ET: Engine (Twin) Exhaust cam follower 7/16 x 20 TPI thread.

[greg brillus]

Hi there All, I'm trying to sort which is the correct thread form for the front cylinder threaded follower spindle. I know it is 7/16 x 20 TPI cycle with a whitworth thread form, but is this 55 degrees or 60 degrees......? .......I have some spindles that will not thread into the cases, and some that do. When you measure the OD of the thread some are about 10 to 15 thou larger in diameter. Everyone tells me that the thread should be 60 degrees same as 7/16 UNF but I don't believe that is the case. So is this thread showing different sizes because of the different angles of the pitch, 55 verses 60 degrees..............If so, which is the correct one............Cheers...............Greg.

 

[davidd]

Know Thy Beast says 55 degrees.

David

 

[bmetcalf]

I'll bet ClevTrev has looked at some NOS items with his optical comparator and has an opinion.

 

[Sakura]

BSCycle X 26tpi is 60° as is 20tpi. 7/16 UNF is 60° but the thread form is slightly different, not enough to make any practical difference. However, Vincent didn't use that many BSC nuts and bolts so they may well have used 55° on items they manufactured themselves. In all cases I doubt there should be 0.015" od difference. More likely you have some poorly manufactured spindles?

 

[Cyborg]

Well I'm afraid I'm no help... When I use two cutters to try and check the angle, I wouldn't bet money on 55, but I'm using an old worn spindle, I'm half blind and challenged when it come to thread cutting.
I did have a problem fitting a new spindle due to the fact that the threads were not concentric with the spindle..... as in the threaded portion would oscillate around ever so slightly preventing the threads from engaging properly. The photos show the threads are wonky in relation to the ground surface. I would tend to agree with Sakura about the manufacturing. BTW, this spindle was manufactured recently.

 

[Vincent Brake]

i find they are all a bit on the piss in the cases, so doe not lay flat on the cam, causing wh#@tever. due to the thread being on: yes where...
so I drill the cases deeper, 9,54 hone, and make me a hardend axle, one end M5 female and make up a the afore mentioned thread on a small piece with 5, 3mm hole. glue in. bolt in.
slightly oversise rod (and hone the follower...) screw in and there we go

 

[greg brillus]

I think there is a cross over on the thread from 55 to 60 degrees. The factory books say whitworth thread as being 55 degrees. I do wonder if it changes on threads on smaller sizes up to say 3/8 and from 7/16 on it is different. I have a set of Norton Roadholders, where the lower yolk pinch studs have this same thread. I made up some reduced head nuts using a 7/16 UNF tap, and although the nuts wind on ok, they appear quite a loose fit, if wound on several threads you can jiggle the nut sideways. Because these nuts need to be done up very tight I do wonder if the grip strength is not so good. So there must be something amiss.......... I have tried looking for taps/dies in 7/16 x 20 TPI with a 55 degree thread..............No success. That particular size seems to be a tricky one, so I'm still looking for an answer..............Where's Trev................????..............

 

[Marcus Bowden]

Greg my handsome, Tracytools is a supplier for nearly every thing I've wanted in way of taps, dies, helicoils, drills mills and much more. < www,tracytools.com > ALLways at reasonable cost. Can not tell if they do international orders, if you don't ask you won't know.
I have always been under the impression that Whitworth thread form was 55 degrees throughout BSW< BSF< BSC< BSP< BSB< or the 20 tpi specials (never known the difference between cycle & brass thread though).
bananaman.

 

[vibrac]

The only divergence in non metric motorcycle threads from 55 are
UNC which has 60 degree threads and a flatter crest than whitworth but except for 1/2" is same TPI as whit
Also BSC or its predecessor CEI is 60 see:

https://britishfasteners.com/threads/bsc.html

 

[ogrilp400]

G'day Greg,
I deal with threads very regularly and have done for decades. Firstly, 7/16" OD is 7/16" OD = .437", UNF, UNC, Whitworth, BSF UNF etc etc, all the same. Now I am sure that some one will come back and say that thread ABC is .428 or some such but you get the idea. Whitworth thread form is 55°. For as long as I have been dealing with this its been 55°. If you are measuring the thread at .010" to .015" larger then there is your problem. Given that the spindle is hardened there is not much you can do with it other wise I would say run a die over it or a thread file. The various thread dimension books give tolerances to some times four decimal places but this is for ground precision fit threads and generally we are no where near those tolerances.