HFT 500 strip down and setting the firing pot

[Archer50]

Even though I have now solved the air blast problem http://www.airarmsownersclub.com/viewtopic.php?t=3257 my HFT 500 has not been running as consistently as I think it should with as much as 20 fps variation between shots! So I decided to give it a thorough service even though the gun is virtually new (probably no more than 2000 shots). I wasn't pleased with what I found:

- The hammer rail had an uneven coating of thick, black grease and even when that was cleaned off, it was rough as a badger's arse!

- The ends of the hammer spring were even rougher and the spring itself had also been smeared all over with grease.

- The firing valve stem was nearly as rough as the hammer rail and didn't run freely in the valve body, although I was surprised to find that the ends of the valve spring were nice and smooth, almost polished.

I got to work with iso-propyl alcohol and Autosol and I'm hoping that will have improved things a lot - I'll find out later this week, when I next get a chance to play with the gun.

Apart from a general rant, the main purpose of this post is to ask for advice about setting the firing pot. I've gone through all the posts I can find on this and most of the other forums but I'm having difficulty understanding how to tune the firing pot or even to understand exactly what it does. I've seen references to changing the power by setting the pot and also to its changing the length of the sweet spot and the shot count. Ideally I would like to reduce full power, as the transfer port screw has to be screwed nearly as far as it will go to give 11.2 fpe with a JSB Exact, but I would also (surprise, surprise) like to get the best sweet spot and shot count I can. I know the normal recommendation is to set it at 57.5 mm, but what happens if it is set longer or shorter?

Alan

[Roger5]

As you say, normal setting is 57 mm, if you screw it in the hammer will have a harder job opening firing valve, and if you screw it out it will do the opposite ie it will be easier for hammer to open the firing valve, I would clean all the grease and crud out of it and polish the striker rod S340 put it back together and set to 57 mm. I should have said screwing it in will reduce the power and unscrewing will up the power, try 56mm and see what happens, back the air screw out when you try it. Rog

[Mark112]

Hi,

You are doing the right thing with the Autosol and degrease. The only way you will know what effect the pot dimension will have is to carry out shotstrings for say 56, 57 and 58mm and compare the graphs. Its a lot of shots though but you could temporarily stuff some pip/rod inside the cylinder to reduce the volume while experimenting to keep air/pellet usage down a bit (I used some blue Mains water pipe in mine). That said the last graph I did for an HFT500 showed it to give 89 shots within 13 fps and this was an-fettled rifle. With a polish and de-gunk it most probably would have been 95 shots. After chasing nirvana though, with my own .177 S410 classic, I finally succumbed this year to fitting a Rob Lane regulator. Now I get 127 shots within 11 fps and 140 within 19fps.

Mark

[Roger5]

I use a short piece of broom handle to reduce the volume. Rog

[Archer50]

Thanks guys.

After the degunk, I think I'm getting around 80 shots in a 15 fps band. However, I'm far from sure as I've had problems with three different chronos today! That's why I haven't bothered to post the graphs themselves as I'm still not sure what's the gun and what's the chrono. However, trying to interpret the masive spikes on the graph, I seem to have a 15 fps 'sweet spot' from 190 bar (ie the first pellet) to about 110 bar, with a power peak at about 160 bar. Until I get the chrono business sorted out there's not much point in doing anything more about tuning the gun, but eventually I will try adjusting the pot to see what effect that has on consistency and sweet spot.

Anybody out there found a really reliable chrono, because it may just be my bad luck, but all the ones I've tried have varied from dodgy to shite!

Alan

[granville]

skan chrono .
if you are close you could borrow mine .
atb paul.

[Archer50]

skan chrono .
if you are close you could borrow mine .
atb paul.

Thanks, mate. Unfortunately I live in Perth (the Scottish one). Just noticed that I didn't show my location.

[Archer50]

I use a short piece of broom handle to reduce the volume. Rog

Roughly how long does the broom handle need to be? Sorry if it's a daft question, but I've never come across this idea before.

[Roger5]

I use a short piece of broom handle to reduce the volume. Rog

Roughly how long does the broom handle need to be? Sorry if it's a daft question, but I've never come across this idea before.

Just under 8.5 inches for a carbine cylinder. Rog

[Mark112]

Hi,

I've read that the HFT500 cylinder is 580mm long and the carbine one is 300mm so you will need to ad 11" (280mm) to the 8 1/2" dimension. I use plastic or aluminium as I was not sure what oozes out of wood under 200 bar pressure!

Mark