Piston Sleeve Issues?

[whipsaw]

I've fitted a TBT kit in my new HW77 and I also made a 0.25mm PTFE sleeve. The problem is, after extended use, the power drops significantly. For example, before I went to the range this morning it was doing 11.4 FPE with JSB express, when I got home it was doing 10.8 FPE. I should add that the shot to shot consistency of the strings is very good at under 10fps. This has happen several times now and if I check the power later, I'm sure it will be higher again. My theory is that the spring is running a bit tight in the sleeve and that heat generated during long shooting sessions is causing expansion and binding. I should also add that the spring is un-lubed. What do you lot reckon?

[imorik]

Try without the sleeve

If still dropping I'd say it's most likely the piston seal getting hot and expanding = drop in power.

[TonyL]

Try without the sleeve

If still dropping I'd say it's most likely the piston seal getting hot and expanding = drop in power.

Agreed.

[Roundshot]

Why fit a piston sleeve in the first place, particularly with a dry spring? Well fitted delrin guides alone should be sufficient.

[whipsaw]

Why fit a piston sleeve in the first place, particularly with a dry spring? Well fitted delrin guides alone should be sufficient.

I fitted a sleeve because there was still a 'buzz' in the firing cycle despite the TBT kit. I only ran a dry spring because the sleeve was fitted, my reasoning being that PTFE was low friction and eliminated metal to metal contact.

[Roundshot]

I fitted a sleeve because there was still a 'buzz' in the firing cycle despite the TBT kit. I only ran a dry spring because the sleeve was fitted, my reasoning being that PTFE was low friction and eliminated metal to metal contact.

I see, and yes I've found a buzz can still persist even with well fitted delrin guides in some rifles.

Some say the purpose of a sleeve is to stop spring grease migrating but I agree with you it can help quieten residual metallic buzz.

Why not experiment with your spring lightly moly'd whilst retaining the sleeve? I never use dry springs with or without a sleeve, although some swear by them.

I hope you find the answer.

[whipsaw]

I stripped down the gun this afternoon to investigate.The sleeve was showing signs of wear even though less than 500 rounds had been fired - I think it was running a bit tight. I polished the full length of the spring,very lightly lubed it with moly and reassemble the gun without the sleeve. I'll take it to the club tomorrow, put some lead down the barrel and take it from there. Initial chono'ing shows a very slight increase in power. Thanks for the replies.

[Shed tuner]

I've never known a sleeve to cause tempeature related power variation... piston seals sure, but not sleeves. Then again, if it's too tight, I guess it's possible.
BTW Is your piston seal fairly tight ?
Also i'd agree with not running 100% dry - even just a wipe of grease will make a difference.

[whipsaw]

The piston seal is a the standard Weihrauch design, I think (in my limited experience) it's ok - it moves freely and smoothly anyway.

[Artfull-Bodger]

freely and smoothly when cold maybe, if you chrono the rifle and start with a cold bore, then see a consistent but slow power drop over 50 shots then it's the seal expanding and dragging.

Bin the ptfe sleeve and make one up out of a beer can or a WD40 can and make sure you don't over lube the spring.

[Shed tuner]

Bin the ptfe sleeve and make one up out of a beer can or a WD40 can

yeah, I keep all my old WD40 cans, good source of steel for sleeves, rather than ally beer cans...

[whipsaw]

Thanks for the tips, I never thought of WD40 cans. Out of interest; how thick is the steel from these?

[whipsaw]

Power is still dropping - it must be the piston seal! What's the best course of action?

[Rickenbacker]

Just a thought - depending on your chrono's setup / lighting solution - are you sure it's the gun and not the chrono results?

[whipsaw]

Just a thought - depending on your chrono's setup / lighting solution - are you sure it's the gun and not the chrono results?

I'm almost certain it's the gun - I chrono in my cellar in the same place, under the same light