B33 cylinder head oil way plugs

[PeteB31]

Can anyone help in identifying the thread plug that should be fitted to the oilway terminations on a 54 B33 cylinder head?

I've removed a couple of bungs made from 5/16" BSW studding that only entered a couple of turns and wept oil. 5/16" BSC seems a marginally better fit, but even that locks up after a couple of turns.

Thanks

Pete

[V500]

I would say that originally it was BSCy and the fool who whanged in the Whit plug has damaged the thread.
A gentle restoration with a BSCy tap?
On second looks is it a Whit helicoil in there?

[Rog1]

As you've probably found, they don't appear in the spares books. If it isn't BSW, then it must be cycle thread, but how many tpi? It may be 20 or 26; which one did you try?

[PeteB31]

V500 / Rog1 thank you for your replies.

It's not a helicoil and surprisingly the thread looks in good order. The best fit I found was a 5/16" x 26 tpi BSC bolt, but it only made a couple of turns before tightening up. 

Could it be 1/16" BSPT? The diameter is about right and 28 tpi looks possible. I have ordered some brass 1/16" BSPT plugs on the basis that they could fit, but if not they shouldn't damage the thread.

Pete

[V500]

BSPT sounds a bit obscure for BSA to use; tapered threads weren't usual on old British classics, but if it fits and does the job...

[Alan]

BSF is 5/16 - 18 . Screw a piece of round softwood in to count the threads.

Alan

[PeteB31]

Thanks Alan.
Excellent idea. Off to try it now.

Pete

[PeteB31]

Problem sorted.
The 1/16" BSPT plugs (7.72 mm dia x 28 tpi) have just been delivered and went straight in. It could be particular to my bike, there are a number of things that look as if they came from a plumbers merchant. Either way, all sorted, thank you to those who contributed.

Pete