by Tom Gaylord
Writing as B.B. Pelletier

Sheridan Supergrade right
Like all Supergrades, my new rifle is graceful and attractive.

Part 1
Part 2

A history of airguns

This report covers:

  • Wise council
  • A special technique for old multi-pumps
  • Is it holding?
  • Test one
  • How is the pump lever?
  • Test 2 — stability
  • Conclusion

Today I’m recovering from the cataract surgery, but I wrote this on Wednesday, so I was still functional. What I thought I would do is try a little experiment that could work. If it does, I will have found a new technique for restoring an old Sheridan Supergrade. Read Part 2 to learn why this multi-pump is so different from all the others.

Wise council

Before I begin, following Part 2 of this report I heard from airgunsmith Tony McDaniel of TMac’s Airgun Service in North Carolina. Tony is the guy who hosts the North Carolina Airgun Show each year (it’s on Oct. 20 & 21, 2017), and the registration form plus show info is on his website.

Tony told me he has worked on a couple Supergrades in the past and one thing he has noticed is the pump arm sometimes comes up after pumping if the inlet valve has a small leak. That would lett compressed air flow back to in front of the pump head, no matter what the head clearance is.

In Part 2 I said it was the pump piston head clearance was causing the pump handle to rise after the first few pumps, and it can be that, but if the inlet valve leaks you will get the same result. One way to test it is to fire a pellet immediately after 5 pumps and then pump the gun 5 more times and wait 10 minutes to fire again. Some velocity is lost through the dissipation of heat from compression, but my testing has shown that that velocity loss is small — perhaps less than 20 f.p.s. If you have a larger loss the inlet valve may be leaking.

One additional thing. Sometimes the inlet valve may leak down to a certain pressure and then seal completely again. The loss of velocity will then be consistent from test to test. Figuring out which it is — the pump head clearance or a leaky inlet valve — can be tricky, but if what I’m showing you today works you may not need to worry about it.

A special technique for old multi-pumps

A reader suggested I try this. He said since I was having such success with Automatic Transmission Sealant repairing leaky CO2 guns, I should try it on the Supergrade. I have tested this stuff over many years and it does not turn o-rings to mush like some people fear, so I believe the risk to the rifle is very small. Therefore, what is there to loose? So, at the beginning of this week (three days ago) I put about 20 drops of ATF sealant into the Supergrade pump tube, ahead of the pump head, and worked it through the valve by pumping and shooting the rifle several times. Then I let the rifle sit with 2 pumps of air in it until today. This will either work or it won’t, but if it does, there are a lot of Supergrades in the same shape as mine that can use it!

Is it holding?

The first test is to find out if the rifle still holds those two pumps I stored it with three days ago. And the answer is, yes, it was still holding. There’s no way to determine if it held all the air that was pumped into it, so this is not a conclusive test. The rifle also held air before I added the ATF sealant, so this also isn’t confirmation of anything. It just makes me feel better. The next test will be far more telling.

Test one

I will run the same first velocity test that I did in Part 2 and compare the numbers. That is testing the velocity with differing numbers of pump strokes, up to the maximum of 8 pumps. This test is performed with 14.3-grain .20-caliber Crosman Premier pellets that are no longer available.

Pump…….Vel. before……Vel. today
3……………..327……………355
4……………..416……………445
5……………..383/353……   .475
6……………..473……………577
7……………..513……………577
8……………..522……………606

I would call this test a positive result! The velocity increased with almost every shot and with each shot I could see excess ATF sealant blowing out the muzzle. So I know all the seals are coated with it.

I tested the rifle by cocking and firing a second time after 6 through 8 pumps and there was no air remaining in the reservoir. So the exhaust valve is performing as it should.

Shots 6 and 7 are a small puzzle. Considering the velocity of shot number 5, shot 6 shouldn’t be as fast as it is and shots 6 and 7 should not be the same velocity, but I’ll take it.

How is the pump lever?

Now I wondered how the pump lever was doing after each pump. Was it still climbing up when I opened it, or was it sitting relatively low and stable? If it sits low and stable, the former problem of it climbing was more than likely a leak at the inlet valve, as Tony described. If it still climbs as it did before, the pump head probably needs some adjustment.

It still climbs. Starting after the third pump stroke, the handle climbs just a little more each time it is opened. I therefore think the problem is with the adjustment of the pump head and not with a leaky inlet valve. At least now I know.

Test 2 — stability

This is where I learn how stable the rifle is. I’ll shoot 5 shots on 4 pumps each with the same Premier pellets.

Four pumps

Shot………Vel.
1………….396
2………….397
3………….409
4………….423
5………….432

The velocity dropped in this test with 4 pumps from what was seen in the first test, but I’ll tell you why I think that is. Notice how the velocity climbs with every shot in this test? I think the pump head is warming up as I pump. The rifle had sat dormant for 15 minutes since the previous test. In other words, it does best with use.

I also continued to see ATF stabilizer being blown out with each shot. I think the seals inside both valves are well saturated with it and that they will continue to be conditioned for some time to come.

Conclusion

I had planned to have this old girl resealed, but after this test I don’t believe that is necessary. However, this is just two tests on one day. My plan now is to proceed on to the accuracy test, once my eye has been fixed.

Do you see why owning a chronograph is so important? Today’s report backs up yesterday’s report.

I think I will retest the velocity again, just before I shoot the rifle for accuracy. That will determine whether the “fix” is holding. If it is I think Sheridan Supergrade owners everywhere have a new trick to put in in their toolkits!