Early Crosman 760 Pumpmaster Before and After Restoration

[GreenbirdII]

Well, I finally managed to get through the (rather arduous) restoration of my First Variant Crosman 760 Pumpmaster rifle…with perhaps a small remaining tweek that I’ll discuss later.

Photos of the rifle before and after restoration are attached.

OK, now it looks pretty, but does it actually work following reassembly you ask?   Answer: Yes! 

As suggested, following a complete disassembly and cleaning, I replaced all the seals using the Mac1 kit:
http://www.mac1airgunshop.com/product-p/cr760rk.htm

And cleaned the barrel using the Mac1 Crown Saver:
http://www.mac1airgunshop.com/jl-crown-saver-p/jlcs.htm

So far I’ve been able to test the gun with BB’s at a range of ~12-13 yards (safe range limit at home), 10 pumps, and it’s holding a nice consistent ~1.5” target circle.  It also holds air overnight.  More testing to do when I can increase the range, but a great start.

Only issue remaining is that the pump handle doesn’t stay locked in position unless it’s pumped and holding pressure.  I’m guessing an adjustment to the piston assy…I’ll address that separately.

I’ve posted a number of times on this forum for help getting this done, and received a ton of advice and comments that were incredibly helpful.  These were the main conversations:

https://www.gatewaytoairguns.org/GTA/index.php?topic=123518.msg1203577#msg1203577

https://www.gatewaytoairguns.org/GTA/index.php?topic=123744.msg1206461#msg1206461

https://www.gatewaytoairguns.org/GTA/index.php?topic=124939.msg1220575#msg1220575

In addition to the comments on GTA, among the more helpful other online links and videos I referenced were the following (some suggested by GTA members, others I found myself):

Assembly/disassembly: 

Metal cleanup and bluing: http://airgunner.org/?p=40 , also the official Birchwood Casey links

Stock and pump handle wood restoration: (all 3 vids)

Again HUGE thanks to David, Rick, Bart, and others for the ongoing sage advice.  Couldn’t have done it without your help!

[Matchstickshooter]

That turned out nice! I have the same wooden stock/handle on my Western Auto Revelation by Crosman. Has the newer type bb cover though.
Super job!

[avator]

Very nice.
I know the first var. was wood and the second was styrene. I wonder when they went from the tootsie roll forearm to the solid wood block? Mine is June of '66. That is the first year of them. ('66 - '70). Funny thing is... the 3rd var. had styrene stock and wood forearm.

[2K1TJ]

Nice work! I had a later variant 760 as a kid that I sent tons of bb's through! Mine had the metal receiver, but plastic stock and forearm.
Enjoy her!

[GreenbirdII]

Based on the "69" stamp on the front of the pump tube plug/barrel band, we were able to arrive at a date of June 1969 for this old rifle (thanks to Bart for the earlier help with that).

The solid wood forearm seems to be somewhat unique, not sure why...I know dad bought it new back in the day, and we've had it in the family unmodified (mostly in storage) ever since.  I was fortunate because it made restoring the wood less of a challenge.

Now if I can just get that fancy wooden pump handle to stay locked in place after the air is discharged, I'll be golden...

[Matchstickshooter]

As I said, I have the same stock/pump handle on a Revelation, made for W.A. by Crosman.  My stock has a missing emblem of some kind on the right side of the stock.  It is a phase one 'Hammer Drop" with what had been a rifled barrel. Riflings  about gone though. Still had bb's in it when I bought it.     

I don't have the rectangular bb door. 

Have to come up with some pics.

You did Great!

[bantam5s]

Based on the "69" stamp on the front of the pump tube plug/barrel band, we were able to arrive at a date of June 1969 for this old rifle (thanks to Bart for the earlier help with that).

The solid wood forearm seems to be somewhat unique, not sure why...I know dad bought it new back in the day, and we've had it in the family unmodified (mostly in storage) ever since.  I was fortunate because it made restoring the wood less of a challenge.

Now if I can just get that fancy wooden pump handle to stay locked in place after the air is discharged, I'll be golden...

Awesome job on the restoration there 👌
Now you just need to adjust the piston head to keep the forearm closed, to do this you just have to screw it out a little bit then use that nut to lock it down. You just have to adjust it a little bit each time until it takes a bit of force to make it snap shut, once you've done this it'll stay closed without air in it.

BTW where did you learn about the number on the barrel band / pomp tube plug to date your gun ?
Mine is a '71-'74 2nd variant and it's got 36 on it.
Maybe the that number only corresponds to the year on the 1st variant guns, but it doesn't appear to correspond with anything on my gun.

[GreenbirdII]

Thanks David...again lots of credit to elite coaching.   

I thought that the adjustment of the (new) piston head was the answer to keeping the (un-pumped) forearm closed.  I vaguely recall it being a growing problem long ago before the gun was retired, and since I simply matched the setting of the new Mac1 piston head with the old worn (rubber cup) piston length, it's understandable that the problem is still there.  Thanks for confirming that. 

The dating information came from Bart back on 3/18...

Look on the front of the pump tube plug/barrel band.   There should be a two digit stamping, first digit mo 1-9 O,N,D, second digit year 1-0.

This key (if universal) would imply that your gun's dated March 1976, but yeah if you know otherwise then hmm, defer to Bart as to whether or not it applies to later variants...

[bantam5s]

Thanks David...again lots of credit to elite coaching.   

I thought that the adjustment of the (new) piston head was the answer to keeping the (un-pumped) forearm closed.  I vaguely recall it being a growing problem long ago before the gun was retired, and since I simply matched the setting of the new Mac1 piston head with the old worn (rubber cup) piston length, it's understandable that the problem is still there.  Thanks for confirming that. 

The dating information came from Bart back on 3/18...

Look on the front of the pump tube plug/barrel band.   There should be a two digit stamping, first digit mo 1-9 O,N,D, second digit year 1-0.

This key (if universal) would imply that your gun's dated March 1976, but yeah if you know otherwise then hmm, defer to Bart as to whether or not it applies to later variants...

thanks !
I also wouldn't think they would still do this when they started to incorporate a date code into all serial numbers after '75 or '76.

[avator]

Yeah, same findings regarding date when I first got mine. 2 digit date stamped on the pump tube end cap. You may have to remove a bit of paint. Mine is 66 making it June of '66. I've often thought of restoring mine but I just don't think it looks bad enough to do it.
Again... nice find and restore.

[GreenbirdII]

Definitely appreciate the endorsements. Almost makes me want to take on another big restoration project...well, in due time. 

David - the adjustments to the piston head seem to have done the trick...forearm locks now without being pressurized, and it even seems to have a bit more juice when it's pumped, with only a relatively minor increase to the resistance at the end of the pump stroke.  The adjustment only cost me a couple noticeable marks in the new bluing on the pump tube...a fair trade-off I'd say.  Hopefully I won't be taking her apart again for a while. 

Hey BTW, related question, did I see on a thread somewhere a suggestion that it's best for the seals to leave 3 pumps of air in the gun when it's being stored/not in use?  Not sure I understand exactly why, but I'd like to confirm that to delay future servicing as much as possible. 

[bantam5s]

Definitely appreciate the endorsements. Almost makes me want to take on another big restoration project...well, in due time. 

David - the adjustments to the piston head seem to have done the trick...forearm locks now without being pressurized, and it even seems to have a bit more juice when it's pumped, with only a relatively minor increase to the resistance at the end of the pump stroke.  The adjustment only cost me a couple noticeable marks in the new bluing on the pump tube...a fair trade-off I'd say.  Hopefully I won't be taking her apart again for a while. 

Hey BTW, related question, did I see on a thread somewhere a suggestion that it's best for the seals to leave 3 pumps of air in the gun when it's being stored/not in use?  Not sure I understand exactly why, but I'd like to confirm that to delay future servicing as much as possible. 

In most multi stroke pneumatic guns it's best to store them with a pump or two of air in them to keep the seals in good shape, however you don't want to do this with a self cocking gun like a 760.
Most other guns require you to actually cock the bolt back before you can fire, but with these ( I've had mine open quite a few times and I'm still not sure how this exactly happens ) the valve cocks itself.
Now I don't know exactly why it's a no go with these self cocking guns, but I've been told this multiple times by people who low their stuff.

[GreenbirdII]

Hmm...well that is an interesting and unexpected twist.   

Regarding how things work, it's interesting how little there is out there that actually describes step-by-step how these self-cocking guns function. In fact I requested that info on a separate thread, and got no responses. So I actually did some reverse engineering when I had the gun apart, to try to understand the mechanics, and I think that I more-ore-less get it now.

In a nutshell as the gun is pumped, pressure increases within the pump valve, putting increased pressure on the exhaust valve (back towards the breech), in turn putting pressure on the adjacent "hammer", which is rather precariously held in place by the top of the (spring-loaded) trigger.  This pressure on the hammer against the top of the trigger is in effect "cocking" the gun as soon as it's pumped, since subsequent pulling of the trigger causes the top of it to slip off the lip of the hammer, releasing it (the hammer) to move back (against the anvil) just far enough to allow pressurized air from the pump valve to escape up into the barrel and out the muzzle with the payload (BB or pellet). Very clever design actually. 

Now, given this design, why keeping it pressurized is a "no-no" still isn't clear to me, aside from the obvious safety issue of having a cocked gun lying around...assuming there isn't a BB or pellet in the breech of course.   

[bantam5s]

Hmm...well that is an interesting and unexpected twist.   

Regarding how things work, it's interesting how little there is out there that actually describes step-by-step how these self-cocking guns function. In fact I requested that info on a separate thread, and got no responses. So I actually did some reverse engineering when I had the gun apart, to try to understand the mechanics, and I think that I more-ore-less get it now.

In a nutshell as the gun is pumped, pressure increases within the pump valve, putting increased pressure on the exhaust valve (back towards the breech), in turn putting pressure on the adjacent "hammer", which is rather precariously held in place by the top of the (spring-loaded) trigger.  This pressure on the hammer against the top of the trigger is in effect "cocking" the gun as soon as it's pumped, since subsequent pulling of the trigger causes the top of it to slip off the lip of the hammer, releasing it (the hammer) to move back (against the anvil) just far enough to allow pressurized air from the pump valve to escape up into the barrel and out the muzzle with the payload (BB or pellet). Very clever design actually. 

Now, given this design, why keeping it pressurized is a "no-no" still isn't clear to me, aside from the obvious safety issue of having a cocked gun lying around...assuming there isn't a BB or pellet in the breech of course.   

That's sort of how I thought the whole thing worked inside, and you're explanation is perfectly clear and is most likely a bull's-eye.

I doesn't appear to me that keeping 1 pump of air in a self cocking gun would be a problem, but I just listen to those who know more than I do.

[Big Bore Bart]

 Thanks for the mention.     The reason for not leaving a pump in the self-cockers is so the Q-ring is relaxed when stored.     If left with pressure, the Q-ring can adhere to the nipple, and get ripped when the next shot is fired.