My petcock is leaking om my 340 and I can't seem to repair it. I see Barratt has them available but was needing to know the thread pitch. I thought maybe I could find a plug for now and not worry about the tap. Is it a common size?

 

It's British Standard Parallel Pipe Thread or BSPP for short.
I can't tell you what the threads per inch they are off hand.

They also have a taper _ BSPT (the "T" stands for taper).
There are adapters out there, but whether or not there's room ?
Others have also used Hylomar in the taper, and that seems to work, although I have tried it on a very small petcock to drain the heater core on my Bentley, and it still leaks.

BSP / BSPT Thread chart | British Metrics
bspp-to-npt pipe fitting adapters | McMaster-Carr

 

Use Yamabond 4 or 5. That's what your favourite F1 team would do.

 

I don't remember the part well and my old radiator is stored and not accessible. However, a tap (faucet?) with a parallel thread would usually have a fibre washer. You could certainly add your favourite sealant to the washer. The softness of the washer allows a good seal and correct orientation. Taps and even end caps are sometimes supplied with a suitable washer. Don't over tighten and destroy the washer - we've all done it and some professionals seem to do it a lot!

 

my radiator faucet was leaking too and I fitted a universal radiator drain tap that has a 1/4" thread like the below picture. Threaded fine and leak stopped. $3.00 at Harbor Freight Tools.

 

If the Yamabond fails which I doubt you can also try some thread tape and as Peter says there should be a fibre washer..

Yamabond available at your Yamaha dealer. Made for precision gasketless casing joints where some movement is possible. Like duct tape every racing team including F1 carries it. Just peels off afterwards. So no scraping required

 

Yes, that will work, but you're distorting the threads in the rad as they are 19 TPI, where the NPT is 18.
The taper in the NPT does enable it to seal with the wrong thread size.

 

Jeff, it threaded without any force and it never leaked again like the original faucet did, I would have preferred to keep the original remote controlled faucet but I could not get it to stop the drop by drop leak, the brass ball and cavity are worn.​​
Curiously, the engine block has an identical faucet in the exhaust side that does not leak. but made of a different silvery material, I am thinking of removing it and using it for the radiator if it threads, on the other hand I am also thinking of replacing the radiator so for now the whole thing is on standby.

 

The standard Jaguar rod operated radiator tap is easily refurbished as long as it has not been butchered by some Plonker.

 

It will start easily as it's only one thread out, and then as the pipe threads increase in size, you won't be able to tell as it's wrenched in.
It's brass on brass, you won't notice any resistance.

The original petcocks can be very frustrating.
I'm dealing with one now.