The 2nd Model 760 might be more challenging than I thought...

[Methuselah]

It appears the exhaust valve stem seal is poured cured in place rubber or RTV compound and no longer available. What I've found online for replacement is one made for the "old" 1377's with a plastic (Derlin?) seal. Some people seemingly had success by "lapping" it to the seat or lightly polishing the brass seat, while others try as they might, couldn't get it to stop leaking and purchased a new valve assembly (which is probably no longer available).

My options are clean it up and see if it seals, find a way to dissolve the rubber and either pour new or try a gasket or o-ring in it, or roll the dice with the 1377 type seal and possibly lap or abrade a flat in the valve body if necessary?

Pics follow. This is going to be interesting. Rifle serial# suggests 1979 and cup looks good.

[Back_Roads]

 I like these rebuild kits. https://www.ebay.com/sch/i.html?item=175668451163&rt=nc&_trksid=p4429486.m3561.l170197&_ssn=crosman600repair

[Methuselah]

I like these rebuild kits. https://www.ebay.com/sch/i.html?item=175668451163&rt=nc&_trksid=p4429486.m3561.l170197&_ssn=crosman600repair

There was no description ebay or crosman600repair.com, but I'm going to use the website to ask if the aluminum valve for the "hd" 760 is compatible with the brass ones for 1977-1983. Their services page seem to indicate they rework valve seating for the plastic exhaust valve stems.

I don't have a lathe but I do have a micro-mill that I might be able to fixture to "carve out" the recess for new RTV/rubber "compound" on the old part. It's not my first preference, but it's one option...

Thanks for the link James 

[eeler1]

Use a torch and burn it out, do it outside.

[Methuselah]

Jon's suggestion is still an option I consider "on the table". My reading suggests valves for the 1377 have an extra hole that can be disregarded and are otherwise compatible - but I'm not sure if that is aftermarket parts for earlier model 1377's than current production.

Vendors are telling me the plastic substitute 760-145 is a direct replacement for my 1322A011 callout, however, I've read some things that give me pause. One wrote asking for help after and could not get it to seat - it kept leaking down. Others say polish the seat as force of habit. I'll attach some pics, but I can tell there is minor damage to the seat so drop-in is unlikely to work. Putting the original back with some silicone grease might work for a little while but even if it did I would expect it to start leaking again - probably soon.

My son (prior chem major) thinks acetone will dissolve the rubber and leave brass and the steel shaft unharmed or possibly with minor etching.

Although I'd desire to keep it as original as possible, I don't know if that is a lost cause. Even though I mentioned a micro-mill to fixture the exhaust valve, it's too heavy and awkward to move right now, so I'm thinking just a drill or HFS mini drill press. The spring is also quite corroded and I tore the tp seal (and might not want to "make" one after the last one I worked on that had low fps which could have been caused by my poorly fitted ice maker line "patch").

Pics...

[Methuselah]

Looks like my better half is putting the kabash on options involving spending more than a few bucks on this 2nd rile. Originally, it was $50 for a forestock, now it just that I hate to see a vintage 760 not go where it belongs, that being in the hands of a young child.

So, tomorrow I call Crosman and see what it costs for a new Derlin exhaust valve assembly, and maybe, maybe, I'll try plumbers grease on the old seal face first to see what happens.

I've heard suggestions that ATF can "rejuvenate" rubber - think it's worth a shot soaking the exhaust valve in it? You can see there's a rough spot on the seat, and that soaking in mineral spirits wasn't quite enough to clean it up (I've googled brass cleaning and lemon juice was suggested).

Ideas and suggestions welcome, I'm really hoping to get it shooting and donate it to a deserving kid, especially since i didn't have one when I was a kid!

[eeler1]

You can cut some of the 3M or scotchbrite pads down, fit them to a dowel or even just by hand, and spin that, or spin the valve parts, to clean up the junk in there.  Doesn't make things perpendicular or flat, but can help clean and polish things up.  Can't help much on the exhaust valve stem, maybe grind the back of the brass off and drive the stem out the face side, then peen or superglue it back in with a new seal. I see a lot of Benji and Sheridan exhaust valve stems coming that way now, and they seem to work ok.

[Methuselah]

Jon, you've given me more to think about. For me it's also tooling and the ever present keeping things as original as practical. I'm just hoping in the end I don't butcher it.

I've been looking up parts for a 1008 also before I call Crosman. Parts list "suggests" I missed an o-ring or one turned up missing somehow. Strange.

Thanks for the reply!

[longhunter]

Clean out the old seal and replace it with a 90 duro o-ring.
I've repaired a lot of them, that way.

[Methuselah]

Clean out the old seal and replace it with a 90 duro o-ring.
I've repaired a lot of them, that way.

Dental pick cleaned it out pretty good, put a coat of 30 weight ND and o-ring from an old HFS kit in it, held 2 pumps overnight.

I haven't cleaned the barrel on this one but it hits higher fps than the 1976 one I did first.

I'll need to post pics. With BB's at 10 pumps, 575.8, 571.1, 578.0 fps, and after 5 minutes for the air to cool in the valve 563.8 fps. Crosman pointed 530.4 fps, Meisterkugeln not chrono'd.

25' groups, bb's 6" with flyers, just under 3" without. The pointed a hair over 2", Meisterkugeln .85" ten shot group, first five pictured.

The 1976 did better on the BB's, but I had scrubbed the barrel and did a 25 shot group instead of 9 or 10. It was getting dark outside.

Other thread (I ended up with three different postings) had a suggestion for trying different brands of BB's but I probably should clean the barrel first (?) --- I did look at Pyramyd Air and they have some of the suggested, but I'd imagine a small order will get swamped by shipping charges. Anybody know if there are chain stores that carry a selection of BB's? I think this only stays a kids gun if it can shoot BB's half way decent - if not maybe a re-barrel is in order.

[Methuselah]

Pics

[splitbeing]

The meisters look pretty good!

[Methuselah]

The meisters look pretty good!

Yup, I wish that was good enough - but I was hoping to gift this for a 5 to 7 year old as a first gun and I'm not liking the idea of the cost of pellets or the lead exposure.

It might do better after a barrel cleaning, or even getting some lead down the barrel with the pellets before switching back to bbs but I was really wanting to put hands on a sample quantity of other bbs like the ones suggested in the thread on the other 760. They seem to only be sold in quantity though...

Pic of the "hogged out" exhaust valve seal to follow (for other's future reference). O-ring choice was 1/8" ID and OD probably didn't matter much, was the only one in the kit seemingly a decent fit.

[eeler1]

That's the upside of bb's.  They aren't very accurate (relatively), but they are pretty cheap and you can take a bunch of shots pretty quickly.  So you are likely to hit your target at some point.  Punching holes in cans and exploding plastic soda bottles is great fun, accurate enough for a kid's 1st air gun.  Nice solution to the seal issue, glad it worked out.

[splitbeing]

See sci guy regarding lead exposure from pellets on this thread.  I can't find thr other thread I was thinking of where he talks about being a toxicologist by trade. Just mentioning it in case it opens the possibility of the kids shooting pellets.

https://www.gatewaytoairguns.org/GTA/index.php?topic=24147.20

[Methuselah]

Nice solution to the seal issue, glad it worked out.

It was Scott's suggestion (@Longhunter)...

[Methuselah]

Dang

I really needed a win. Just one.

It quit holding air. I've added some oil and stood it up in the corner. Doubtful.

Maybe some washers under the next o-ring...

[PBguns]

Here is a great video (I have used Mac1s method many many times over the years).

[Matchstickshooter]

Dang

I really needed a win. Just one.

It quit holding air. I've added some oil and stood it up in the corner. Doubtful.

Maybe some washers under the next o-ring...

Steve,
 Try Rick Willnecker at Precision Pellet (airgunshop@aol.com)
as he rebuilds the Crosman valve stems.
 He rebuilt the stem for me on my 400 Repeater.
  I had even ordered a new Delrin  on Ebay  that did not work.
 

[threesuns]

Steve,
here’s a link to my experience with the 760. https://www.gatewaytoairguns.org/GTA/index.php?topic=204133.20

I rebuilt a ‘78 and ‘80 both using the current 1377 valve stem and had no issues.