Thread Size for Roadster Petcock

[JimR]

Hi! I'm a new poster but have been looking at the site for some time. I have a '74 850 Roadster which I've had since it came out of the box. Been off the street for the past 25 years. 3 children, 3 job changes and 4 moves later I'm starting to put it back on the road. Everything going well but I made a small, correctible mistake with the tank. When I had it painted (great job by VintageVendor) I had the inside Caswell coated and forgot about plugging the petcock holes. Does anyone know what size and thread these are since I need to chase the threads to get the petcocks in. There was an old post on the board but it seemed to indicate 1/4" which doesn't seem right.

The fitting appears to measure 1/2" and threads look to be parallel so I am thinking they are 1/2-14 BSPP.

 

[GavinJuice]

The tap is 1/8 BSP (parallel), but its not cheap. You would be better off modifying an old petcock but cutting slots perpendicular in the threads and running it in and out, or i believe MEK (METHYL ETHYL KETONE) will remove caswell. Maybe swab it in there with a q-tip and let it do it's thing, nasty stuff tho.

 

[Rich_j]

Hi! I'm a new poster but have been looking at the site for some time. I have a '74 850 Roadster which I've had since it came out of the box. Been off the street for the past 25 years. 3 children, 3 job changes and 4 moves later I'm starting to put it back on the road. Everything going well but I made a small, correctible mistake with the tank. When I had it painted (great job by VintageVendor) I had the inside Caswell coated and forgot about plugging the petcock holes. Does anyone know what size and thread these are since I need to chase the threads to get the petcocks in. There was an old post on the board but it seemed to indicate 1/4" which doesn't seem right.

The fitting appears to measure 1/2" and threads look to be parallel so I am thinking they are 1/2-14 BSPP.

With BSP threads the nominal size doesn't actually correspond to any dimension of the actual thread, a bit confusing I know. A 1/4 bsp thread has a roughly 1/2" major diameter.
Google BSP and you'll find more info.

 

[L.A.B.]

The tap is 1/8 BSP (parallel),

The thread is actually parallel 1/4" - 19 tpi BSP (or also known as 1/4" Gas thread) and, as pipe threads generally refer to the pipe bore size, the actual thread diameter is 0.518".

http://www.britishfasteners.com/threads/bsp.html

 

[Rich_j]

The tap is 1/8 BSP (parallel),

The thread is actually parallel 1/4" - 19 tpi BSP (or also known as 1/4" Gas thread) and, as pipe threads generally refer to the pipe bore size, the actual thread diameter is 0.518".

http://www.britishfasteners.com/threads/bsp.html

Be careful when you start calling BSP gas threads, there is a completely different range of fine threads called "gas threads" fitted to pressure gauges.

In UK/Europe you'll also find BSPP (parallel) referenced as G i.e. G1/4, G1/2 etc and BSPT (taper) as R..

 

[L.A.B.]

Be careful when you start calling BSP gas threads, there is a completely different range of fine threads called "gas threads" fitted to pressure gauges.

Well, maybe so, but I think you will find that 1/4" BSP is also commonly referred to as " 1/4" Gas" for motorcycle pipe fittings.

There are 13 instances here of it being referred to as "Gas" (not to be confused with gasoline) on this British website page of petrol taps: http://www.burtonbikebits.net/Taps.htm

http://www.burtonbikebits.net/tap_size_guide.htm

"Petrol Tap Size Guide

In order to determine the gas thread needed please see the guide below:

Gas Thread Measurements
1/8" Gas 3/8" dia x 28 TPI
1/4" Gas 1/2" dia x 19 TPI
3/8" Gas 21/32" dia x 19 TPI
7/16" Gas 7/16" dia x 19 TPI "

http://www.newmantools.com/taps/bsp.htm

"G: Series British Standard Pipe - Parallel ( Straight ) BSP or BSPF

Also referred to as British Gas, British Pipe Parallel or Parallel Fastening thread.

 

[JimR]

Thanks for great explanation of BSP. I would never have guessed 1/4 x 1/4. Confirmed petcock size through burtonbikes. Will try the lacquer thinner on the coating.

 

[swooshdave]

Will try the lacquer thinner on the coating.

Be prepared to use something more potent.

 

[maylar]

I have used a 1/2-20 UNF tap on them in the past and it's close enough for me. I've plugged the threads with 1/2-20 bolts many times when painting or sealing a tank and they fit nicely.

 

[debby]

I have used a 1/2-20 UNF tap on them in the past and it's close enough for me. I've plugged the threads with 1/2-20 bolts many times when painting or sealing a tank and they fit nicely.

Me too. When I coated my 750's tank with Caswell I used a 1/2-20 tap to clean the epoxy out of the threads. It worked just fine.

Debby

 

[Cookie]

A third on that, I'm not sure it is perfect but close enough that the sealer I use on it makes a good seal and it has never leaked. currently I'm using a remote petcock and an American plumbing fitting in the tank.

 

[JimR]

Took a few hours but the paint stripper worked great. Used the old petcocks for a thread chaser which cleaned out the threads nicely. Little elbow grease along the way and the new petcocks are in real nice. Thanks alot for all the advice. Hope to have the bike fired up in a few weeks, or at least before winter here in New Hampshire.