Quote Originally Posted by Phil Russell View Post
Thanks for this very detailed explanation Rob. It goes further than I expected but is all the better for it. I would be very surprised if any problems were caused by publishing a charge pressure ... I am sure it is available elsewhere on BBS threads ..... but I thought it would complete the picture of an excellent thread. I always find it interesting that the HW90 uses a lower pressure but have not really investigated why ... are cylinder sizes different?
Overpressurising can be a real issue as I know from experience. Many years ago I bought a cosmetically perfect .177 HE at auction for a very good price. Full of anticipation when I got home, I chono'd it ... a measly 5ft lbs with a very hard cocking action. I had no experience of the Theoben gas ram so did not know the correct cocking effort but was surprised it was so hard. Similarly I had never serviced a ram so took it to Theoben. The diagnosis was over pressurising and a burnt out piston head. In this case I think it may have not been possible to pump it up over 12. My auction buy was still good value but it was a salutory experience.
So ... thank you,
Cheers, Phil
I can only comment for the size of my piston crown which is approx 27mm dia. Other theobens may have had a 31mm dia one. The 90 i understand has a 30mm one.
The 90 I have shot is a lot harsher than my fenman but as I stated above, a wide range of internal parts have been used over the years in theoben gas ram rifles which may have an effect on how they perform. Also drilled and undrilled inertia weights have been used. Mine is undrilled.