mk1-forum.net

Bit of an obscure one - the internal thread on the SU dashpot which the damper cap screws into is 11/16 inch with a 26tpi thread, but there are two possible thread forms.
- is it 'brass' thread with 55 degree angle?
- or 'cycle' thread with 60 degree angle?

Many thanks, and Happy New Year!

My guess would be brass since the dampers were at one time made of brass before they went plastic......

Its cycle, I've got a tap for it from when I shortened the dashpots on my sprint

Thanks Rich,

My first guess was for brass thread, but good to know that it in fact cycle thread!

And you're also right about the reason for wanting to know the thread type, I have a lack of clearance between carb and bonnet...

I bought and have used several times a 5/8" x 16 UN tap in my crankshafts.
The thread angle on the tap is 60 degrees.

Apparently the correct thread is 5/8" x 16 Whitworth (55 degrees).

Rich, why would SU use "Cycle Thread" and not "Brass"?
The jet winder threads on HS (and before) carbs are allegedly a Whitworth form,
thus suggesting that "Brass" may be correct...

My view here is that as all of the materials are soft on the carb's suction
chamber assembly, it is not going to make much difference which tap is used.

Ian

I am not familiar with seeing 55 degree threads referred to as "brass" and/or 60 degree threads as "cycle". Can you elaborate on the origins of those classification terms?

BSB (brass) threads are always 26tpi due to brass tube always having the same wall thickness irrespective of diameter.
BSC (cycle) is generally 26tpi, but there are exceptions.

This is a good website about British threadworms Doug (and is actually from your side of the pond!):
http://www.britishfasteners.com/threads/bsc.html
There more good stuff on their homepage too.

dklawson wrote:I am not familiar with seeing 55 degree threads referred to as "brass"
and/or 60 degree threads as "cycle".
Can you elaborate on the origins of those classification terms?

Brass threads "just happen to be" 55 degrees, the same as Whitworth
Cycle threads "just happen to be" 60 degrees, the same as Unified & Metric.

If a thread is "Brass" it will be 55 degrees, but a thread with 55 degrees
it not necessarily "Brass". It could be Brass, Whitworth, or anything else
with that angle...

A 60 degree thread could be Unified, Metric, "Cycle" or something else...

We are not saying that ALL 55 degree threads are referred to as "Brass",
just the ones in the context of the subject being discussed. Ditto, we are
not saying that all 60 degree threads are referred to as "Cycle".

NB: For both "Brass" and "Cycle" most of the pitches are 26TPI regardless
of the diameter.

...and I know that Rich is going to say that the thread in the dashpot is not
"Brass" because it's made from ally...

Ian
[PS I think I was typing at the same time as GraemeC...]

Hi all,

It's Whitworth 55 deg.

su_mark wrote:Hi all,

It's Whitworth 55 deg.

dammit


wrong again

su_mark wrote:Hi all,
It's Whitworth 55 deg.

Please state your source/evidence...
(I am inclined to believe that this is correct because the winder
mechanisms for the jet are Whitworth-Form threads and the bolt
that holds the float chamber on is 1/4" BSF, ie "Whitworth-Fine").

Ian

ianh1968 wrote:

su_mark wrote:Hi all,
It's Whitworth 55 deg.

Please state your source/evidence...

ian

as mark is 'mr. su' I would be inclined to believe him.........