Gearbox drain plug fix

Bob Z. · Apr 5, 2012

When doing spring maintenance on my daily driver a 1972 750, I changed the gearbox oil. The drain plug felt a bit weird when removing, then when replacing it I got a mushy feeling when tightening. Yep, stripped hole. After all these years. Steel plug in alloy threaded hole.
The stock drain plug has odd threads so I tried to figure an easy and secure repair.

An 1/8 inch NPT (pipe thread) tap cleaned up the drain hole threads with minimal metal removal. A brass pipe plug wrapped with teflon tape fit nicely and tightened with a firm feel. After hours of riding not a trace of oil weep.

Has this drain plug stripped on anyone else? What would be a more traditional repair?

Diablouph · Apr 5, 2012

An 1/8 inch NPT (pipe thread) tap cleaned up the drain hole threads with minimal metal removal.

You ruined the bike, you fool. I'll give you a grand for it.

1up3down · Apr 5, 2012

You should be proud, we salute you!

A "traditional repair" is exactly what you did.

You used your creative sense of Nortonese.

Your solution is entirely and wonderfully British!

Well done old chap!

bwolfie · Apr 5, 2012

Only could be better if it was done in a shed with a hammer :lol:

swooshdave · Apr 5, 2012

If it doesn't leak then how will you know if there's oil in the gearbox? :mrgreen:

Brly · Apr 5, 2012

If it doesn't leak then how will you know if there's oil in the gearbox?

Exactly!!

Bob Z. · Apr 5, 2012

swooshdave said:
If it doesn't leak then how will you know if there's oil in the gearbox? :mrgreen:

Hmmm. I thought that a 20 year old O-ring seal on the kickstart shaft would allow a visual oil check!

swooshdave · Apr 5, 2012

Bob Z. said:
swooshdave said:

If it doesn't leak then how will you know if there's oil in the gearbox? :mrgreen:

Click to expand...

Hmmm. I thought that a 20 year old O-ring seal on the kickstart shaft would allow a visual oil check!

Good note, now you know why there are so many holes in the gearbox, seal one up and there are others to check the oil at!

macca49 · Jul 11, 2012

Bob Z. said:
When doing spring maintenance on my daily driver a 1972 750, I changed the gearbox oil. The drain plug felt a bit weird when removing, then when replacing it I got a mushy feeling when tightening. Yep, stripped hole. After all these years. Steel plug in alloy threaded hole.
The stock drain plug has odd threads so I tried to figure an easy and secure repair.

An 1/8 inch NPT (pipe thread) tap cleaned up the drain hole threads with minimal metal removal. A brass pipe plug wrapped with teflon tape fit nicely and tightened with a firm feel. After hours of riding not a trace of oil weep.

Has this drain plug stripped on anyone else? What would be a more traditional repair?

hey bob z
how has the repair held up ? as i have the same problem and plan on repairing it this week
cheers
macca

concours · Jul 11, 2012

Pipe plug is a great choice for a drain plug. Just a word of CAUTION to the ham-fisted amongst us.... the tapered design of a pipe thread makes them seal well, BUT, it can exert a lot of expanding force as well. So, don't over tighten it. Three fingers, gently. It wont leak OR fall out. To over tighten it, the case can and will crack. Seen it a hundred times in an industrial setting.

Bob Z. · Jul 11, 2012

The brass plug has held perfectly.
It's a good point to not over torque the plug.

I also did the outercover kickstart shaft seal with the SKF 9815 lipped seal. It's an easy no machining required replacement for the stock O-ring.

Brendan S · Jul 12, 2012

I have exactly the same problem. I used the less elegant solution of wrapping hockey tape around the gearbox to keep the plug from dropping out. I may have to try your fix.

speirmoor · Jul 11, 2014

Now I have the same issue. I felt something wasnt 100% when I replaced the drain screw and sure enough it was stripped. I removed the washer and fiber washer and caught the last couple of threads and got a snug fit on it. Filled it up went for a ride and all is good no leaks. However this will be on my mind to be taken care of in the winter. So tap and plug seems easy enough but I'll go with a regular bolt and not a pipe plug?

Bob Z. · Jul 11, 2014

The repair is still good after several gearbox oil drains, removing and replacing the plug.
One advantage is that the 1/8" NPT tap and brass or steel plugs are easily available at any hardware store.

Remember not to over-torque!

A standard threaded oversize hole drill and tap would probably produce more chips inside the gearbox.

pete.v · Jul 11, 2014

Bob Z. said:
Has this drain plug stripped on anyone else? What would be a more traditional repair?

Maybe a threaded insert would be a more traditional repair as done on my crankcase, but in a pinch your solution is quite viable.
As concours points out, there are issues to consider.

Gearbox drain plug fix

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