Tuning the Crosman 140

[GBertolet]

I have rebuilt my 1968 140. New seals, and a Mac1 flat head piston. I adjusted the pump rod, so the piston just taps the valve at the end of the stroke. The gun holds air, with no leaks. Pump effort is considerable after several pumps, so I know I am building good pressure. I hung a piece of tissue over the breach when firing, so no air leakage there was detected. I made the transfer port seal from some 1/4" tubing. After chronographing, I am still disappointed with the velocity. 6 pumps deliver 500 fps with the Crosman 14.3 pellets.

8 pumps is around 535 fps, which I think is still low. Pump effort is extremely high at 8 pumps. I am hearing of others getting nearly 600 fps with 8 pumps. I cannot find anything wrong to look for.

Does anyone have any tuning tips for this airgun?

[Blowpipe Sam]

I rebuilt my 1968 manufactured 1st model 1400 a couple of years ago with the Mac1 piston.  I didn't save the chronograph numbers for CPHPs but this gun will hit 593 fps with 17.6gr Norma GTFT (the most accurate pellet in my gun) at 8 pumps and 622 fps at ten pumps.  That's 13.7 FPE and 15.1 FPE respectively.  Your gun should be able to hit those energy levels.    If you did the 1/4" tubing mod to the transfer port you might need to adjust the length of your seal.  Too long and it will compress when you torque the barrel down and actually impede air flow.  Too short and it doesn't seal adequately.  Getting the seal just the right length is the trick and it may take several tries to get it right.  I cut several pieces of slightly different length and experimented til I got the perfect fit.

[JayDee24ca]

Man, that is so obvious when I think about it, I can't believe it never dawned on me! I don't have a 140, but I have other guns that I have used fridge tubing for and not got the results I expected. I'll bet that compressed and distorted tubing could well be the issue. Thanks for that!

[Van]

  I do the 1/4" icemaker tube mod on all of my Crosman CO2, pump and PCP guns. As Blowpipe Sam said the tube should just take a little pressure to crush fit and seal between the valve pocket and barrel pocket. Too long and it can close up or kink when the breech is cranked down. Too short and it will leak when you do the tissue test.  It's trial and error to find the right length. For .22cal I drill the port opening on the barrel and valve to 11/64" diameter per Roy at Mountain Air gun. Any larger will cause loading problems with a .22 pellet. Then use a 1/4" end mill bit to get a flat bottom pocket drilled in the valve and the barrel.  It is easy to do if you have access to a drill press and vice to hold the parts rock steady. It's hard to control if you just try with a hand drill.  I have done some with a regular drill bit, but the end mill works best for me. Without the 1/4" pocket drilled in the barrel it is harder to seal to the tubing as the stock port opening is too small. This is a picture of the modded barrel with a stock barrel for comparison at the bottom. You can see that the end mill bit drills a flat bottom pocket to hold the tube between the barrel/valve. It works best on guns with high flow custom valves and performance mods.

[splitbeing]

My resealed 120 gets low numbers like the OP, too, with the same pellets.

[Mzq284]

There was an excellent thread by RSterne a few years ago on his 140/1400, there was some good info there.
On my own 140, I drilled all the exhaust passages from the valve to the barrel port at 5/32". There isn't any more room for bigger ports. I used a modern transfer port and a 362 barrel, cut to be a direct replacement for the original barrel. I don't remember the numbers, I would have to chrono it again, but they were decent and the gun is a shooter. I used a peep sight from a Daisy 99, Iirc.

[HPP-Mechanic]

Ok, this post got me curious. I just so happen to have a 140 & 1400. Both are freshly re-sealed & with less than 10 shots each. I didnt mess with pump length or anything else, except lighter trigger springs.

1965 Crosman 140    6pumps=400fps      8pumps=453fps

1971 Crosman 1400  6pumps=450fps      8pumps=503fp

These numbers were measured with a Chinese $25 clamp-on chrony.

Here are some ideas for my low #'s, (and maybe yours too). Junk chrony? mine is questionable.  December 18- its cold out. My guns were in an un-heated back porch, and its maybe 50 degrees in there. I was seeing 5fps increases with every shot. Maybe due to gun sealing better, or slightly getting warmer?  How cold is your gun, and how many shots do you have thru it?

Temperature is a big variable. When testing/tuning a Dragonfly mk2 last summer, (75-85degrees outside), the pump tube would get very warm after several hundred pumps.

[OTmachine]

There was an excellent thread by RSterne a few years ago on his 140/1400, there was some good info there.
On my own 140, I drilled all the exhaust passages from the valve to the barrel port at 5/32". There isn't any more room for bigger ports. I used a modern transfer port and a 362 barrel, cut to be a direct replacement for the original barrel. I don't remember the numbers, I would have to chrono it again, but they were decent and the gun is a shooter. I used a peep sight from a Daisy 99, Iirc.

Here is one link:
https://www.airgunforum.ca/forums/viewtopic.php?t=36633

[Van]

  A few years ago, I resealed a Sears 126 version of the 1400 with a Mac1 kit and at 8 pumps got 605fps with 14.3g pellets. It was for a friend and when we checked it later, we got 625fps, so it went up a bit after breaking in.  This was taken on an F1 Chrony.

[GBertolet]

I tore down my 140 tonight. The tubing used for the transfer port seal was mashed, restricted, and deformed. I think that could be a factor in the reduced performance. I am surprised it worked as well as it did. I can't find any replacement tubing in the shop of the right size, so a trip to the home center is warranted. I measured the orifice size for the barrel with a pin gauge. It is .130. I may up it to around .140 for starters, and see where it takes me. I like the idea of milling a pocket to keep the tubing aligned with the port.

[GBertolet]

This might seem like a dumb question, but I had been using rubber fuel line for my transfer port seal. Is the 1/4" ice maker line described, made of some other material?

[HPP-Mechanic]

This might seem like a dumb question, but I had been using rubber fuel line for my transfer port seal. Is the 1/4" ice maker line described, made of some other material?

Its more like a clear plastic/vinyl vacuum hose than the black rubber fuel line. Its softer.

[Van]

   Ice maker tube usually comes in a roll at Lowes or Home Depot.  To get precise cuts I like to insert a brass rod into the tubing to keep it sturdy and use a tubing cutter to get an even cut. This will make a good seal into the 1/4" pocket made by the end mill bit. I never have a leak if I get the length right. The brass rod does not dull the cutter like a steel rod.

[Blowpipe Sam]

   Ice maker tube usually comes in a roll at Lowes or Home Depot.  To get precise cuts I like to insert a brass rod into the tubing to keep it sturdy and use a tubing cutter to get an even cut. This will make a good seal into the 1/4" pocket made by the end mill bit. I never have a leak if I get the length right.

My method is a somewhat cruder.  I slip a piece of tubing over a drill bit.  I spin the bit with the drill and use an Exacto knife to cut an assortment of seals.  Does a fair job.

[GBertolet]

Well, I did the surgery on the 140 today. I bought some ice maker tubing, and went to work. I found that the transfer port was partially blocked. With what I don't know. A .130 pin gauge barely fit through, but a #28 drill bit .1405 blew right through without cutting any metal, cleaning out the hole. I milled the 1/4" pocket for the tubing, and cut some pieces of tubing, and found one that lightly compressed, upon reassembly, and used that one.

I tried a few shots outside my basement door, using Crosman 14.3 gr pellets. All I can say is WOW. 605 fps, 6 pumps, 3 shot average. A 100 fps increase. My Competition Electronics chrono, does not like cold weather. It doesn't last long once the cold temp gets to it. It was 20 degrees outside. Readings were still consistent, but I will retest when warmer. I got one shot off at 8 pumps, before the chrono died. That was at 645 fps. My Crosman now rivals my Sheridan. I don't know if it would be worthwhile to to do anymore enlargement of the transfer port. Maybe just leave well enough alone.

[Mzq284]

Good job, they're great rifles that deserve to be used and enjoyed.

Now you're making me wonder what mine is shooting at. Guess I'll have to chrono it.

[HPP-Mechanic]

Well, I let the 1400 warm up in the house, pumped slower, & no increase in fps. Maybe I should look at my transfer port too... The original transfer port seal had a metal sleeve that fit inside the seal/barrel recess.