68AUTOMAG Power Tube Spacer Kit from Pro-Team Products
© Ravi Chopra, 1997
I'd like to tell you about Pro-Team's Power Tube Spacer Kit for the '68 68AUTOMAG and related 'guns. But I can't until you understand the import of the power tube and the components that interact with it.
If you're not familiar with the 'Mag power tube and componenets, I'd advise you to retrieve your owner's manual and refer to the appropriate pages to help you understand what I'll be covering here.
The power tube is the forward extension of the air chamber behind the bolt. The end of the power tube has an o-ring, a spring, and a screw-in cap. The o-ring rests on a ledge inside the tube, the spring holds the o-ring back against the ledge, and the screw-in power tube end compresses the spring against the o-ring. The 'Mag's bolt has a stem down its middle which fits into the power tube. If you look at the end of the bolt stem, you will note that its tail-end is slightly narrower than the rest of the stem. When the bolt is placed over the power tube, the stem slides down into the power tube, the narrow end fits into the o-ring, and the ledge of the stem where it widens out seats against the o-ring. When the 'gun is gassed up, the gas pressure in the air chamber pushes the o-ring forward against the spring and seals around the bolt stem. The bolt is held back against the pressure in the air chamber by the sear.
If the bolt-stem does not seat properly against the power tube o-ring there are a couple of problems you can encounter. If the bolt comes to rest too far forward, the stem ledge will not seat firmly against the o-ring and gas will leak around it and down the barrel. If you look at an 68AUTOMAG sear, you'll notice that it is angled back such that when you pull the trigger, it actually forces the bolt further back against the o-ring before releasing the bolt. Thus, if the bolt rests too far back with respect to the o-ring, the trigger pull will feel overly stiff, and in severe cases can actually shove the wider part of the bolt stem through the o-ring. When this occurs, you get a condition called "bolt stick" wherein the bolt stem is stuck so tightly in the o-ring that even though the trigger is pulled and the sear no longer holds the bolt back, the gas pressure behind the bolt can not blow it forward.
Thus, you can see that the bolt stem-power tube o-ring relationship is critical to good, reliable 68AUTOMAG trigger function. There are several variables which can affect this relationship. Factors that can allow the bolt to seat too far forward with respect to the o-ring include a worn or shaved sear (as with many trigger jobs), a too-long power tube spring, or a loose regulator retention thumb-screw (which would allow the regulator to sit slightly further back). The biggest factor that causes the bolt to seat too close to the o-ring is a too-short power tube spring.
Until recently, if you had problems with your power tube, the standard remedy was to try first replacing the power tube o-ring. If that didn't work, you would have to hunt around for a new power tube spring that would remedy the situation. Those of you who have not yet had the opportunity to experience the transcendental joy of the great power tube spring hunt don't know what you're missing.
Enter the Pro-Team Power Tube Spacer Kit. This kit includes a new power tube o-ring and five aluminum rings (spacers) of the correct diameter to fit into your power tube to replace the power tube spring. These spacers are of five different lengths and are color coded to follow the same convention used in standard Nelson spring kits (from longest to shortest they are: purple, red, blue, green, yellow) so you can easily remember which is which. You may notice that all five spacers are much shorter than any relaxed power tube spring. The key is that they are of comparable length to a COMPRESSED power tube spring as it would be when installed in the power tube. Using different spacers, you set the distance back that the o-ring seats. The idea is to use the longest spacer that doesn't allow your 'gun to leak down the barrel. Simply install the new o-ring and the purple (longest) spacer. If it doesn't leak, you're good to go. If it does leak, put in the red spacer and check it again. Repeat as necessary until you don't get any more leakage down the barrel.
Typically you'll find that two or three of the spacers fit into the range where you do not suffer either barrel leaks or bolt stick. You may want to experiment with each of these spacers to find the one that gives the best trigger feel for you. In general, longer spacers will result in lighter trigger pull (you're not pushing the bolt stem as hard against the o-ring when you pull the trigger) while shorter spacers will result in a stiffer, longer pull.
There is one thing to be careful of when using this kit. While the 68AUTOMAGs are built to extremely exacting standards, there is some inevitable variability in their construction. It is this variability that makes this kit so useful. At the same time, it also means that a small percentage of the 'Mags out there have either an unusually shallow power tube shelf (on which the o-ring sits), an unusually long power tube screw-in end, or a combination of both. In either of these cases, the longest power tube spacer can actually be short enough to prevent barrel leaks, but be too long for the space between the power tube end and the o-ring. When this happens, the o-ring can be crushed against the ledge and flatten out, narrowing the power tube diameter to the point where even the narrow end of the bolt stem will stick. If you find that you are experiencing bolt stick after installing the longest spacer, try using a shorter spacer to relieve the o-ring compression. This should resolve the problem.
Is Pro-Team's spacer kit for everyone? Of course not. The best rule of thumb is this: if it works fine, don't screw around with it. People who want nothing more than a 'gun that works (and who already have that) probably won't find much use for the spacer kit. On the other hand, people who have barrel leaks or bolt stick will see this as a God-send. And players with trigger jobs who like to tinker with their triggers to get the best feel will absolutely love it.
All material at this site is © Ravi Chopra, 1999