Respringing a HW55T

[Phil Russell]

In the recent thread ' Projects for 2015' I indicated that I was fettling a HW55T that was a delight to shoot but was sadly down on power at c. 350fps or so.
I had tried a used mainspring from my spares box but it made little difference so I had fitted a cut down HW77 spring. Performance was back to normal; most likely a bit over. So I thought a new 'proper' spring would be best. Knibbs did not list them, Chambers were out of stock. Protek listed them but I thought I would keep looking. I should say that I had a Titan No. 7 that is listed as fitting the 55 but the original spring guide was too big for the id of the Titan so I did not continue down that route. Investigations took me to Versandhaus Schneider. They listed two springs for the 50, 55, 57 and 95: a standard 7.5J and the export version. As the 55T is a target rifle I thought the standard 7.5J would be just right. I ordered two and fitted one today. Delivery was superb and the cost was no more than buying from the UK.
Now the original spring I took out was 32 coils, 2.75 / 2.8mm wire, 20mm od, 14.2 id ... I can only think discrepancies were caused by the end coils. this size was confirmed from information obtained from an article sourced via Pyramidair in the USA. The new 7.5J spring was only 28 coils, 2.8mm wire with 14.1id and 20.1od. Again no doubt errors creep in due to the end coils. The lack of the other 4 coils did concern me a bit, but I figured it might be OK. After fitting, a chrono test was disappointing ... c. 350fps. Mmmmmmm....... But then you might remember that I thought the piston, fitted with what could well have been a new leather washer, was a tad tight in the cylinder. It would move when pushed but was on the tight side. So out it came and I sized it down to a much better sliding fit. A new chrono test showed great improvement; c 520fps with Hobbys. But still a little low. I had added no preload even though with 4 coils less than the original spec there was plenty of space. So, all apart again. A 1.5mm slip washer added inside the piston head and a further 5mm added to the spring guide. Hobby now a far more respectable 580fps with AA Field at 545; both with a 6fps spread over a fair few shots. I am certain that if I added more preload I could get closer to my initial target of c. 600fps with Hobby but at present I am happy with it 'as is'.

Interestingly the biggest factor in this operation was sizing the piston seal. Maybe if I loosened it a little more the fps would increase with no further action; or maybe I will just let it wear in.

I wonder what spec the Schneider export spring is?

Cheers, Phil

[T 20]

It would be interesting to know what it does with the cut down 77 spring now Phil ?



All the best Mick

[flatrajectory]

Let the seal wear in.......you know it makes sense.

Pete.

[Rickenbacker]

Your own post # 7 on the mainspring data thread includes some VS springs Phil

Link: http://www.airgunbbs.com/showthread....ainspring+data

Would the "20.7 14.5 3.1 255 Versandhaus" likely be the export (12 FPE) spring?

[Phil Russell]

Your own post # 7 on the mainspring data thread includes some VS springs Phil

Link: http://www.airgunbbs.com/showthread....ainspring+data

Would the "20.7 14.5 3.1 255 Versandhaus" likely be the export (12 FPE) spring?

You may well be correct. But it comes to something when I cannot remember my own posts! Looking at the spec you quote it is that given for the 50 and 57, but as many springs lump the 50, 55 and 57 together this spec could well be the export version.
The 7.5J spring I had from Versandhaus was a little loose on the original spring guide (a typical early HW splayed steel tube) and while cocking was fine and smooth, returning the lever to rest gave a little lever rattle as the cocking link moved over spring coils. So today I made a new, synthetic, guide. It is a good sliding fit but takes very little effort to push into the spring. The lever rattle has now gone and, interestingly, I have gained about 25fps with Hobby pellets.

Cheers, Phil

[wonky donky]

Why Hobby? They can either give the highest reading for light weight pellets or the lowest!. It all depends on compression?.... In a HW99s I would say tune or take readings with RWS,.. but not all HW's Phill,.... Some would do better with Air Arms diablo Express or for papaer punching RWS R10, H&N Finale Match, JSB Match... or the lighter versions from RWS, JSB or H&N.

Too much is made from testing with Hobby because they are light......They are also tight, cheap & inconsistent, this may mean other pellets will give higher readings depending upon compression...Sorry but I feel I'm trying to teach my granny to suck eggs here, I feel the same when I reading some of Jims articles......sorry Jim, but it's true!

[Phil Russell]

Hi Wonky Donkey. I have no issues with your comments, thanks. But why not Hobby? I use those pellets because they are a relatively cheap flat head that I use for short range paper punching, plinking at short range and bell target at home. Yes, I could have used another make but I did not intend my tests to turn into an encyclopedia of what different pellet brands do. As it was, over several shots the difference between lowest and highest was 6fps.
In the end the HW55T is currently running around 600 fps with Hobby. Would that change with different pellets? Of course it will ... from experience I would expect around 570 with AA Field for instance. Others I could not predict, have not tried, and am not particularly interested. Now if I was fettling for a sub 12 rifle ... say 11.3, then other pellets would certainly come into the equation, but in those cases long range accuracy would be more important than chrono performance.

Now, time for supper: more eggs I guess.
Cheers, Phil