I'm working on the oil tank cap for the early 1950's bikes (ES2 / Model 7) and have found that the brass "nut" that the butterfly bolt screws into is cracked. I've just tried adding a photo but a warning message has just come up?
In the hope that you can see the photo, it is the the second item from the left. It is 18 TPI and appears to be 5/16" BSW, but a die will not screw onto the thread. On closer inspection it looks like the thread on the butterfly bolt has a steeper thread angle, possibly to open and close the cap quickly.
I've tried measuring the angle of the thread and 5/16" BSW = 4.5 degrees and the original is closer to 5 degrees.
Has anyone got any information about this thread form?
Also, is the smaller thread on the end for the locknut 0BA?
Enots Cap Threads
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Looks to be a two start…
Looks to be a two start thread. Make a tap from silver steel and harden it to tap a new nut. Or screwcut a new nut in the lathe. Either way means the use of a lathe and screwcutting skills
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Two start thread
You're both right, on close inspection it is a two start thread.
Unfortunately my lathe skills have only just begun in my retirement, so screw-cutting, especially two start threads is beyond me for the time being!
Thanks for your help.
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A challenge
Wow. There are some real experts on this site. Always amazed. Well done Phil, Richard and Peter.
I know you live in the rural parts Phil, but this sounds like a challenge for a 'Model Engineering' mate. If you know anyone, probably making a 4 foot steam locomotive or traction engine
as you read, they might be more than happy to take it on. It just takes time and the matching parts to get best thread and hex sizes.
Let us know how you get on.
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Peter, I happen to live next…
Peter,
I happen to live next door to a Model Engineer with two 4 foot steam engine projects in his workshop. The trouble is that with advancing age he is losing interest in completing due to body aches and motivation!
My brother is also a precision machinist so will see if his aches and pains will allow him to have a go.
Otherwise, as Richard says, I might look out for a secondhand cap to use the butterfly bolt and brass nut for my repairs.
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Parts Availability
I'm sure that I have a box of dismantled caps, but life is complicated by the fact that I've just picked up a van-load of storage boxes and am still sorting things. I no longer know where anything is. I'm sure they're somewhere...They must be !
Keep reminding me and I'll post one off next time I'm over.
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Richard, I went through the…
Richard,
I went through the storage boxes stage during Lockdown so that I could sort out my garage with shelving. There's still a few things that I cannot find! I'm sure I can find a spare cap at the VMCC autojumble in mid September.
Meanwhile, my brother showed an interest in the two start thread and pointed out that it is 9 TPI, not 18 TPI, hence the quick action to open and close the cap. He's willing to have a go at making a suitable nut, but a spare cap seems a simpler option.
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Making a male thread...
... would be relatively straightforward but a nut would be difficult given the small dimensions.
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Model engineer
Ian, his job included model engineering to replicate armaments at small scale, so if anyone can do it then I'm willing to say "yes please" to him!
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I had some head-scratching with these as the pre-war butterflies were a different shape and steel which means they're usually rusty and mine needed replacement.
If you look closely, you'll see that they have a double-start thread !
Your best option is probably to find another nut.