QB78 using Crosman TP seal

[cobalt327]

Is the highlighted part number correct for the Crosman TP seal I need for my QB78?

[Motorhead]

Many have used it ...
personally don't like it used in the QB being said seal is very soft and easily blown out between the gap of receiver & air tube on some QB guns.

But YES, same seal also used on M-rods X2 etc ....

[Nvreloader]

Scott

Question for you,
Which seal would you use for the QB series?

Tia,
Don

[Motorhead]

Scott

Question for you,
Which seal would you use for the QB series?

Tia,
Don

I make my own from Teflon rod or Acetal sizing them to be in slight crush as receiver is tightened down to main tube.
Also I spot face the bottom of receiver so BOTH seal surfaces are flat bottom and a crush transfer tube better bridges & seals the gap.

[WesBob]

I guess I got lucky, the crosman seal has worked great on mine and I've taken the breech off multiple times with no troubles yet.
From what Don has reported though, it sounds like a multi shot breech doesn't fit the same and won't seal properly..
For those wanting a cheap multi shot system I'd encourage you to email Lukáš at amerlock@volny.cz. He sells auto indexing magazines for the QB line for $59 shipped. Eight shot in 22, works great! I've had mine quite awhile now, still working perfect.

[Nvreloader]

Thanks Scott,

That is my next step, waiting for the materials etc. to get here. 

Tia,
Don

[rsterne]

I like the Crosman seal if you leave both the valve and receiver curved, it bends to conform, and is thick enough to have slight compression when installed.... If you machine flats on the valve and receiver, it is too thin, you will need a thicker seal, as Scott suggests Teflon is a good choice, with about 0.010" compression....

I understand some later QB valves are flat on top instead of curved, and if that is the case, the Crosman seal may not be thick enough in the middle to seal properly.... My QB valves are round, they have no flat at the exhaust port location....

Bob

[Ribbonstone]

Have read where this option has been used, and for many it seems to work. Not real sure why it would be better than other choices, but if it does have some advantage, I'd like to hear it.

[WesBob]

For me the crosman seal was handy since I had an extra and didn't have to cut something to try fit properly. Made a huge difference in power as well! Not that something else wouldn't do just as well

[Nvreloader]

Guys

For what it is worth, here is the design of TP seal I building, It will fit the repeater breech, and won't leak, I hope. 
http://www.ebay.com/itm/172908246099?ul_noapp=true

I am waiting for him to reply back, as to the dimensions spec's are a little lacking in info etc.
And I do not know what the price would be in US funds etc.

Tia,
Don

[rsterne]

The largest size is 3.5mm = 0.138".... perfect for a .177 cal.... but IMO small for good performance from a .22 cal.... I don't see why you need the smaller diameter section at all, just a short seal should do the job, and you won't be limited by the wall thickness of the small portion....

Bob

[Nvreloader]

Bob

I believe that my leaking problem is a compound of several small items etc.
The off set/angles of the 3 dimensions/holes between the valve, psi tube and bottom breech radius etc.

And the main problem is with the repeater breech flat bottom, which is cut with a ball nose end mill,
the outside radius of this cut edge of the trough cut does not closely match the psi tube diameter,
creating a arch/gap between the breech trough/psi tube, that the Crosman tube "O" ring cannot fit properly,
which keeps leaking at this point.

I have used this Crosman tube "O" ring on the OEM 79 breech and it seals perfectly, with Co2/Hpa.

Tia,
Don