Crosman Transfer Ports - How Many Sizes? - Test Results Now Included

[rsterne]

HI Guys.... I just bought a couple more transfer ports from the Canadian 1377s, and received yet another size.... As most of you will know, the standard TP diameter on the 22XXs (CO2) and 13XXs (pumpers) is 0.140" ID (9/64"), and that is also shared by the Disco.... I got some a while back that were 0.109" (7/64") that were supposedly from an under 500 fps version of the 1377.... When I reordered them (I didn't have a part number) I received a couple that are only 0.070" ID (a #50 drill, in between 1/16" and 5/64").... These ports are actualy stepped, they are only the small size on the bottom of the port, next to the valve....

Now most of you power mongers won't be interested in restricting the flow in your guns, but bear with me for a minute.... There are a couple of uses for smaller ports.... One is on a PCP where you want to use a higher pressure fill and yet get lots of shots at relatively low power (compared to what you could get flat out).... Normally, if you just back down the hammer preload, you get less power, and the shot count goes up, but you can't fill to the same pressure.... If you do, the first few shots are dismally slow, or the valve may even lock up.... Reducing the fill pressure reduces the number of shots you can get.... If instead, you reduce the power with a smaller transfer port, you can go back to the hammer preload that gives you you full fill pressure, and benefit from a flatter shot string as well....

The other use is on CO2 if you are interested in lots of shots per cartridge at reduced power.... With careful balancing of the hammer and valve springs, you can actually make the gun self-regulating over a relatively wide range of temperature, something which is unusual for a CO2 gun.... Varying the port size allows you to further control the velocity without upsetting that balance.... I suppose it's even possible that on a pumper, if you were going for a retained air setup.... ie more than one shot before repumping.... a restricted transfer port might help there as well....

Anyway, if any of you have run across other sizes of transfer ports that fit the 13XX / 22XX / Disco guns.... please take the time to measure then and let us know the sizes you have.... If you have part numbers for them, so much the better....

Bob

[Muppit]

bob this is great info thank you........ dave

[rsterne]

In case you were wondering how to detune a Disco for more shots, here is some information you may find useful.... I have been able to get three sizes of ports, the stock 0.140" (9/64"), one that measures 0.109" (7/64"), and a stepped one from a Canadian 1377 that measures a tiny 0.070".... I made some intermediate sizes by drilling out some of these, ending up with additional ports measuring 0.166" (as big as you can go on a steel Crosman port, and the same size as a poly port), 0.156" (5/32"), 0.125" (1/8"), 0.094" (3/32") and also one measuring 0.081".... I used a tethered Disco running on a regulator, and tested all the ports at 1600 psi, which is the top of the sweet spot in a stock Disco.... I then reduced the regulator pressure to 1200 psi, and then to 850 psi, and tested the commercially available ports.... A stock Disco hammer spring and rear plug were used for all tests.... I used 14.3 gr. JSB Express pellets, and shot 10 shots with each combination, recording the average velocity with the following results....



You will note that increasing the transfer port size without also increasing the size of the barrel port and/or the valve porting pays no benefit.... This makes sense, as the barrel port measures only 0.134", slightly smaller than a stock transfer port, and is obviouly limiting the flow.... Reducing the diameter, however, does reduce the velocity, although the 1/8" port hardly makes any difference at all.... When you fit the 0.109" port, the gun is noticably quieter, losing about 6% in velocity and 12% in energy.... From that point on, losses are pretty linear, with the smallest port resulting in a very much quieter report, a loss of about 25% in velocity, and about 45% in energy.... Very similar losses are observed at the lower pressures, starting from a lower level with the stock (0.140") port.... It is interesting to note that the gun is louder at 1200 psi than it is at 1600 psi with a stock port, as is observed with an unregulated Disco....

I hope that the above information will be helpful to anyone contemplating detuning a Disco to increase the shot count.... I did not try weaker hammer springs, they would, of course, increase the shot count at pressures below 1600 psi.... A spring from a 22XX can be made to work at about 1200 psi, and one from a 13XX at about 1400 psi.... or an RVA which allows the preload to be reduced would be beneficial as well.... Too much hammer strike results in wasted air and excess noise, so it must be balanced against the pressure used....

Bob

[SAADE]

Nice Bob, great info