found my air leak= clutch side crank seal, first place I looked....not. Working on my 1975 MX400 , had the yamaha double lip seal and the o ring that goes behind the primary. I thought the o ring just goes on the crank against the main bearing so thats what I did. Air test and the key on the shaft that the primary fits on is whistling air. What should I do ?

So I put the old o ring on top of the new one. No more leak. Any thoughts?

Wrong Oring?, it's quite a fat one that sits in the recess of the crank gear. I'm surprised it leaked as i always thought if the crank gear nut was tight it shouldn't leak even without an Oring as all surfaces that bolt up are flat, or should be.

Is the new o-ring an OEM part? Also, I've heard on good authority that some new Yamaha o-rings for our bikes of this era may not be exact. Close, but not exact. Did you compare the old o-ring to the new one. Was the old one 'formed/misshapen' by getting sucked into the key way?

Yes yamaha OEM Rechecked the numbers , looked the same as old, old looked good.

crank nut was not on , I held the primary gear in place an pumped it up. sounds like thats no good ? Well its back together no leaks there . I must have a case leak cause the gauge is still dropping like a rock I've soaped everything then wd40 every thing. I put the breather hose under water, nothing. Thanks for the help guys. need to take a break . Getting discouraged

Intake manifold rubber? I had one that didn't show with a soap test, but I happened to notice when I pumped it up there were tiny micro fissures all over the old manifold rubber that could barely be seen with the eye.

It is in good shape no cracking, I could try a different one

Check the front motor mount with lots of soapy water. Maybe remove the bolt. I've seen a few with massive leaks there.

I've just done the magneto side on my DT. I was getting rev hangs but not all the time. Sometimes I'd go out on the bike and it was fine and other times it did it after ~15mins of warming up properly. IMO the seal might have eventually failed entirely but I just caught it in time. I suspect if I pressurised the cylinder this seal would be fine but pressurising the case side might have highlighted there was a problem.