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PostPosted: Sun Jan 18, 2015 4:51 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8979
Location: Coalmont BC
A lot of my modified Discos use a larger transfer port, made from a piece of 5/16" OD Teflon rod.... To fit that, the valve and barrel have the flat increased to that diameter, and the breech and main tube are drilled out to 5/16" as well.... Drilling the holes in the tube and breech out are not that difficult to do, and can be done with a drill press, or even with a portable drill, if care is taken to make sure you keep the drill square when enlarging the hole.... Going up 1/64" at a time helps you to correct any mistake in the angle as you go, and keeps the hole centered as per the original....

I have a mill, so spot facing the valve and barrel isn't that much of a trick, but most people don't have access to a mill, so here is a way you can make a flat big enough for the 5/16" OD transfer port with just a file.... The barrel port on a stock Crosman barrel has a drilled recess in it around the actual barrel port.... It is 0.200" OD, and tapered on the bottom (drilled, not milled), and provides a location and seal (on the taper) for the steel Crosman transfer port.... It turns out that if you simply file a flat to remove the vertical part of that recess, but leave the taper at the bottom that leads to the actual barrel port, you end up with a flat that is 5/16" wide.... If you file it also 5/16" long, then the end of the new, larger transfer port has a place to seal.... In the photo below, the tube and breech have been drilled through to 5/16", the valve (with larger, angled ports and a large flat) is visible inside the tube, the 5/16" OD Teflon transfer port is shown sitting in the hole in the breech, and the proportions and location of the flat on the barrel are clearly shown.... The barrel port has NOT been drilled out yet....


In the photo above, you can see the flat I filed in the barrel, and that the taper at the bottom of the original recess for the stock transfer port is still intact, but the straight portion has been filed away.... Yes, the flat is square instead of round, but the hole in the breech will hold the transfer port lined up with the barrel port, so that doesn't matter.... If you have modified your own valve to have larger ports, you can do the same thing on the valve.... Just file off the top of the valve around the 1/4" recess until you end up with a flat that is 5/16" square (and deep enough to equal the original recess).... If you have an aftermarket valve which already has a 5/16" diameter flat milled in it, you don't have to do that, of course.... Then simply make a transfer port from a piece of 5/16" Teflon rod (Delrin is a bit hard, IMO) with the ends cut square and flat, and drilled to the diameter you need for the port, and drill the barrel port out as well.... I wouldn't drill the barrel port out until after you file the flat, as you can use the original hole to visually check on the location of the flat as you file it, for parallel and square in both directions.... Just work slowly and carefully, and you should have no problem getting it flat enough to seal.... I find having the transfer port about 0.005-0.010" longer than the space from the valve flat to the barrel flat provides the proper "crush" to get a good seal, but without it holding the breech up or kinking and collapsing the wall of the port....

The barrel port can be drilled out to 75% of the caliber, and with proper deburring shouldn't cause any loading problems, and will have enough flow for pretty much any pellet shooter.... The transfer port and valve port should be the same size, to provide unrestricted flow.... That works out to a #20 drill for a .22 cal (0.161" - you can use 5/32"), 3/16" (0.188") for a .25 cal, and 7/32" (0.219") for a .30 cal or larger, which is about the biggest hole you can use in a 5/16" OD transfer port.... A .177 cal should be left the stock size (0.134") or you may create loading problems.... I hope that makes you confident enough to try using a larger transfer port, which is pretty much necessary for .25 cal and larger builds, or if you have an aftermarket valve with an exhaust port larger than 3/16"....


Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

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