Originally Posted by Tin Woodman
Thanks everyone for your input.
I don't believe the existing threads are salvageable and even if they were, there would always be lingering doubt. BMW has an oversize plug for a good reason - I'm willing to bet ham fisted mechanics screw up these threads all the time. There's plenty of material on the housing to allow going to an oversize plug. The trick will be to drill and tap squarely without showering the internals with chips and debris. I watched a You Tube video tonight highlighting the grease method of catching chips - it was hilarious. All he succeeded in doing was dropping a big blob of contaminated grease into the spark plug hole he was repairing. He eventually retrieved it with a shop vac.
I like your idea, Plaka, of using positive compressed air (I believe that's what you mean) to keep debris out as I drill and tap. Could probably do it with the drain hole on its side rather than it facing up to lessen the effects of gravity.
Alternately, I could open up the gearbox again (already replaced the seals and gasket) but would prefer not to.
Overall, the larger plug has the advantage of being a more permanent solution - there's evidence to suggest inserts sometimes fail on components that are often removed and everything I've read about airhead transmissions indicates oil should be changed regularly.
No, not air. I'm grinding on an airhead strait oil filter with a high speed burr. it makes very small particles that won't go upstream against an air flow. I have an electric leaf blower, a shopvac that blows and a big gas compressor to choose from.
For larger particles you want something with more mass than air to provide the flow you work against. if you have a parts washer that's ideal. Stuff the nozzle in the fill opening. You can also use a garden hose with the sweeper nozzle. You don't need a blast, just a strong fluid flow. Using water you would finish by a flush with a water displacing solvent like WD40 ($20/gal). Then mount it, put the driveshaft on with the old bolts, take the rear wheel off and load it with cheap engine oil. Spin it with the engine for a minute, change the oil, repeat several times until the oil comes out clear. Then a flush or two with 90 wt and you're done. You can reclaim your flushing materials by heating them to 160 - 200F (use a candy thermometer or similar that registers degrees) for a few hours to drive off the moisture.
The thing is, you need the tranny oriented so you get your best shot with the drill and tap. I would bolt a square of plywood to the output end so it stands nicely wherever you're working (driveway, bathtub, etc). This is the problem with gravity and grease methods. you want to be looking strait down at the hole with the unit on the ground or a low bench so you can get over it.
You can use a drill press. Just take the press outside and then hose it with spray wd40 so it doesn't rust.
You want to use a 15mm drill bit for an m16 thread.
A high spiral flute drill would be ideal because they clear the chips upwards out of the hole more strongly than other types. But costly and I didn't spot one in a reduced shank, which means you need a 3/4" chuck. A reduced shank bit will work in a 1/2" drill. Rent one if you don't have one.
You might be able to rent taps too. Call around . Spiral flute tap would be ideal, same reason as the drill..., but very expensive. May need a semi-bottoming tap depending on how much past the hole you can go. This is critical, how much clearance do you have on the other side of the hole? If you are essentially doing a blind hole you will need two taps, a plug tap to get started and a bottoming tap to finish. This is a plug tap, pretty general purpose type.
You need a big tap wrench and unfortunately these can be the most difficult to get started strait. But you can do it if you go slow and step back and eyeball it as soon as you have enough thread for the tap to stand in the work. You can still re-align at this point.
Or you can get a helicoil kit for $45 that has everything but the tap handle. Again, beware the depth behind the hole. I can look at my kits and tell you what kind of taps they use. Installed correctly, they are a lot stronger than the original hole (stainless steel threads and much bigger) and are often used for this reason, not to make a repair.. You can make them fail, but it will take a lot more than what failed the original threads. You hear stories of them coming out---they weren't put in correctly. I put a 10mm in I-forget-what the other day where I needed to reinforce a very thin walled piece. That helicoil in red locktite was too perfect. Timeserts are even better, but costly. (If only I could remember what I was working on
. Now this is really gonna bug me...)