Idiots guide to classic airgun strips (pt 1)

[Phil Russell]

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

[robs5230]

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.

[The Squire]

Well done Rob, some great info on here. Earlier this evening I rebuilt a .177 HE Evo and now that is back together the next job is overhaul of a .177 Sirocco with the lower pressure ram.

[robs5230]

Just to update on this rebuild, Thanks to the cracking lads at Impact Airguns this fenman is now back up and running and smooth as Silk. Impact have really pulled all the stops out to find me the bits I needed and I can't thanks them enough.
Oh and this rifle with the 27mm piston is very smooth

WORD TO THE WISE ..... If you do need Theoben gas ram parts they will surely be able to provide what you need, however you need to be very sure of sizes etc as there were definitely different parts used through the earlier years of production. So strip and measure the parts carefully before you order.