career / sumatra tuning for 12 FP efficiency...

[Shed tuner]

I get a surprising number of question about how I used to tune these. I purchased a (new to me) sumatra a little while back, so actually took a piccy or three for a change.

It's pretty simple, but here goes:

1) The valve is 8mm bore. Stupidly large unless you are shooting monkeys as they were originally designed for. So I make up a little top hat with an undercut o-ring and sleave the valve down to 6mm. This reduced the force required to open it from around 230 lbs to 130. it also halves the airflow through the valve, saves wasting it. See pics when I find a hosting site.

2) The hammer is stupid heavy - so just make a new one out of ally. Just add a steel striking piece at the end that hits the valve, threaded into the nose of the hammer. On the sumatra with the adjustable stroke hammer, you just re-use the same steel adjustable nose, m6 thread.

3) Slightly reduce the valve return spring strength, just a touch to allow for the extra thickness of the top hat (#1)

4) If you have done the valve sleave (#1 above), do not add a restrictor / spring guide for the valve retrun spring where air enters the valve return spring chamber. If you haven't, then fine, you should

5) Reduce the hammer spring strength. With the above mods, you should get a nice power curve in the +-15 fps range between 140 and 200 BAR, with a huge number of shots, all at around 11 FP.

6) leave TP small around 1.2mm

The light hammer opens the valve really fast, and most importantly, closes it really fast too, so a nice narrow pressure pulse = minimal wasted air/noise. The TH chokes off the flow into the valve, as well as drastically reducing the valve opening forces (allowing the use of a lighter hammer / / spring)

The sumatra now shoots really fast, really quiet (I don't even bother with a silencer) does gazzilions of shots per charge, and is easy to cock. Only problem is the bugger is too front heavy, so I'm probably going to completely remove the underslung ally air tube (need to find what that 22mm thread is in the end of the steel tube and get a new end piece made to seal it and house a filler) and run with just the single steel tube. Even if it's only 130 or so ccs, it'll be plenty.

Pics to come in a sec..

OK, try this shared link - will it work - who knows:
https://goo.gl/photos/ghx5S7ZmQABRiunm7

[WILBA]

Two questions jon.
1. Would the hammer be better if made out of titanium than aluminium?
2. My gun like tinners is power locked. Tinners said he found the problem layed with the hammer spring being too soft. So fitting a stronger one cured the issue but had to make adjustments to keep legal. If you do as you say and reduce it a bit to adjust for the top hat thickness could this make that power locking problem more prone to happening?
Reason i asked about the Ti hammer is i can get hold of some Ti round bar. Regards. Chris

[tinbum]

Two questions jon.
1. Would the hammer be better if made out of titanium than aluminium?
2. My gun like tinners is power locked. Tinners said he found the problem layed with the hammer spring being too soft. So fitting a stronger one cured the issue but had to make adjustments to keep legal. If you do as you say and reduce it a bit to adjust for the top hat thickness could this make that power locking problem more prone to happening?
Reason i asked about the Ti hammer is i can get hold of some Ti round bar. Regards. Chris

If you can machine it you can use it! It may still need a steel cap though as titanium is relatively soft.

[WILBA]

If you can machine it you can use it! It may still need a steel cap though as titanium is relatively soft.

Ok thanks tinners

[kevG]

did this type of mod on mine a career.like you say really quiet and so much sweeter to shoot.one gun I regret selling.i was in the middle of making a reg for it when I sold it.if I could get a nice example of one I would love to carry on with further mods.
kevG

[Shed tuner]

did this type of mod on mine a career.like you say really quiet and so much sweeter to shoot.one gun I regret selling.i was in the middle of making a reg for it when I sold it.if I could get a nice example of one I would love to carry on with further mods.
kevG

long time kev, how are you pal ?

I didn't realise you ventured off the Rapid forum

[kevG]

look in a lot jon m8.just don't post.unless interesting stuff.
kevG