Velocity vs barrel length in QB

[nervoustrigger]

William, that's right.  I cut a shallow groove to accept the E-clip.  Can do it by chucking it up in a drill press and cut it with a thin emery wheel in a Dremel.  The valve backer block does not have to be drilled out.  Both the E-clip and spring clear easily.  A dry fit will allow you to confirm for your particular parts.

Troy, the backer block does not provide any support to the valve stem.  Not radially in the context of your wear question, anyway.  I would say its roles are for safety by ensuring the valve can't eject out the back of the tube and it limits the max valve lift.  Secondarily, it provides attachment points for the receiver and stock.

You also asked if others are using this type of valve mod.  I've seen a few examples and I think you also said you've seen it before as well.  I would not suggest it for an unregulated rifle because a weak spring tends to produce a narrower bell curve, but it's great for a regulated rifle.  Also, if anything I would expect a 79 to benefit more from it than a 78 but not dramatically so.  Or if you can get your hands on a suitable conical spring and butt it against the tank block as shown on my DIY, that will produce nearly as much energy.  So that may be the route to take if it is a more comfortable approach for you.   I just think the parts for the reverse spring will be easier to find for most folks.

[wll2506]

William, that's right.  I cut a shallow groove to accept the E-clip.  Can do it by chucking it up in a drill press and cut it with a thin emery wheel in a Dremel.  The valve backer block does not have to be drilled out.  Both the E-clip and spring clear easily.  A dry fit will allow you to confirm for your particular parts.

Troy, the backer block does not provide any support to the valve stem.  Not radially in the context of your wear question, anyway.  I would say its roles are for safety by ensuring the valve can't eject out the back of the tube and it limits the max valve lift.  Secondarily, it provides attachment points for the receiver and stock.

You also asked if others are using this type of valve mod.  I've seen a few examples and I think you also said you've seen it before as well.  I would not suggest it for an unregulated rifle because a weak spring tends to produce a narrower bell curve, but it's great for a regulated rifle.  Also, if anything I would expect a 79 to benefit more from it than a 78 but not dramatically so.  Or if you can get your hands on a suitable conical spring and butt it against the tank block as shown on my DIY, that will produce nearly as much energy.  So that may be the route to take if it is a more comfortable approach for you.   I just think the parts for the reverse spring will be easier to find for most folks.

From your pics, it looks as if you are .175- .200 back from the shoulder of the nose ... is that about right ?

Also do you remember the dimensions of the conical spring you guys used, I may just do that if McMaster-carr has them ?

wll

[mackeral5]

McMaster carr's conical spring selection is very limited.  The one they offer that is close enough to the right size is a very lightweight/weak spring.  I ordered one and IMHO it was too weak for my liking so I didn't use it..

[nervoustrigger]

William, yes your estimation looks correct.  The stem is 3mm in diameter so looking at the photo, that's about right. 

To be clear, it does not have to be precise.  You just don't want the spring to get to coil bind if the hammer drives all the way flush to the backer block, else it could cause the E-clip to dislodge as previously mentioned.  And all you need is a small amount of preload to help hold the poppet against its seat so you can fill from zero PSI.  So long as you don't have an old gnarly poppet or damaged seat, the pressure takes care of the rest.

By the same token, a conical spring does not have to be strong either.

Some rifles have no valve spring at all.  You pretty much have to use a tank to fill from empty though, basically slamming the poppet against its seat.

[wll2506]

William, yes your estimation looks correct.  The stem is 3mm in diameter so looking at the photo, that's about right. 

To be clear, it does not have to be precise.  You just don't want the spring to get to coil bind if the hammer drives all the way flush to the backer block, else it could cause the E-clip to dislodge as previously mentioned.  And all you need is a small amount of preload to help hold the poppet against its seat so you can fill from zero PSI.  So long as you don't have an old gnarly poppet or damaged seat, the pressure takes care of the rest.

By the same token, a conical spring does not have to be strong either.

Some rifles have no valve spring at all.  You pretty much have to use a tank to fill from empty though, basically slamming the poppet against its seat.

McMaster car has an Inch conical that I think will work, I may try that, It is going to be stronger than a pen spring, but no where near as heavy as the stock spring.

wll

[Austringer]

Jason
In the name of gaining plenum volume, I think I will try your rear sprung valve mod, however I thought for sure the valve block on my 78 had a hole which fit the valve stem snugly. Guess I can drill it out if need be. Another question, in your QB79 build post, you hogged out the tank block by drilling a 7/16 hole a little ways into the block to gain volume. Do you think there may be more you can do to the block to gain another cc or so?

Like William, I found a Big 5 which reportedly has some  QB78D’s in stock. Against my common sense, I may have to pick one up just to have spare parts on hand and a longer tube in the case the 79 tube simply will not cut the mustard. Still waiting for my 79 to arrive from AGD.

Built me a shorter and lighter LDC which sounds only slightly louder than my longer one. I’m thinking since I have a significant aversion to my bent barrel (bore misalignment), I might experiment with cutting that barrel down in the case I get another 78 in 22 cal.

Can anyone tell me the actual total weight difference between a 78D and a 79? All these questions on airguns, but nothing but lamb butchering going on in my shop. Unfortunately, the wife has other plans for me once the meat is in the freezer. I’ll be lucky to have my next gun together by fall.

Thanks Guys
Troy

Troy

[rsterne]

You have to be careful drilling out the tank block for two reasons.... You are creating a thin wall under the O-ring grooves, which may cause them to fail.... and eventually, you will hit the screws that mount the tank block to the tube (or weaken where they anchor into the block).... Know what you are doing before you modify any parts dealing with HPA....

Bob

[Buldawg76]

Just adding a comment to follow this post.

[Gertrude]

I spoke with "me" today,... and "me" said that Ron is looking very forward to using this during Whack-A-Squirrel season !     

[nervoustrigger]

Me too, brother.  Me too.

I'll be there with you in spirit.

Although I am still jealous of that spread of cotton tails you bagged a while back, but it gets easier to accept with each passing day.

[southern gun]

I don't have an addictive personality BUT ya"ll are kiiiiiiillllling me, I have had enough cant pass up the fun, two weeks ago I thought a QB 78-79 was in the NFL with a screwed up number! I just ordered one of each. THANKS A LOT.

[Austringer]

Bob

I understand the importance of retaining proper wall thickness in the tank block when boring it out. However I will admit I do not know what that thickness should be. Having never seen a QB tank block, and knowing I’ll be looking for all the volume I can find, I naturally am thinking of ways I might be able to improve the situation. I’m guessing if there was volume to be found, Jason would have already explored that route. Thank you for reminding me and all the other folks out there the importance of retaining sufficient safety factors when dealing with high pressure air. I’m guessing most of us, even if we think we know, really have no clue what can happen in a a catastrophic part failure at 3000 psi air. We also probably think it will never happen to us. The reality is most of us (especially us followers) have no clue about considerations and calculations given to safety in commercially manufactured modern HPA airguns.

A very good friend of mine is a hydraulics and air technical advisor for a major airplane manufacture. (he’s chomping at the bit to dive into this) He sent me a safety advisory primarily on the dangers of hydraulic injection but also the power of HPA. The paper gives examples of industrial accidents resulting in both debilitating injury and even death. It’s an eye opening paper. Unfortunately, (in my opinion) with all the interest in HPA guns these days, and the people like me willing to tinker and modify them, a lethal accident is inevitable.

I appreciate your your monitoring of this forum and your attention to safety. I see it in most every thread. Your comments are always backed up by common sense and often mathematic calculations which make your input carry that much more weight.  We are all very lucky to have such a knowledgeable, informative and respected member of this group to keep us on track. You, above all others, are keeping ME in line and reminding me safety is paramount.  Thank you.

Troy