Discussion in 'Piston & Spring' started by Adam, Feb 28, 2019.
i see what you mean
When I read...
...earlier today, I realised that none of my posts on the thread to that point had any relevance whatsoever to your rifle and the POI shift.
My old MarkIII TX in .177 FAC was shooting around 945-920 fps with Exacts. It wasn't the tightest spread, but then it would throw out a shot right around 860 fps. Sometimes 2 shots in a row at that speed and it did this every 10-20 shots. I replaced all internals multiple times with zero change. One day I decided to flare the pellet skirts open a bit. The avg. spread seemed a bit better, but it still had the 860 fps shot every 10 to 20 shots. Flaring the skirts and it's slight improvement made me suspect the barrel. I slid the internals out as a unit and replaced the barrel. I slid the internals back in and the spread was down to 9 fps in 10 shots, straight from the tin. It never acted up again and won me many FT matches. The original barrel had a very tight spot about 3-4 inches in and the pellets were sized down to the point they nearly fell to the choke. I don't understand why the gun acted the way it did, but it is now totally cured. Funny part is, it's the only new TX I ever bought and the other 9 I've had were all great, even if they were well used when I got them.
Boil this down for me . I got a bit lost .
So , the problem tune is a skirtless 25mm with std. comp tube ,stroke and spring ?
With para ? or o ring ? Or other seal?
Ok, it’s a skirtless 25mm with sized down std seal and o-ring behind it. Delrin bearing on the piston and one on large steel top hat. 3.6mm port, LGU std spring.
Quite a lot of cyl contact there Adam
The bearing on the top hat could have variable contact . Do you really need it ?
Bearing on the piston ? At the back end ? Prob good enough.
As nick says is the latch rod concentric ?
O ring and para seal ? This is the bigtoe idea. Right ?
extra o ring is a belt and braces approach in my opinion, also with o ring y the crush and the groove width are critical for them to work well in air guns . If the bearing on the top hat in your set up isn't concentric it will be fighting against the bearing on the piston , as it rotates and could well be the cause of your problems .
That’s right it’s Tony’s. Rod looks ok to the eye but can’t tell if is concentric as there’s nothing to measure it against. As I could with a skirted piston measure the gaps all round. I don’t have a lathe.
You using a std comp tube Adam ?
And std stroke ?
Yes std comp tube but 3.6mm port. Far as I know it’s std stroke, that is something I can measure but forgot to do.
Good point Nick. The top hat is a very tight fit on the spring, so is locked to the spring's rotation, if any. The piston will be free to rotate independently of the spring. I also note that the para seal is free to turn on the piston quite freely.
On a general point I'm not sure at what point in the cycle that rotation happens. I presume it cannot happen on cocking due to the opposing forces exerted by the comp tube end and the spring. That being the case if there is rotation it must be during firing. But during the main piston acceleration the spring is still exerting massive force on the back of the piston through the top hat. Could it be during piston rebound that the spring force lessens enough to let the two surfaces slip?
That tight spring to top hat could well be your problem
Springs rotate when they expand. If they can't rotate both ends then the whole spring rotates from the fixed end transfering the rotation into the thing it touches (the piston) due to friction forces which increase with pressure. The only thing that can prevent that is either increasing the friction of the piston on the comp tube wall (which isn't ideal) or trying to find a way of lowering the piston to spring friction below that (which might not be possible) or using a piston design which doesn't rotate.
I'm resisting going on my usual jaunt of why rotating pistons cause a problem that was fixed in the first place. But it's quite hard work which is why a little just leaked out.
As a first check , if you don't have any measuring kit, take your seal off and insert the piston in the tube , rotate it to see if there are any tight spots, that will check your two bearings are concentric to each other , you then need to clock your piston rod , but without a dti, put your comp tube with piston in the action then fit the trigger block with the guide, but no spring , do the rotation check again with the rod entered into the guide almost fully but not latched ( probably have to rotate the comp tube as you wont have access to the piston at this point ), this is a bit rough but will show you if you have any problems with run out. If all that checks out, you can look up at the front of the piston in case your seal fitting hasn't been machined concentric, after that checks out look at the spring, process of elimination .
Just thinking online... if the spring isn't straight or finished flat, could that not help either? Just thinking either the spring getting kinked if it's compressed, or just having a kink, or if the ends aren't staying flat then it could rub or push a sloppy piston off axis?
That's 2 Kinks . A Rub . And Something Sloppy all in one paragraph
Just needs someone to polish the end off maybe...
You're the badddest man I know
Did the standard piston go back in to do the chrono test?
It went back in for the PJBRC. I ran a few shots over to check it wasn’t too high or low (about 770) then re-zeroed at club on Saturday. I transplanted the entire actions between that and my spare LGU / silhouette gun. Comp tube, piston, spring, guide, trigger block. That way it’s a ten minute job. The two springs don’t fit well on each other’s guides.
Though I needn’t have bothered as it turned out. Lost confidence and feeling a bit crap anyway do not a high score make.