I am trying to build an ultimate wheel using 6061 aluminum rectangle tubing and an aluminum rim. One problem I found is that tapped thread of aluminum is not strong enough to “hold” the the pedal. Initially I can tighten the pedal to the tapped socket, but after just several rides the pedal bolt started wobbling.
I checked KH spirit crank arm, which is made from aluminum, and found the thread on the crank arm is made from a different (stainless steel?) metal. Anyone know where to get that?
6061 is standard aluminum for bike rims and frames
7005 is better and used for KH frames + Moment cranks but not so easy to bend as 6061
7075 is even better and used for Spirit cranks but it’s not possible to weld
Thanks all for the replies. I did not tap directly to the thin-walled (1/8") aluminum rectangular tubing. I drilled 1" holes and then inserted and welded a section of 1" round tubing (1/4" wall) in the 1" holes. I then tapped the 1/2" hole in the 1" round tubing with 9/16-20 left and right hand taps.
I don’t think the 1" round aluminum tubing (1/4" wall) is hardened.
I do not think I rode backwards, but I tightened the pedal again and will see if the problem happen again.
I rode the ultimate wheel a couple more times and during which I had to tighten both pedals several times. The left pedal thread on the UW (9/16-20) is now completely stripped. The right pedal thread on the UW (5/8-24 with a 5/8-24 to 9/16-20 thread insert/helicoil bushing) is still fine. Looks like the thread insert/helicoil busing does make the thread stronger.