Helicoil

Sorry to repost but it looks like the last one got lost in the shuffle.

I need to helicoil my case to fit the front center barrel to case stud.
Can anyone confirm the thread. The stud is 3/8 x 16 x 26 - the 16 being the case end - Is that UNC and the 26 is BSC (cylce)?

Any suggestions on the best place to buy?
Thanks,

Phil

fastback said:

Can anyone confirm the thread. The stud is 3/8 x 16 x 26 - the 16 being the case end - Is that UNC and the 26 is BSC (cylce)?

Click to expand...

Up to the 1971 models, it would appear that the stud threads were 3/8" x 16 x 26, and the 16 tpi thread would have very likely been Whitworth, the 26 tpi thread is Cycle, this stud is part number NM 18943 so the crankcase thread *should* I believe be 16 tpi Whitworth?

However, from some point in production (1972-on models?) the stud changed to part number 062641, this stud being 3/8" x 16 x 24, the 24 tpi end being UNF, the part is recognisable by it having the first turn of thread machined off the UNF end. So the 16 tpi crankcase/stud thread could well have changed to Unified thread at that time, 3/8" Unified coarse (UNC) also being 16 tpi -the same as Whitworth.

Whitworth and Unified thread profiles differ slightly, so you could use a 3/8" UNC helicoil, as that will may be correct for a 72-on model? A UNC helicoil could still be used on a pre-'71 model crankcase and the stud changed to the 062641 part?

Whitworth thread (55 degrees)
http://www.boltscience.com/pages/screw4.htm

Metric/Unified thread (60 degrees)
http://www.boltscience.com/pages/screw6.htm

Wow - thanks for the great response!
I've got a '70, so I guess it is Whitworth.
Armed with that info I found Whitworth (BSW) helicoils at the uni-thread site. broken link removed
I will confirm the thread with Nick at Andover - where I got the replacement stud from.

fastback said:

I will confirm the thread with Nick at Andover - where I got the replacement stud from.

Click to expand...

It would be interesting to know *if and when* the crankcase/stud coarse thread changed from BSW to UNC?
I noticed on the BSA Regal list that the NM18943 stud has an updated part number = 067726.

Starting with engine number 200001 the case to cylinder studs were UNC/UNF while earlier models were 16 and 26 tpi.

Starting with engine number 200001 the case to cylinder studs were UNC/UNF while earlier models were 16 and 26 tpi.

Nick confirmed that the threads on 06.7726 - NM18943 are indeed Whitworth on the case end.

I guess it is a set of Whitworth helicoils for me :?

You might find it easier to get the UNC Helicoil, the UNC/UNF stud and nut.
Just a thought.
Cash