Rocker box thread

[andrew.chapman]

Does anybody know what the thread is on the rocker-box bolts on an VH? It is 1939, if that makes a difference.

[andrew.chapman]

Well, it looks like I may know the answer but I would appreciate some confirmation. I think it is 5/16" BSCY (26tpi). Does that sound right?

[david.anderson]

Andrew
yes 5/16 cei / bscy
David

[andrew.chapman]

Many thanks David.

[chris.read]

by chris read. thread is 5/16 26tpi BSB not BSCY wrong thread angle. Ariel did not use BSCY but BSA did!

[simon.holyfield]

BSB? Are you sure?

[john.whiting]

BSB and BSC are considered to be fully interchangeable in corresponding sizes.There may be some minor differences in rounding,but the pressure angles are the same,as of course is the pitch.BSC is considered to be a later version,increased in scope to include the heavier standard fasteners required by motorcycles.Many manufacturers used their own specialized combinations as well.Regards John.

[nevhunter]

I should know better than contest anything written by many here but I understood the angle difference between Brass 26 tpi and british cycle is 5 degrees. I think brass is 55 and cycle 60. Not that it matters in a practical sense unless one is making these things new. By the way it is necessary to clean out the threads fully with a plug tap, with cast iron heads or you run the risk of cracking the head which is common on the thread bosses.
The alloy heads have a coarser thread and always make sure the longest bolt that can be used is used or it risks stripping. 2 lengths are used in all heads. Nev

[Leejm]

I'm an engineer by trade and I grind threads for a living. Thread flank angle as never said for cei threads are 55 degrees. Not 60 degrees. Whitworth thread is that. Cei stands for cycle engineers institute. It was invented as motorcycle trade found a need for fine threads on a large bolt size. With Whitworth the thread size goes up with bolt diameter. So the tpi goes down. Cei you could get any tpi on any bolt size. Eg. 20, 24,26,32 there are more!! They found some tpi that were not being used so they slimmed down on the tpi list. Then it become bsc.British standard cycle! 26 tpi being the most common used. But beware it may not be 26 tpi. 9 times out of 10 it will be but you need to bear that in mind. As for the angle flanks the flanks need to be the correct angle. Its the flanks that hold the bolt! So do not try a bsf in cycle thread yes it will screw on but bsf is basically a Whitworth thread so 60 degrees angle.

[Leejm]

Ariel never used bsb thread. Please don't use it.