Theoben Rapid Forum

My Rapid 12/250 had sprung a bad leak on refilling the buddy bottle , the steel valve which has worked ok for many years, the 4BA grub screw with worn thread also blew out.
I fitted a New Valve from T Robb which cured the leak out of the barrel , but on firing the grub screw blows out of the worn thread.
The 4BA thread regarding the air leak on firing sound like bottle pressure passing up the stem of the valve? or is this normal with a good thread holding the pressure off.
This is only on firing and no leak otherwise.

https://rapidforum.co.uk/forum/index.php?topic=4035.msg24364#msg24364

Thread goes over the same thing.

Advice from KevG was to tap with another BA thread. I could not find that and tapped M4 and that worked great.
Think 4AB is also not original.

There shouldn't be pressure behind that screw,
my guess would be that the hammer is knocking the whole valve body forward, so the ports don't align properly & the air can then get behind the valve O ring.
Take the valve out & re-tap the thread with the next size up, remember to chamfer the grubscrew to fit the valve recess