Oval sprocket bolt holes

[brian kirby]

Both of the bikes I have with conical hubs have ovaled out sprocket holes. Has anyone ever tried drilling out the holes to use a 7mm or 8mm bolt?

Brian

[Daniel P. McEntee]

That's funny you should ask this question. I was going through my parts to sort out what I need for the 250 and 400 I got going back together now, and have run across two different size holes in the sprockets. I think my '77 400 already has 7mm bolts in it. I would have to measure to be sure, but opening them up to 8mm might be pushing things a bit, but I don't know for sure. 7mm bolts are kind of an odd ball size, but not unobtainable. If you ever replace the wheels, welding them up and redrilling at that time is an option. I'll be watching this thread and see what others find or have.
   Type at you later,
   Dan McEntee

[brian kirby]

One wheel is from the '74.5 250 I recently bought, but I checked the '77 MC5 400 wheel and its bolts are loose too, and I am sure the holes are ovaled. The '77 definitely has bigger bolts, but I did not take them out to verify 7mm or 8mm. My '79 Maico also had 7mm sprocket bolts but the replacement wheel I got for it (when the ovaled bolt holes in the original wheel caused the bolts to come loose, which in turn sheared off all the bolts and ruined the hub) was off of an '80 which had 8mm bolts.

The back side of the hub has a shoulder which the nut flats sit against, this would have to have a small area machined out for a bigger nut to clear, but I am also thinking of getting 7mm bolts, drill the hub and sprocket, then run a 6mm die over the exposed part of the 7mm bolt so I can use the original 6mm nuts so I wouldnt have to cut recesses for a bigger nut.

Brian

[Rocket]

The early one's had 6mm bolts and the later one's had 7mm from what I have found.  If yours are ovaled out 6mm, drill them to 7mm.  You can buy 7mm bolts from Fastenal.

[Daniel P. McEntee]

Another possible option is to drill out the holes to 7mm, and if you have access to a lathe or know someone who does, have them make you some steel bushings that are an interference fit into the 7mm hole and loctited, and drilled to take the 6mm bolt again. The walls will be pretty thin, but the steel will take the beating better than the aluminum, and you won't need extra clearance for the nuts like you described.I there is room in the hub, drill them out to 8mm and put in 6mm by 8mm bushings. That way you end up with a thicker wall on the bushing. If you have some old hubs with really bad holes, that would be the ideal thing to try. Some one with more intimate knowledge of the rear hubs may know if this all will work or not. Just sittin' here at my lap top brain stormin' it!
  Keep us posted,
   Dan McEntee

[brian kirby]

The conical KTM hubs are magnesium.

Brian

[Daniel P. McEntee]

Don't give up on the hub just because they are magnesium. If you need to weld one, any decent TIG welder should be able to handle it if he has mag rod. To me, that's the hardest part about welding magnesium, finding filler rod in small quantities! Most welding supply shops tell me that they can order all I want at 50 bucks a pound, and a ten pound minimum!! I lucked into a pound at a local welding supply shop about three or four years ago, so I have some. I welded up a cracked rear hub that was in the batch of parts that my younger brother picked up a few years ago. I've dome cracked magnesium fork legs for a couple of guys on the VINDURO List. It's pretty much like welding aluminum, you just need the proper rod. Drilling and tapping is a bit of an adventure because it likes to get brittle as it ages in some instances. I drilled and tapped my 250 Hercules ignition cover for a compression fitting so I could vent it, and it sounded like I was tapping glass! When drilling, just be careful with drill speeds and feeds. Working with magnesium is a mystery to a lot of people and sometimes freaks them out, because it's just not a common metal for them to work with. It has it's good and bad properties. Even as far back as WW-2 they were fazing it out for use on aircraft parts because of the tendency to get brittle, and even though it's nice and light, most times it's not as strong as a good grade of aluminum. I can't get to that repaired hub right now or a wheel to look at to check, but I think the only real problem in welding up the holes is the depth of the hole. It's difficult work down into a long, small diameter hole like that, and the way the hubs are constructed with the cooling fins that run the circumference of the hub, welding up the holes solid might not be practical.
  Type at you later,
    Dan McEntee

[Kip Kern]

Yes, I have drilled them for 7mm with no issue.  Bolts can be had at any Fastenal or good Auto Parts store. Happy holidays