Thread repair with Heli-coil

[mean gene]

Got machined parts back for Atlas rebuild. The threads that attach the primary and alternator have been run loose and threads are egged out. The big question is Can I redo them in 1/4 nf for more surface area or 1/4 nc.. I have noticed most manufacturers use NC thread in cast aluminum but is there a reason? Before someone questions, this will be done before assembling case halves.

 

[concours]

Coarse is better in aluminum. Yes, Heli-Coil will give a lasting repair. Industry standard for decades.

 

[mean gene]

Thanks guys your opinion is always valued. Fine threads with Heli-coil it is. I'll watch that the holes clean up when I rebore holes to 6.6mm

 

[Ron L]

Thanks guys your opinion is always valued. Fine threads with Heli-coil it is. ......

What??!!

 

[Richard Tool]

Coarse is better in aluminum.

See above

 

[mean gene]

What you all didn't see is a private conversation that "didn't want to get in a war" of right or wrong thread! His opinion, that I agree with, is that yes course thread is best for raw aluminum but the use if the Heli- Coil changes that in the fact you were turning/pulling on the stainless Heli-Coil not the raw aluminum. No disrespect to anyone. If I was wrong and the threads do not last I will be the first to write a post to that effect.

 

[Dbhinkel]

Always use course threads in soft materials.

 

[marshg246]

Always use course threads in soft materials.

Not the issue - that's fully agreed by most other than Norton (and AMC?) engineers. Once heli-coiled, fine threads are fine. Most cases have already been done over the years and have been done with the originally specified fine threads - at least on every Norton engine I've built.

 

[mean gene]

While we are having this warm and fuzzy conversation, which lock washer, NOT Loctite, is best? Internal serration or external serration? Or good old split lock washer?

 

[marshg246]

While we are having this warm and fuzzy conversation, which lock washer, NOT Loctite, is best? Internal serration or external serration? Or good old split lock washer?

The primary reason I do rebuilds rather than restores is that I use nylock nuts most places - so I don't have decide that question The only places I deviate are those where Norton said to use an internal or external serrated such as the timing chain adjustment locknuts - IMHO they specified those to make sure the adjustment didn't move (digs into the adjuster and nut), and keep the nuts on.

If I were doing a restore, I would use whatever the parts manual specified under the assumption that the Norton engineers know what they were doing.

All that said, I'm not sure there is a clear answer. Depends on whether bare metal or not, metal type, and when not bare metal painted or powder coated and I'm guessing that high or low torque makes a difference.

 

[mean gene]

I'm not doing a points restore, Just want it to stay together