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#839772 02/12/21 3:07 am
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Finally 15 years after I started dismantling, I am starting to re-build my 1960 C15. One of the drain plugs at bottom of the tubes has a bad thread. But this plug has an extended end. Does the point have a point? smile i.e. is it supposed to lock or locate the bottom bush in the tube? Or can I just replace it with a short 3/16" BSF bolt?

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The drain screws do/should have the peg extension on C15 road bike forks. It fits into a notch on the center "restrictor rod". the peg keeps the rod from turning when tightening the screw that comes up from the bottom for the fork leg that holds the restrictor rod. It does not lock any bushes in place as the bush has to travel up and down with the stanchion tube. Also the thread form is 2BA so why mess it up by screwing in a 3/16 screw? And yeah I know that the fit is "pretty close" or "close enough". Do the next owner a favor and just get the correct item. Part # 40-5046

Peter


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Thanks, will do

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Originally Posted by Peter Quick
The drain screws do/should have the peg extension on C15 road bike forks. It fits into a notch on the center "restrictor rod". the peg keeps the rod from turning when tightening the screw that comes up from the bottom for the fork leg that holds the restrictor rod. It does not lock any bushes in place as the bush has to travel up and down with the stanchion tube. Also the thread form is 2BA so why mess it up by screwing in a 3/16 screw? And yeah I know that the fit is "pretty close" or "close enough". Do the next owner a favor and just get the correct item. Part # 40-5046

Peter


3/16bsf is 2ba


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Originally Posted by Allan G
Originally Posted by Peter Quick
The drain screws do/should have the peg extension on C15 road bike forks. It fits into a notch on the center "restrictor rod". the peg keeps the rod from turning when tightening the screw that comes up from the bottom for the fork leg that holds the restrictor rod. It does not lock any bushes in place as the bush has to travel up and down with the stanchion tube. Also the thread form is 2BA so why mess it up by screwing in a 3/16 screw? And yeah I know that the fit is "pretty close" or "close enough". Do the next owner a favor and just get the correct item. Part # 40-5046

Peter


3/16bsf is 2ba

No it isn't
2BA is 4.7mm (0.185") x OD 0.81mm pitch (31.36 TPI) x 47.5 deg thread angle
3/16" BSF is 0.1875 OD x 32TPI x 55 deg thread angle


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BSF is 55 degree thread form, BA is 47.5 degree thread form. Sizes and TPI's are also different even if marginal.

You can stuff a 3/16 BSF bolt into a worn 2BA hole and as a bodge it will work but 3/16 BSF is not 2BA and the distorted thread will not be 2BA.

https://www.trfastenings.com/products/knowledgebase/thread-geometry/british-standard-fine-bsf

https://www.trfastenings.com/products/knowledgebase/thread-geometry/british-association-ba

3/16 BSF Major dia 0.1875 TPI 32

BA2 Major dia 0.18504 TPI 31

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In a convoluted way BA threads are metric, the BA series is based on 0BA being 6mm dia x 1mm pitch. BSF are imperial


BSA B31 500 "Stargazer"
Greeves 200 "Blue Meanie"
Greeves 350
Greeves 360
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500 sprint/LSR bike "Deofol"
Jawa 500 "Llareggub"
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Soooooo, still a bit confused!
I took a closer look and realize the female thread in the leg is just as bad as the screw. I will have to buy a thread insert to repair that as well as the new screw. I don't want to insert the wrong thread into the fork leg, and stuff up another screw or worse.
I know the screw is approximately 32 tpi and approximately 3/16", but I don't have a micrometer and I can't tell from my cheap thread gauge and my poor eyesight whether the thread form is a greater or lesser angle than my 55 degree gauge. Thanks to Peter for alerting me that it is not BSF thread with 55 degree form.

I double checked on British Threads website and they confirm Kommando (well almost, as 2BA is actually 31.4 TPI which is good enough for me). Now, my question is more about what to buy for the insert. I was not aware that many threads were BA, although I understand Cycle thread (CEI) is used extensively on these old BSAs. So is it actually:
2BA = 0.1850 dia. & 31.4 TPI at 47.5 deg or
3/16 CEI = 0.1875 dia & 32 TPI at 60 deg
I suppose I could wait until the new screw arrives and borrow a micrometer?

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CEI is less used on the “modern” BSA’s like the C15.

On C11/C12 there is more because thy were pretty much hangovers from prewar. Your C15 will be unit construction so quite a different beast. A whole re-design of the bike really.

Fork legs I think will still be the BA thread form.

Can you chase the threads with a 2BA tap and a die nut? Perhaps add some thread tape to take up the wear.

After all, it’s just a drain plug and a locator for the restrictor rod.

A screw insert that small might be a fiddly fix ( unless you have the gear to keep it nice and accurate)


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Got hold of a dial caliber and the old screw is 0.183 to 0.185, so absolutely 2BA. So you were 100% correct Peter & Ginge, and thanks to all for helping me get educated a bit about different thread details.


Moderated by  Allan G, Jon W. Whitley 

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