After I got the handlebar assembly castings back from powder coating and put them back together, I discovered a problem with the left side clutch assembly. Referring to the diagram below,
the pinch bolt (5) is supposed to be a 6 mm x 1 socket head cap screw that squeezes the bracket around the handlebar so the serrated perch (16) (see diagram below) grabs the bar keeping the assembly from rotating.
Instead of the socket head cap screw, there was a smaller diameter hex bolt with a nut to tighten the assembly. On the throttle assembly on the right, there was a 6mm hex bolt so I thought the socket head cap screws had been lost and this was the fix. I bought new socket head cap screws and as I screwed in a new one, it wouldn’t tight up. Ah, so the threads are stripped and that’s why the small diameter bolt and nut was used.
My first thought was to tap the hole with a 7 mm tap. The drill diameter for that is 6.1 mm, so that should work. I posted a note to the Airhead BMW Club and sent an Email to a technical discussion forum for airhead owners with my idea. Tom Cutter replied with “helicoil is best” and someone else replied that M7 bolts are made from Unobtainium. So, instead of a $0.65 bolt, I bought a $40.00 6mm x 1 helicoil kit at my local NAPA store. But on the bright side, now I can repair another 11 stripped 6 mm x 1 threads. When I look at it that way the kit only costs $3.33 per stripped thread. 🙂
Honestly, in all the years I’ve been tinkering, I never had to do a HeliCoil repair. But through the magic of YouTube, I saw how to do it on a car engine block, and that’s close enough to a BMW handlebar assembly to make it look pretty easy.
Here’s the NAPA kit I bought.
Here is the assembly showing the stripped thread.
The instructions said to drill out the existing hole with a 1/4 inch drill. Hmm, nice new powder coated assembly and here I am attacking it with a drill … sigh.
The drill is variable speed so I started at low speed so when the bit grabbed it wouldn’t fling the assembly across the room. Carefully working the bit into the threads, I got the hole drilled without damaging the powder coat.
I used the special HeliCoil tap to cut the threads for the insert.
I put a little tap cutting oil on the tap threads and used a tap handle to cut them.
The assembly is split so the front side where the bolt head rests doesn’t have any threads and the back side does. So I tapped the hole from the backside.
I went slowly and was careful to keep the tap square to the hole on the first few turns. And, voila, a HeliCoil threaded hole.
The kit includes a tool for installing a thread insert. The insert goes inside the tool through a cutout on the side with the tang on the bottom of the insert pointing to the bottom of the tool.
There is a threaded rod inside the Installation tool that screws through the inside of the insert until a little ridge on the end of the threaded rod catches on the tang at the bottom of the insert.
The end of the threaded rod is square so it will fit a small tap handle.
I mounted the assembly in the vice using rubber jaws. As the threaded rod is screwed down, the insert coils are compressed so the coils will engage with the the threads in the hole. There are no threads on the front side of the hole, so I threaded the insert into the backside threads. But the diameter of the Installation tool is large enough that I couldn’t keep it square with the hole. I figured the insert coils would engage with the threads and since the coils are flexible the insert should thread into the hole correctly.
I used a punch to knock the tang off the end of the insert and then threaded the new socket head cap screw into the HeliCoil and tightened it up enough to see the ends of the slot start to close. Success.
Here are the finished left and right hand handlebar assemblies with new powder coating and polished levers. I think they are going to look very nice on the new handlebar.