On my last trip to the dunes my
Powerbloc ALT clutch flew off the crankshaft sending pucks, clutch faces and shit flying everywhere! The Powerbloc clutch bolt is an M12mm x 1.25, I think the length is 140mm. I tried using a 150mm to get a few more threads but still not enough to get 75lbs torque much less 50 lbs before the last threads remaining in the crank stripped.
I decided to tap the crankshaft hole to a 1/2-20 thread size since the 1/2 bolt is just slightly larger in diamater than the 12mm and the 1/2 bolt will fit in the Powerbloc clutch assembly.
For this repair I used the following: Reiff & Nestor HSS or HSGH-3 1/2-20 taps(this included the taper tap, plug tap and bottoming tap), a Starrett 91B tap holder, a Greenfield HSS 29/64th X 7" or 7-1/2" drill bit, and a can of Tap Magic cutting fluid, see pics. The new bolt will be a 1/2-20X6" Grade 8 bolt. I made the repair a few weeks ago so Im going by memory on this procedure, im sure i will leave something out but here goes.
First I removed the pull start cover and inserted a piece of metal stock through the pull start sprocket square holes to keep the crankshaft from spinning while drilling and tapping, see pics.
Next I set the fixed face of the clutch on the crankshaft just tight enough so that I can insert the drill bit inside to begin drilling the crankshaft for tapping, the clutch face will act as a drill bit guide so that the drill bit will cut into the crankshaft straight, see pic. Before drilling, I marked the end of the drill bit with some aluminum tape so I don't drill too far into the crankshaft, this also keeps the bits sharp edges from biting into the fixed face "clutch removal" threads, I then squirted a bit of cutting fluid on the end of the drill bit, the bit will want to bite quick so I set my Dewalts clutch at about half setting and adjusting as needed. The drill bit will just barely cut the old 12mm threads out of the crankshaft. Once drilled to the proper depth I cleaned the debris from the crankshaft hole will my compressors air tool and then followed with some carb cleaner.
Now its time for tapping. My Starrett tap holder's arm was a little long and was hitting the Pilot's frame as I turned the tap, so i used some shims to lift the
motor ((
Internal Combustion Engine ? )) up a bit so that the tap holder will clear when turning the tap, see pic. Next I chucked up the taper tap, see pic(1st tap on the left), into the Starrett tap holder and applied cutting fluid to the end of tap and the crankshaft hole. Then I inserted the tap into the hole, I had to bear down hard on the tap while slowly turning and waiting for the tap to bite, when it did, I beared down again and turned the tap about a 1/8 to 1/4 turn, then reversed the tap to clear the debris,I would do this 3 or 4 times, then removed the tap completely, sprayed air then followed with carb cleaner to clean the debris from the hole and tap, then go back to the 1/8 turn 1/4 turn procedure, it was really hard to keep the tap centered as well as applying pressure while tapping. Once the taper tap has tapped about 8 or 9 threads, I felt comfortable letting the tap pull itself into the hole from the threads it made. Still applying cutting fluid I was able to get about a 1/2 turn before reversing the tap to clear debris, the tap was getting easier to turn by this time. When I hit the end of the hole, by this time the tap was beginning to get easier to turn and when it bottomed out, it was easy to feel this and the tap just stopped turning. Once the taper tap had tapped a far as it would go, I again cleaned the hole and taps with air and carb cleaner, next I chucked up the plug tap, see pic (middle tap), still using cutting fluid, when the plug tap started cutting, I turned 1/2 half turn then reversed, following that procedure till it bottomed out, then cleaned the debris in the hole and tap, Next I chucked up the bottoming tap in the holder, see pic (far right tap), same procedure as plug tap. After bottoming tap was finished cutting, I cleaned the hole again and then inserted the new bolt with some anti sieze lube hoping to remove any remaining debris. I then cleaned the anti sieze lube from the hole and bolt. I then mounted the clutch with the new 1/2-20 X 6" bolt, I also used a 1/8to5/32 thick washer to keep the bolt threads from bottoming out(not sure if they will, its close) and help keep that ALT clutch cover from wallering out, see pic. I was able to apply 95lbs of torque to the bolt in the newly tapped hole. Feel free to add or scrutinize, I like to learn.