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