 rubber rings that come straight out of the head, its a different set up

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Thereb is no removable exhaust flange on the 97. The exhaust pushes into a recess in the cylinder. Make sure to check your o rings frequently to prevent damage to your cylinder.

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I just bought the Pro Circuit one for my 03, it is cheaper and will prevent the cracking that the OEM part is supposed to have.  If the Boyesen part does anything to increase low end it will have to be longer in order to change port timing.  The Pro circuit part was identical to the OEM part but was billet.  The Pro Circuit part is only $30 on Ebay.

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