New gas cap seal


XS400 Addict
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Philadelphia, PA
So the gas cap seal on my 1978 XS400 has seen better days. It was very cracked and dry rotted and when standing near the bike, released some noticeable gas fumes, though never leaked any gas, thankfully. Googling for a new seal brought up plenty of replacements for the XS650 and other models, or complete gas cap assemblies of questionable quality (and now needing a third key for the new cap) for our XS400's. I had noticed that MikesXS sold just a replacement seal for the XS650, and it looked identical to the seal on our 400's. So I took the $9 gamble and ordered one. It fit perfect! It's maybe a touch thicker than the factory seal, but the cap is spring loaded and has more than enough movement to compensate. Replacing it is a matter of 4 screws, one to remove the cap from the tank and three to hold the seal plate to the cap. You cold probably try to replace it with cap in place too, but I'd rather not risk dropping the tiny screws into the tank opening. No more gas smell while standing next to the bike!

So if you're looking to replace just the rubber seal on your gas cap, go get you one of them. The MikeXS part number is 20-0036. BTW, I'm not a sponsor.
It fit good on mine as well but after a few days it seemed to deform/stretch out. Almost like the rubber is a bit soft.
It fit good on mine as well but after a few days it seemed to deform/stretch out. Almost like the rubber is a bit soft.
Huh, I filled my tank after putting it on and haven't put but 30 miles on it. I'll check mine, but still don't have any fuel smells.
Maybe I'll try to boil it for a few minutes, hope it shrinks back down to size and take a good look at the rest of the cap. It's a bit more snug pushing and opening the cap, obviously more than it was with the dry rotted seal that looked very similar to yours. I see other seals online with the yamaha part number 2m0-24641-00 for much more than what Mike XS is sold for but it's a gamble.
I just checked mine since I rode into work and it looks fine and still feels tight. I run 91 octane on my bike and don't add anything. Maybe some additive in your gas is affecting it?
No additives other than seafoam a month ago, I use 90 ethanol free gasoline. Maybe I'm just overthinking it. It's still looking like it's going to seal, the gasket is just not tight around the cap as it was when I stuck it on. Kind of hard to see in the picture. Thanks for checking.

That might be the extra squish from it being thicker. Maybe a little washer between the cap hinge and tank to lower the tension?
I may have opened my rebuild thread on this forum with a negative comment about the longevity of OEM rubber, however, after cleaning up all of the rubber from the bike, I was dumbfounded to find that most of it was in really great condition. I completely reversed course on my feelings about Yamaha rubber parts and searched out genuine Yamaha and Mikuni rubber where I could if I needed a replacement. However, I see that the Yamaha gas cap gasket has REALLY gone up in price since I bought one. But, I think I would still go for the OEM rubber since I think that the Yamaha stuff is some pretty good shite.

I would be good to see how long the MikesXS stuff lasts as it is very low cost and can be replaced several times before approaching the price of the OEM part.