[shorrog]

A bit of an aside but do you know we have a comprehensive set of thread data on the main Website. https://www.marston-sunbeam.org/threads/thread_ba.php.
Graham

[andythomp]

Many thanks Steve and Rick for your help.
The holes and threads don't look mucked about, but they are what they are, so I will make up some studs with M10 to fit them and 3/8 BSF at the other end for the head nuts.
Cheers Andy

[Rick Parkington]

Ha ha, yes I know the feeling, Steve! People who were working piecework making these bikes at the factory would think we are all completely barmy!
But when it comes to it, since nobody seems to have figured out 'the meaning of life' making radiused head bolts seems a worthwhile as anything!
cheers R

[Greybeard]

I only noticed because I'd spent all day making various 20tpi radiused head bolts, Rick - then polishing each one  Sometimes wonder whether I'm going over the top. Im quite sure that Albert John Stevens wouldn't have spent this much time building the bikes in the midst of a depression!

Steve

[Rick Parkington]

Oops! Sorry, thanks Steve - I meant to double check that before posting!
I'll get my coat ...
R

[Greybeard]

3/8th BSF is 20tpi, chaps. 20tpi also used for Whitform threads of various sizes just to sprinkle grit in life's Nivea  I've uncovered a variety during my current restoration of my 1926 AJS 500 - those builders had an evil sense of humour!

Steve

[Rick Parkington]

Hi Andy,
Certainly wouldn't have been Metric from new - the 1932 parts list describes these as 'screw 3/8' but that doesn't mean it hasn't been tapped out to M10 in more recent years. A mate of mine did that to his Commando because he found it easier to work with Metric spanners...!
I think 1932 was when Sunbeam changed most of the fasteners on the bikes from the fine 'Cycle' thread to British Standard Fine - which is really the 'medium' thread pitch, Whitworth being the 'coarse' thread - and too coarse for this job. Usually Whit is used into soft materials - ie alloy castings and I'd guess your bolts were originally 3/8 BSF - which is 18tpi.
1.5mm pitch is 16.9tpi so that's pretty close  but 3/8" is 0.375", 10mm is just under 0.400", so I don't think tapping to M10 would get around a stripped hole so I'd say you just have worn 3/8 BSF threads, unless someone has run a 10 x 1.5 tap into the holes to give them a bit more life. Have you tried a 10 x1.5 bolt in the hole?

If you want to put it back to standard and BSF bolts won't hold, you can get a wire insert (helicoil) kit from Unithread but beware if there are any blind holes. The tap supplied in these kits is a taper cut - no2 profile meaning that it doesn't cut all the way down. That means that the wire insert closes in toward the bottom of the hole and if the bolt it long enough it can jam and being hard, the insert can damage the end of the bolt enough to make it hard to get out again. So whenever blind holes are involved always ask for a no3 cut, 'bottoming' or 'plug', as well as the one in the kit - they're not very expensive.

Cheers Rick     

[andythomp]

Hi can anyone let me know what the thread of the barrel head studs for a 1932 Lion 500cc s/v are?
The threads in the barrel I have look ok and I thought they might be 3/8" Whit but the stud is far too loose and
an M10 x 1.5 is a better fit. Is this the correct size?
Thank you
Andy