Vintage Dirt Bike Q & A

Does anyone have a cheat for the asbestos like exhaust gaskets on the MR and MT's? They are pretty disposable, and getting scarce.

cmsnl.com has the ring gaskets in stock, I believe. If they're left in place they tend to last nearly forever, which is good - they're not that easy to install on the bike (they tend to bunch up and snag), but if you install and then pre-shape them with the (removed) exhaust manifold before putting the pipe on, they're fine. I think their main function is as a mechanical buffer and preliminary seal between the pipe and manifold, with the piston ring for centering and additional sealing.

I found this setup to be leaky, so now I use a Honda 91305-HA0-680 O-ring instead of the piston ring, along with hi-temp sealant. This is a different O-ring than I've recommended previously; I thought it was too thin, but it expands with heat and seals fine even without the sealant, provided everything's clean and in good shape. I always leave the ring gasket in place, though.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Hi there Ray , that was going to be my next question , what is the correct procedure in installing the pipe with the asbestos seal and the metal ring gasket?

Could you perhaps elaborate for me please!

Thank you

Well... for an all-OEM installation I guess I'd say

a) Clean everything well, including the exhaust manifold groove and the pipe spigot
b) If the spigot-to-pipe weld area is rough inside, grind it smooth
c) Push the asbestos seal (what I called the 'ring gasket') fully into the spigot, making sure it's round and seated well
d) Rotate the tapered end of the exhaust manifold into the pipe spigot and asbestos seal, forming the seal into a mating shape
e) Reinstall the exhaust manifold on the cylinder
f) Liberally coat the manifold, the metal ring gasket (what I called the 'piston ring', since it looks identical to one), and the pipe spigot/asbestos seal with hi-temp sealant
g) Reinstall the pipe onto the manifold, using a twisting/pushing motion
h) Use strong springs - if they're old and/or easy to install, they're not strong enough

The procedure I use on my bikes is the same, except I replace the metal ring gasket/'piston ring' with the 91305-HA0-680 O-ring I mentioned earlier.

Hey, I found out how to make a new smiley face.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Perfect Ray thank you very much! Will take some pics when my bits arrive and I complete the process , then post it here to benefit other members.

Has anyone tried using Teflon Tape? I have seen some postings and they note that the tape is good up to 500F. Does the MR175 exhaust manifold get hotter than that? I checked with CSMNL and they want $27 to ship the gasket. I haven't found it anywhere else. I hate to start using a sealant because I don't know which mufflers will end up on which bikes.

Any thoughts on the tape or substitute for asbestos?

The highest temp I've read on the pipe spigot was 330 degrees F. I'm sure it's hotter inside, how much I don't know. The Viton O-rings in the manifold groove eventually get hard and brittle, and the O-ring I use inside the cylinder/manifold joint (instead of the OEM copper flange gasket) bakes into chunks, but the sealant survives and prevents leakage.

The Teflon tape sounds interesting. I'm using Teflon core/braided stainless tubing from the pipe to the silencer on my other 175 and it held up fine for two short test rides, for what that's worth (probably 250F - 300F max). I imagine the tape would seal OK, but the pipe/manifold abrasion might chew it up; the steel-screen core of the asbeston ring definitely wears a bit. It's certainly worth a try, and might work great!

If the O-ring didn't work, my next plan was to use a rolled-up ring of lead or aluminum wool instead of the asbestos gasket, which should have sealed extremely well. I never had to try it, though. Another thing that might work well is just a carefully cut strip of hi-temp semi-hard gasket material formed into a flat ring and fitted into the spigot; I think the OEM asbestos gasket is initially made this way, then bonded at the seam to form a continuous ring.

The sealant I use (Dynatex Dynablack) sets up but never really hardens 100%, and comes off easily with a dry paper towel upon disassembly. It might work fine alone with just the pipe and manifold and no other parts, but I haven't tried that either.

One other thing I forgot to mention... it's critical that the pipe spigot mouth doesn't bottom on the manifold flange, otherwise the springs exert no pressure on the spigot/manifold joint and asbestos gasket, and exhaust pours out onto the cylinder. I had to grind down both my OEM pipe spigots to prevent this. Most of the aftermarket pipes use shorter spigots, and don't have this problem.

Having a good pipe/manifold seal really makes a difference in power output and throttle response.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric