Tapping aluminum for pedal thread

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?

I googled and found this helicoil bushing (https://www.amazon.com/Unior-616551-Right-Helicoil-Bushing/dp/B001C6DK6W/ref=pd_nav_hcs_rp_1?_encoding=UTF8&pd_rd_i=B001C6DK6W&pd_rd_r=98ba6f25-d0f2-4936-9020-0409244f002a&pd_rd_w=y83qx&pd_rd_wg=MgCJ9&pf_rd_p=1f389e35-a4a8-4fe9-bcb3-352102c5df87&pf_rd_r=J345Q6RFFYPJZ9XCAMN3&psc=1&refRID=J345Q6RFFYPJZ9XCAMN3) and installed it, but I found the problem remains. Maybe this bushing is too thin?

Could anyone please shed some light?


I’m no expert, so I don’t know about “6061” but I do know that most “aluminum” cycle components are aluminium alloy, and therefore harder.

I think, 6061 is not a bad material to start with. Is it hardened? Nimbus VCX Cranks are 6061 too, but hardened to T6, wich is as hard as 6061 can get. unhardened 6061 is much weaker

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

Looks more like the problem is using tubing. On a crank, the thread is through a plain block, almost half an inch, not just the outer part of the tube which can’t be that thick.

Do you have a block that your are tapping into or is it just the wall section of the tubing? Can you post a picture?

If it’s just tubing then yes you need support for the entire length of the threaded section of the pedal. Even relativity soft aluminum should hold OK if you aren’t hopping much or riding backwards.

You aren’t riding with the wheel backwards are you? If the pedals are on the wrong side, that would also loosen the pedals and wreck the threads on any ultimate wheel.

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.

After you welded the tubing into rectangular channel none of the aluminum was hard anymore. The rectangular stuff and the round tubing were probably T6 when you started and T0 after you welded them.

Good point!

I am now thinking of welding a steel nut with 9/16-20 thread to a steel plate and then bolt the plate to the aluminum tubing. However I googled but could not find left hand 9/16-20 nut anywhere.

You could use a 1/2" nut and drill/ream it out to 1/2" (or 33/64") Then use the 9/16-20 taps you have to cut new threads.

Or maybe an option would be to use a threaded insert in the aluminum.

Thanks for the suggestion. another option would be using 1/2" inside diameter round steel tubing and tap 9/16-20 thread.

I already tried the thread insert/helicoil bushing. It could make the tread a little stronger but I think its main purpose is for repairing damaged thread.



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.

Now I need to find a lefthand 5/8-24 tap.

You can pick up 5/8x24 left hand tap on Ebay and many other places on line.