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CRF450L Exhaust Coupler


Feb 2, 2022
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The CRF450L/RL Exhaust Coupler allow the fitment of the FMF Q4 or HEX on the OEM CRF450 L/RL header.

Keep your factory header and lose the heavy silencer by adding the FMF


Parts in stock, ready to ship.

$94.95 each, plus flat fee of $20 for shipping and insurance to the lower 48 States.

Shipping quotes for AK/HI and outside the USA, upon Request.


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On Bike testing performed by Mitch Winder, bike photos courtesy of M. Winder


Ordering is simple.
Order by PM, phone call or email, the quote/invoice will be emailed to the customer, along with instructions for payment.
We accept the following methods of payment:
  • VENMO, PAYPAL , Credit Cards via SquareUp

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Here's a video about installing an FMF Q4 muffler on a Honda CRF450L. Note the problem he runs into at 1:10 is what MVI solved with his exhaust coupler. In the video, he ended up with an FMF 450X header, which is currently selling for $315. I'd rather use the L header + MVI coupler, and spend the difference on tires and gas.

I ran into a few problems while installing the FMF muffler that had nothing to do with the MVI part; it fits perfectly. The FMF muffler comes with a coupler that's sized to fit the 450X header, which is a larger diameter than the 450L header. I measured the FMF & MVI couplers and they're the same OD. The parts don't need to be pressed together, but the fit is intentionally tight to prevent play in the connection, and the escape of exhaust gases.

I test fit the MVI coupler onto the 450L header. Test fitting the part makes it easier to fix any problems. It has to fit snugly to prevent leaking and to properly support the connection. I added a light coating of anti-seize compound to the header and muffler sides of the connection, but that's not required. The MVI coupler went on smoothly with and without any lubrication. If it doesn't slide on easy, clean up the header. Any grit in the joint can make it difficult to slide the coupler into place.

It may be necessary to deburr the end of OEM header, polish it with emery cloth, and lube it. Anti-seize compound was my choice, but a light coat of grease will work - it'll smoke as it burns off later. If all of that has been done and you still can't slide the coupler to the stop on the header, try carefully tapping the coupler with a brass hammer until it's fully seated. It's possible that the OEM header is slightly out-of-round, which would be why it won't slide into the coupler more easily.


MVI coupler started onto 450L header. Note the silver band on the header


MVI coupler fully seated onto 450L header (side view) - no gap between MVI coupler and header band

Inserting the MVI coupler into the muffler inlet took a few minutes because the tubing was out of round. It's a tight fit, and even though the aluminum tubing was somewhat flexible, I couldn't get the coupler started by hand or by tapping it with a mallet.


FMF muffler inlet

I used a bar clamp to apply light pressure to the tubing, which made it round enough to get the MVI coupler started.


Bar clamp positioned 1/2" from the end of the tubing to allow the tubing to flex as the coupler was started


MVI coupler fully seated inside FMF muffler inlet - no gap

As mentioned in the video, there's a small tab on the FMF muffler that interferes with the 450L mounting point. It's just tacked on, and I removed it as it was in the way.


Forward muffler mount, with FMF tab interfering with 450L


FMF tab removed

With the 450L header fully inserted into the MVI coupler, I still needed to enlarge the FMF muffler holes slightly to line up with the bike's mounting holes. Not every bike will need to have these FMF holes enlarged. The forward FMF mount is stainless steel, which is tougher to cut than the soft aluminum used for the rear FMF mounting hole. For the front mount, slide the speed nut off and adjust the hole as needed. For the rear FMF hole, adjust as needed - there are no threads. I used a 3/8" drill bit and some cutting fluid, but any rotary tool (Dremel, die grinder) can do the job.

An alternative to modifying the holes would be to trim the required amount from the inlet pipe on the muffler so that it can slide forward on the MVI coupler enough to align with the existing holes. The FMF inlet pipe is fairly flexible, so care must be taken to prevent crushing it while cutting. A cutoff disc on a grinder or a band saw could be used, or for a small amount, a flap disc on a grinder might be effective.


Front mounting hole misalignment


FMF front mount - I had to remove approximately 1/4" of material from this side of the slot to align with the bike's mounting point. Another option would be to remove the speed nut and use a regular nut+bolt+washers at this mounting point.

With the holes adjusted, fit the muffler (with MVI coupler already installed) onto the 450L header, making sure that the header is fully inserted into the coupler. Use the original bolt for the front muffler mount, and the longer bolt supplied by FMF for the rear muffler mount, with the washer from the original bolt with the FMF bolt.

I like to use medium strength thread locker gel, especially when threading a steel fastener into aluminum.



Longer FMF bolt with Honda washer (L) / shorter Honda bolt (R)


Medium grade thread locker gel applied to rear mounting bolt

Tighten the two mounting bolts and re-install body parts. I tightened the bolts partially and worked between them to prevent adding stress to the exhaust. Don't forget to add appropriate heat shielding to the muffler.


Muffler mounting bolt locations

If you have any questions, please let me know.
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I've heard from another person using the coupler that combining the FMF muffler with the OEM header maintains low end torque that could be lost by installing a larger diameter header. I want low end torque, so that suits me fine. I'm boxing up the OE muffler today to add to my collection of Things that Weigh More than they Appear.
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I finally broke down and ordered the FMF Q4 Hex pipe to go along with the coupler I bought months ago. I installed it today. Mine was the MkII one discussed, with the chamfers to help with the install. I couldn't easily push it in by hand. Like ZoomerP , the muffler was slightly out of round. I used the quick clamp just like he did, and threw the MVI coupler in the freezer for 30 min. This made it go in quite easily with some hammer taps and a piece of wood padding. It took a bit longer than I thought to do the hole clearancing on the muffler. Strangely, I had no issues at all with the thin stainless front mount, but I fought with the aluminum sleeve at the rear for some time. All told, it's fully installed, and wasn't bad at all. I truly think those chamfers made a big difference in getting the thing aligned right at the start--the only part that took any persuasion was the first half inch or so.

It's still strange to me that these aftermarket pipes go together without a normal exhaust coupler gasket or clamp, but that's apparently how it's done. Oh, and I don't have my reflashed GET ECU back just yet, so I couldn't start the bike up. I'm really crossing my fingers that it's not much louder, since I want all my toys as quiet as possible.

Nice product, MVI!
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