Selecting Orings for the Hatsan Nova

[AmBraCol]

The Hatsan Nova saga continues.  I've got her apart now ( https://www.gatewaytoairguns.org/GTA/index.php?topic=143407.0 ) so the process of selecting the proper orings to reseal her is next.  This is a separate post as hopefully we'll find the info necessary and it'll come up on 'net searches for folks with the same issue. 

I'm operating on the assumption that the Nova uses metric orings.  So the question then arises - WHAT orings does she use?  The ones that came with the rifle, in a box labeled "4.5 mm" don't appear to be the correct ones.  At least, I've not found any place where they'd appear to work for sealing the rifle.  Using my trusty dusty micrometer (dusty meaning rarely used, not physical condition) the gland for the valve oring that seals the valve to the tank block is 20.60 mm (0.811") The cross section is (as near as I can measure it) 2 mm.  The bottle side of the valve that seals the transfer ports is 16.38 mm (0.645") and 2 mm cross section.  The other end of the same part of the valve is 9.5 mm 0.375") and 3 mm cross section. The oring that seals the bottle itself is also the same size as the first one mentioned above,  20.60 mm (0.811") The cross section is 2 mm.

Oh, the check valve o-ring, I wasn't able to get it off the check valve to measure it.  Not sure what to do there yet, don't want to damage something that would completely blow me out of the water.  It might be the source of my problems, at which point it will become necessary to get it off as well and measure it as I've not found any place on the 'net that gives the appropriate sizes.

So, given those measurements, which orings should I purchase?  I'm not sure how much they should stretch so don't know how much under the gland size the oring should be.  Also, Buna-N 70 duro, or something else would be an appropriate material?  Thanks in advance for helping to solve this situation. 

[nervoustrigger]

I haven’t worked on a Nova but other Hatsan PCPs I’ve worked on have indeed used metric O-rings.  You’re also correct that Buna-N in 70 durometer is a good material.

For A large-ish O-ring like the 20.6mm you measured, you’ll be fine to round down to 20mm.  I don’t know if you were giving the ID or the OD, but I wanted to mention they are spec’d
by the ID and cross section.  So for example, you would order an M2 x 16 if you wanted the OD to be 20mm.  16 + 2 + 2

[Privateer]

Paul,
Jason knows his stuff! I blame him for getting me started with the AT44 platform!!
 

[nervoustrigger]

Haha, I still don’t know how I got blamed for that.  The way I remember it, I told you and Jay it’s a persnickety PITA with a wandering POI.  But then you and he proceeded to vanquish house flies with it at 43 yards.  So that’s on you, my friend.

[Privateer]

Ok. You got me started down the path.
 
And you are correct! They can be a PITA at times!! But once you get one shooting good?
 
And what size were those breech seals you sent me? I know they were not stock as you pointed that weakness out to me!
(And sent me some that have vanished!) (Well, Jay and I used them for probe seals as I had a burr)

[nervoustrigger]

Hi Jeff, I remember there was some noticeable blow by, and the problem was the original breech O-ring was a standard metric having a 6.5mm ID.  However the bolt is sized precisely to the caliber which is 6.35mm so it didn’t seal properly.   I’ll look that up tonight and reply back.   

[AmBraCol]

I haven’t worked on a Nova but other Hatsan PCPs I’ve worked on have indeed used metric O-rings.  You’re also correct that Buna-N in 70 durometer is a good material.

For A large-ish O-ring like the 20.6mm you measured, you’ll be fine to round down to 20mm.  I don’t know if you were giving the ID or the OD, but I wanted to mention they are spec’d
by the ID and cross section.  So for example, you would order an M2 x 16 if you wanted the OD to be 20mm.  16 + 2 + 2

Thanks, Jason.  I measured the gland as best I could so it's an inside measurement for the oring.  So I need to order the following for the valve:

20 x 2 mm
16 X2 mm
9.5 mm X 3 mm
20 X 2 mm

Looking online I see that there's a 9.5 X 3 and then 9 X 3 mm.  .5 mm difference in diameter is substantial at that size. I'm guessing I'd go with the 9.5, but as I'll be ordering internationally I don't want to mess it up.

Do you have any idea what size the barrel orings are?  Both the outside ones and the one for the bolt probe?  My Nova is a .177 and I've no idea how to measure the inside gland for it. 

[nervoustrigger]

Looking online I see that there's a 9.5 X 3 and then 9 X 3 mm.  .5 mm difference in diameter is substantial at that size. I'm guessing I'd go with the 9.5, but as I'll be ordering internationally I don't want to mess it up.

Actually I would go with the 9mm.  Better that it stretch a tiny amount than be even the slightest bit loose, because if it is loose it is more likely to snag and tear when being assembled.  0.5mm is only about .020" which is about the thickness of 4-5 sheets of photocopy paper.  The O-ring's 3mm cross section will not thin out any appreciable amount when very lightly stretched over a 9.5mm groove.

Do you have any idea what size the barrel orings are?  Both the outside ones and the one for the bolt probe?  My Nova is a .177 and I've no idea how to measure the inside gland for it.

In my notes for the AT44, the O-rings on the outside of the barrel are M1.5 x 8.  Its breech and barrel assembly looks very much like the Nova so perhaps they use the same O-rings in that location.

For your breech O-ring, can you measure the bolt probe's OD?  That would represent the O-ring's ID since it needs to seal against the bolt.  I suspect it's 4.5mm to match the .177 caliber.  For the OD, measure the recess in the barrel.  If you haven't noticed, look closely and I think you'll find a small threaded collar that retains the O-ring.  It's easy to miss.  Once that's out of the way, you can take your calipers and measure the recess to get the OD of your O-ring.

[nervoustrigger]

Jeff, the breech/bolt O-rings I used to eliminate the blow by on the .25 cal was a #902 with a 0.239" ID and 0.367" OD (6.07mm ID, 9.32mm OD).  Fits and works great.