Tin Bum Tuning - Short Stroke fitting.

[Jason Middleton]

Just bought a second hand TX Mk3 with a TBT kit fitted, however, putting it over the Combro and it's all over the place. Inside is the top hat and piston guide, a single nylon washer, 3 steel washers and Delrin guide that fits down the piston rod. I have had it all out and degreased/relubed everything but it still gives the odd low power shot. I also have the short stroke extension piece but fitting it drops the power right down and makes it way harder to cock, is this normal. With it already having 4 spacers I'm reluctant to add more spacers. I also noticed that the spring guide and top hat are really tight fitting, I was under the assumption that these should be able to rotate within the spring, is that a correct assumption? I have a suspicious feeling that the spring and guides are mismatched, what's the general consensus?

Thanks in advance,

Jay

[Blackbaronfish]

Have you tried removing some of the metal washers and putting it over the chrono

BBF

[Jason Middleton]

Yes, it dropped the power down to around 8 ft/lb.

[Timmytree]

Hi Jason,
Fitting a short stroke nose means the spring has to work harder to produce the power, probably why there are extra washers fitted. Some people like short stroke, personally I don't.
The spring guide should be a snug fit into the spring but not tight so judicious use of sandpaper will help. Try swapping the guide to the other end of the spring first, that often helps.
I would check the spring against a new one, number of coils, internal and external diameter and wire thickness, Julian (bgaltd) may be able to help with that. The original spring from AA is good, as good if not better than many after market replacements.
Power loss can have multiple causes but I would start with the cheapest options first, check the piston seal, if in any doubt replace it anyway, similar with breach seals, there are two seals stacked one on top of the other.
Check the lock up of the compression tube in the closed position, maybe wear in the cocking shoe?
Too much lube can also rob power.
Finally, pellets! Some pellets are abysmal! I don't know what you're using but try with a known good pellet, usually something mid to lightweight from AA or JSB

[Jason Middleton]

I have ordered some new breech seals. I unlatched the cocking arm from the shoe and there is a discernible movement of the shoe in the carrier, so, I ordered a new cocking shoe. I put in the original AA guide and piston weight and the three metal spacers and this takes the rifle up to 9 ft/lbs on average, though this is fairly consistent and adding delrin spacers on top seems to rob even more power. I have also ordered an apex seal from maccari so I can remove the short stroke extension. The one thing that does confuse me is, with the tissue over the breech there seems to be no escaping air as it doesn't move.
I haven't measured the spring, but even to get it to run at 9ft/lb I am having to really compress the spring to refit the trigger block.
I have ordered a new spring, this second-hand rifle is getting expensive

Have I missed anything?

Jay

[Timmytree]

That's the effect of the short stroke nose extension. More preload needed to try and make power.
I can almost guarantee that with a standard spring, maybe 2 washers and no short stroke you'll be back up to 11 ftlb+
Just out of interest why a maccari seal instead of the standard item?

[Jason Middleton]

I ordered Maccari because I always used to use them on my old Mk1 and they're nice colours.

[TrevorM]

Tin Bum supply the FAC rated spring with their short stroke kit to keep the power at 12 foot pounds, if the spring is the original it will not be stiff enough.

[Blackbaronfish]

Give Nick a ring and ask him whats the best thing to do

BBF

[FPoole]

That's the effect of the short stroke nose extension. More preload needed to try and make power.
I can almost guarantee that with a standard spring, maybe 2 washers and no short stroke you'll be back up to 11 ftlb+
Just out of interest why a maccari seal instead of the standard item?

Timmy, I feel that the Maccari seal is better than the factory. They last for years without getting hard and crumbling. I've left guns for several years, without shooting, and they just start right where I left off. I've had the original seal go hard and stop sealing or even break up after a couple of years non use. Also seen Maccari seals go past 50,000 pellets in about 3 years of constant FT use. The seal seemed fine even after all this.