Valve cover stud thread repair

[Towner]

Hello,

after refitting the head of my commando for fixing of a leak, one lower stud of the front valve cover comes out while tightening :? .
It is 1/4" UNC. There is not much material left to insert a helicoil. Has someone experience ?

Thanks !

Ralf

 

[L.A.B.]

The thread is 1/4 - 20 Whitworth, not UNC.

 

[Towner]

The thread is 1/4 - 20 Whitworth, not UNC.

Yes, but the thread inside the is UNC.

Have you experience with Helicoil on these threads ? Helcoil recommends 3mm wall thickness, but there are about 2,5mm.

 

[L.A.B.]

The thread is 1/4 - 20 Whitworth, not UNC.

Yes, but the thread inside the is UNC.

The stud inner end thread is 1/4 - 20 Whitworth, the outer end thread is 1/4 - 26.

http://www.britishfasteners.com/threads/bsw.html

 

[Towner]

The stud inner end thread is 1/4 - 20 Whitworth, the outer end thread is 1/4 - 26.

Thanks for this info. I was told that it is UNC, maybe that's wrong. It is most likely that both threads are Whitworth.
So I have to look for someone who has BSW helicoils.

But for this insert the recommended wall thickness is 3mm as well. Do you think that is still practicable ?

The alternative would be welding.

Ralf

 

[L.A.B.]

Thanks for this info. I was told that it is UNC, maybe that's wrong.

Well, the same stud (NMT2252/06-7582) was used from 1956 to the end of Commando production, so there's no evidence-as far as I'm aware, that the original head/stud inner thread form was ever changed to UNC.

It is most likely that both threads are Whitworth.

1/4 -26 is probably 60 degree BSCy/CEI so not Whitworth (although it could also be 55 degree 1/4 -26 BSF) .

So I have to look for someone who has BSW helicoils.

You could use UNC if Whitworth is unavailable-although technically UNC is a 60 degree thread form, and Whitworth is 55 degree.

But for this insert the recommended wall thickness is 3mm as well. Do you think that is still practicable ?

Would I personally attempt to Helicoil it if it were my own, then yes, I probably would, but the final decision whether to go ahead or not must be yours.

 

[nortonspeed]

Ralf, 1/4" UNC helicoil will do the job just fine, gently mount the stud with blue loctite and don't tighten the domed nut too much.

 

[Towner]

You could use UNC if Whitworth is unavailable-although technically UNC is a 60 degree thread form, and Whitworth is 55 degree.

Thank you very much for this competent informations. Do you think the difference between UNC and BSW would be acceptable in this case ?

I am considering to insert a M6 helicoil. It is a bit smaller than 1/4" and the wall thickness would be ok. A friend of mine has metric helicoils.
But I have to look for someone to manufacture a stud with M6 on one side and 1/4" - 26 on the other side. But that could be possible.

Ralf

 

[L.A.B.]

You could use UNC if Whitworth is unavailable-although technically UNC is a 60 degree thread form, and Whitworth is 55 degree.

Thank you very much for this competent informations. Do you think the difference between UNC and BSW would be acceptable in this case ?

Yes, a UNC Helicoil will do, if Whitworth Helicoils are not easy to get.

I am considering to insert a M6 helicoil. It is a bit smaller than 1/4" and the wall thickness would be ok. A friend of mine has metric helicoils.
But I have to look for someone to manufacture a stud with M6 on one side and 1/4" - 26 on the other side. But that could be possible.

Yes I'm sure that would work out OK.

 

[toppy]

If you got your old stud you should be able to use a 6mm spilt die to recut the 1/4 bsw end if you are careful. It will not be a prefect thread but should hold enough with some locktight and if you did over tighten then the stud would hopefully fail before the helicoil