On any repair that involves tapping new threads into the case (oversize thread, Helicoil, weld and re-tap), the answer to your question will be determined by the proximity of the contents inside the gearbox. All these repairs count on having enough clearance inside the case for a tap to be run though the existing drain hole. So if you can't run a tap in, the repair (done in or out of the frame) will be unsuccessful.

I would imagine that the material thickness at the location of the drain is minimal, and that all repairs in thin case material locations (done in or out of the frame) would be unsubstantial. If I went though the trouble to pull the g/b, I'd not be settling for an "iffy" repair. I believe, eventually the case will ultimately need to be replaced.

In the interim, to get you going this Spring may I suggest.... Pulling the drain plug. (I know you said it spun in the hole, but if you pull and turn at the same time it may come loose.) Then clean the case and plug until all traces of oil are gone. Then use an industrial grade of epoxy to hold the plug back in the hole on a permanent basis. This won't be any of the household "5 minute" types of epoxy, but at a minimum the thick (viscous) 2-part, dark gray "filler" types, like J-B Weld. Be sure and check the useful temperature range. You may need to place a wedge or jack under the plug for 24 hours while the epoxy cures. This move makes it so that you don't need to buy the first case you find for $500, but allows you time to discover one for $25.

While the drain is permanently installed, you can use vacuum to pull oil out if you need to change the gearbox oil. Such manual siphons and electric vacuums are sold at boating supply stores. Or, you can buy a "fork oil filling syringe" which is smaller, but can also be useful when you change your fork oils.

Link to Fork Oil Syringe on Amazon

Link to Larger capacity Oil Siphon on Amazon

Maybe not the ultimate answer, but hopefully there's an idea in there to help.

Thanks Wobbly! Very good suggestion to epoxy the plug in. I may go that route for now.

Hank Pfister
#2365
Roanoke, VA

Thanks Wobbly! Very good suggestion to epoxy the plug in. I may go that route for now.

Hank -
You might get used to this oil sucking idea and never go back. I'm 68 now and still change the oil on 6 family cars & trucks, but I haven't been under a car in years. I purchased a nice used medical vacuum pump on Ebay years ago, and have been sucking the engine and transmission oils out through the dip stick tubes ever since !

That's nice because newer cars are much lower, and a lot of automatic transmissions don't even have drain plugs. Time for new brake fluid? Just suck the reservoir empty, refill and start bleeding. Lawn mowers? No need to make a mess, just suck it out ! Need to empty a fuel tank for painting or winter storage? No problem and zero mess. Whatever you got, it all goes neatly into the storage bottle.

😉

I used a what is called a piggy-back plug with oversized threads on an R75/5 with satisfactory results.
What you get is a plug threaded in to a plug that has threads the same pitch as the original except the threads themselves are taller and bite in to the bore where the original threads are damaged. WI. Airmarshal Randy Boris is the person who clued me in to the existence of such plugs.

Thanks for all the good suggestions. I feel like I will soon pull the tranny out and not attempt a fix on the frame. It has almost 70,000 miles, and was rebuilt about 17 years ago when I returned the bike to service after a 20 year rest. I'll either plug it with a piggy back plug like 8053 suggests, or JB weld a drain plug in it.
I also have a spare transmission that has been gone thru that I may put in it, but it does not have the kick start mechanism. It would be possible to take the kicker off one tranny and put on the spare, but I don't feel qualified to do that.