Hitchcocks Forum

Hi guys. The threads of a couple of the rocker cover studs on my 1996 Iron barrel are coming out of the head. Each time I refit the studs (currently after some head work) another piece of thread breaks loose (and no, I haven’t swapped the ends around – I marked them before removing).
All the rocker studs have been helicoiled already, so this is not new. But I don’t know what size they should be. The studs have threads that have remarkably similar spacing to 6mm metric – if I put the threads together, I can see no difference. So can I use metric helicoils (so I don’t have to replace the studs and their rocker cover nuts) and then run a 6mm die over the studs to ensure they fit OK?
Hope you can advise.
Cheers, ChrisD

Hi Chris, the studs should be 1/4" BSF as standard, and it has the same thread pitch as the metric M6, but is of a slightly larger diameter. You might be able to Helicoil to M6, but if you use the original 1/4" dimensions, DONT use M6 studs. If you have M6 studs in there already, that is probably why your threads are stripping, as they will screw in, but will be too slack a fit diametrically. If the threads were M6 originally and I doubt it, then you might be able to tap out to 1/4" BSF and screw studs of this size straight in to the head and they should hold. Regards, Paul.

Thread data comparison charts, as has been stated largish difference between the two threads from an engineering point of view:-
http://emkaysupplies.co.uk/chart.htm

helicoils are not the most worthwhile reparation, a threaded bush is a definitive reparation.

I would get yourself a thread gauge. They're cheap and will let you know what you have thread-wise.
I thread is either right or wrong - near enough wont do and running a M6 die down a BSF thread will just make it worse.
I agree with mauri that inserts are better than helicoils though they would do the job here OK. The trouble is that if they are already helicoiled, you would need to go up a size to redo them (unless you can get them out without damaging the "parent" thread)

Running an M6 tap down a 1/4" BSF thread will not do anything - the 1/4" size is larger than 6mm. If, however, in the unlikely event the rocker cover studs and their holes have been M6 size, running a 1/4" tap down the holes might just mean you can then successfully screw a 1/4" stud into the hole, which is what I was trying to say in my post above.

Guys – thanks for the quick replies. The rocker cover studs are standard – not metric and not 6mm – in fact a 6mm nut will not screw onto the stud as it is slightly too small – so I assume I have the standard ¼”BSF studs. I agree it has the same thread pitch as 6mm metric, hence my initial thought about a quick-fix.

The rocker studs that were helicoiled are 6mm metric and use the original studs so that seems to have worked OK (at least up to 7ft-lb) – and they’ve been there for some 5 years now.


Whatever I do, I will have to either find a way to use the studs as they are or make new one’s to suit whatever thread can be fitted – I don’t want to change the tube nuts as well and so must keep the original studs. So a plug may be the best way - should last forever (but wait, this is an Indian enfield - nothing else of the original bike will last forever). I’ll discuss it with my engineering expert who did the previous inserts.
Cheers, ChrisD

A correctly fitted helicoil will be stronger than the original thread, if the helicoil is either the wrong thread or fitted by Mr Hamfist it will be next to useless, if it has been done by either of these two latter methods the only real answer is as Mauri says a threaded insert.

"Running an M6 tap down a 1/4" BSF thread will not do anything"
You're right of course BW, the slight difference in thread makes no difference (M6 = 25.4tpi 1/4 BSF = 26tpi)

Hi all – I have had 6mm helicoils fitted to the head for the two tall rocker cover studs that were damaged) and the engineer made new 6mm studs made to match the ¼“ BSF tube nuts.

Incidentally, comparison of the OEM alloy conrod (1996 500cc Iron barrel) and the new Hitchcocks steel conrod (PN 90125) shows the latter is 20thou shorter eye-to-eye than the OEM one. I’d understood that Indian parts were a bit suspect but half a millimetre out?
Cheers, ChrisD