JohnDThompson 

While I'd also be reluctant to ride with only four bolts, I should also point out that the inner ring mounting is quite different on the Avocet touring triple and the Sugino AT triple. The Avocet inner ring uses long bolts with stand-off spacers to position the inner ring. This makes for a long lever arm on the mounting bolts and increased stress under load. The Sugino, OTOH, has the stand-offs for the inner ring forged integrally with the arm, and provides much more support for the inner ring mounting bolts.

Avocet triple:



Sugino AT triple:



If chasing the threads is enough to allow all the bolts to engage securely, I'd anticipate no problems. Otherwise, a helicoil repair is a likely solution.

noobinsf 

According to this Park Tool table, I need a tap that's 8mm x 0.75mm. Does that ring true?

https://www.parktool.com/blog/repair...hread-concepts

JohnDThompson 

Yes, that sounds right. 8mm x 0.75mm is the standard thread for a regular chainring bolt, so you can check by threading a regular chainring bolt into one of the undamaged inner ring mounting holes.

OTOH, the aforementioned Avocet triple crank used narrower 6mm x 1mm thread for its inner ring mounting bolts; another strength compromise.

t_e_r_r_y

it does. a quick google search shows the standard chainring bolt is 8mm x 0.75mm, and heli-coils are available in this size.

t_e_r_r_y

also, a proper heli-coil job would replace the trashed aluminum threads with permanent steel threads, making the crank better than new. you wouldn't have to worry about future cross threading or damage to chainring bolts or the existing threads

that said, you could get away with a longer bolt and chasing the remaining threads and if/when that eventually fails the heli-coil solution will still be there as a backup.

dddd

That's a WOW, didn't know that this thread specification wasn't limited to the bicycle world.
And as long as there is enough "meat" for the slight enlargement of the threaded hole, a simple and reliable fix.

But I still suspect strongly that this crank needs no thread restoration beyond running a longer bolt in and out a few times.
The original bolt does seem to have been aluminum, and the bolt threads do appear to have surrendered to the harder forged alloy of the crankarm.

seypat

I also just acquired a 110/74 Sugino AT. The info I am reading is for 3 rings, the BB spindle length is 124.5.

noobinsf 

Just wanted to post a quick update to this thread. I bought a tap (M8 x 0.75) and ran it through the hole. While this “after” pic shows clear thread damage about 3-4 threads down, an undamaged bolt now runs cleanly in and out, and there is plenty of thread engagement to make me confident in its integrity. Thanks for your advice, everyone!

seypat

Glad it worked out for you.