I finally received my MK2 piston and rings from overseas and was able to get going on short stroking my TX200HC MK3. The path I decided to take was to use a fairly low rate spring with around 1.75" of preload in order to maintain pressure on the piston at end of stroke. The theory is that this helps control piston bounce and the associated "surge". To accomplish this I replaced the 27 1/2 coil spring that came with the Vortek PG3 12 ft-lb kit with a 31 coil spring of the same configuration. The plan was that the additional preload would make up for the shorter power stroke and bring the rifle in just under 12 ft-lb. Here is a comparison of the old and new components, with the old components in the center, and the replacements top and bottom.
You can see that stroke is being reduced by lengthening the latch rod around 14 mm. Did you ever wonder why the rear beartrap notch on the MK3 cylinder is spaced back further than the other two? This is so the anti-beartrap can still engage that one notch when the MK2 piston is installed. The new seal is a Vortek 25 mm made from their moly impregnated urethane.
Because of the longer preload, which ended up being 1.820", I decided to go ahead and cobble up a spring compressor. Since I am a woodworker, I started with a 4 foot beam clamp and added some padded vee blocks to support the receiver. It made the job mush easier. In use, the receiver was held in place using another clamp (not shown).
After a short break in, I took some velocity measurements (20 shots each pellet type):
JSB 8.44/4.53
JSB 7.87
In this new configuration, the gun appears to prefer the 8.44 pellet weight. Energy is higher and standard deviation is lower. The gun also seems to shoot a little smoother with the heavier pellet. In terms of accuracy improvement, it is too early to tell how much the shorter stroke has helped. The firing cycle does feel quicker, and if I can manage a steady hold, which isn't as easy in shorts and a tee shirt, the pellets will go where the gun is pointed. Once cooler weather comes around I will be able to shoot it more and see if my scores have actually benefited from the change.
You can see that stroke is being reduced by lengthening the latch rod around 14 mm. Did you ever wonder why the rear beartrap notch on the MK3 cylinder is spaced back further than the other two? This is so the anti-beartrap can still engage that one notch when the MK2 piston is installed. The new seal is a Vortek 25 mm made from their moly impregnated urethane.
Because of the longer preload, which ended up being 1.820", I decided to go ahead and cobble up a spring compressor. Since I am a woodworker, I started with a 4 foot beam clamp and added some padded vee blocks to support the receiver. It made the job mush easier. In use, the receiver was held in place using another clamp (not shown).
After a short break in, I took some velocity measurements (20 shots each pellet type):
JSB 8.44/4.53
- Average velocity 796 fps
- ES 11.4
- SD 3.0
- Energy 11.9 ft-lb
JSB 7.87
- Average velocity 807 fps
- ES 15.5
- SD 4.1
- Energy 11.4 ft-lb
In this new configuration, the gun appears to prefer the 8.44 pellet weight. Energy is higher and standard deviation is lower. The gun also seems to shoot a little smoother with the heavier pellet. In terms of accuracy improvement, it is too early to tell how much the shorter stroke has helped. The firing cycle does feel quicker, and if I can manage a steady hold, which isn't as easy in shorts and a tee shirt, the pellets will go where the gun is pointed. Once cooler weather comes around I will be able to shoot it more and see if my scores have actually benefited from the change.
