Stripped cylinder head sleeve stud Bolt

Good afternoon Maurice.

You need a helicoil repair. It sounds as though you're not too confident of your own abilities so I would take the head to an engineering place - but only one that understands that you need the correct thread in the head. From memory this would be a 5/16" BSW (Whitworth) but someone may well correct me on this.

The last thing you want is for someone to put a UNF or even worse metric helicoil in there.

Where in the world are you? Someone here may be able to make a recommendation.

Maurice

I've no experience of helicoils but when this problem occurred on my 750 a brass insert was made up and inserted.

This was fitted by BLR Engineering in London who trade as JD Autoworx. Did a good job in 2015 which is fine today. They are experienced in working on Commando cylinder heads.

The stud referred to is part no. 06.7885 and from my notes has a 3/8"BSF 20 tpi thread for the cylinder head end and a 3/8" BScY 26 tpi thread for the underside fixing nuts.

Please will someone confirm this as varying thread types occur on our Nortons!

In any event ensure that whoever takes on the job understands British thread types - assuming you wish to stick with standard threads. The slightly longer stud from RGM Norton is worth a try as it makes full use of the available threads. I'd suggest handing the stud and nut to the engineer doing the job.

Andy

I would check carefully the thread spec into the alloy head, BSW is usually recommended for alloy. I used BLR many years ago and they did a good rebore for me.

I would alwYs go for a brass insert over a helicoil on something like a head or barrel.

Maurice & Ian

I have checked the threads on new and old studs. These are definitely 26 tpi for the nut end of the stud. This is consistent with 3/8" BScy.

https://britishfasteners.com/threads/bsc.html

The cylinder head end of the stud is 20 tpi which is consistent with 3/8" BSF.

https://en.wikipedia.org/wiki/British_Standard_Fine

3/8" BSW is 16 tpi https://en.wikipedia.org/wiki/British_Standard_Whitworth

I agree with Ian that BSW would be better but it doesn't look like that's what we've got. This use of BSF probably contributes to these stud threads failing so often.

I'm prepared to be corrected by any definitive information. It actually disappoints me greatly (understatement) that there is no definitive info readily available.

RGM Norton list this part with info consistent with the thread characteristics I've quoted https://www.rgmnorton.co.uk/buy/cylinder-head-to-barrel-stud-3-8-stainless_156.htm

The stud is listed in this database http://www.gregmarsh.com/MC/FastenersList.aspx under the old part number 24389 (NM) with these dimensions 3/8"x20-26x1-7/8" which is similar to what I'm saying but is an incomplete description.

Hope this clarifies but I've been corrected before by those with more experience and knowledge and would be happy to be corrected again.

Andy

It's is 20 TPI, but should have been UNC, in the US the bronze repair inserts use UNC and come with the studs to match.

John

Does this mean that Norton should have (i.e. could have better) used 3/8" UNC (which is 16 tpi) see https://en.wikipedia.org/wiki/Unified_Thread_Standard ?

Do you agree with the dimensions I have given above for the stud?

Just clarifying

Andy

Norton were changing to Unified threads, BSF thread is too prone to stripping in this position so UNC is better and the taps and dies are readily available.

Duplicate post

I remember BLR - they did the exhaust threads on my first Commando in 1981. I rode the bike down from Brum, they did the job while I waited then I rode back home.....