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Stripped Piston
#1
Here's what happened to my SCAR-L at a recent game...basically high ROF caused the 5-6x piston teeth to strip. This is from extended 11.1V Lipo full auto over a period of time :-

[Image: StrippedPiston.jpg]
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#2
Ahahahahah!!! Damn Q, what the hell?! I can only imagine your rate of fire to get your piston is such a condition... 20BB/seconds Big Grin
You need to get a full metal teethed piston next, but not sure your gearbox is gonna stay in one piece with such stress on it Undecided
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#3
Q,
that looks just like the piston in my M4 that was stripped in my lipo test video. I had it replaced with a short stroke piston. I was getting 22 r/s. Will a short stroke reduce the power of the gun?
Hope you get it fixed.
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#4
Oh it's fixed already. All that's needed here are harder polymer pistons such has stock Tokyo Marui, System Super Core, or Prometheus & probably something shaving off the 2nd tooth would have also helped. I've got lotsa spares at home so I replaced it with a Prometheus. The red one above is Systema is isn't very strong. I should add that this Systema red piston lasted 2 years before the above happened.

At 20-22's ROF, it's still quite low and doesn't warrant a full metal teeth piston, nor short stroking. Just remove the 2nd tooth & better quality piston would suffice. Once you start aiming for 30-40+ ROF, then short stroking becomes relevant - I'm happy to show you the ROF of my P90 (once I've fixed my BB feed issues). As for full metal teeth, they are more for higher strength springs (above J limits so no real need here).
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#5
Q,
About my question...
Will a short stroke reduce the power of the gun?

What do you think?
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#6
Just use a stronger spring. My P90, I shaved off 4 teeth from the gears & used a Angel Hi Speed piston (already matching 12 teeth too). Managed to secure a M130 spring from HK, cut it down 1/2 ring by 1/2 ring until I got close to 98m/s.

I never measured my ROF of my P90 but maybe I'll ask your help in checking via audio recording - looking at using Audacity as the software.
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#7
hi sean,

whats happening is the gears are coming back around before the piston has returned to its resting position. If if was stationary there would be no wear on any teeth, the fact that it progressively moves up the piston tells us this.

It will do the same to your promy piston as its even heavier, it will not be able to return in enough time to be ready for when the gears come round to pick it up in the correct postion.

You need to lighten the piston a bit, the tm ones a good start, take a drill and put holes in the back side of the piston removing weight.

Removing the bearing on the piston heads a good move to.

and removing half of the hieght of the second tooth, enough so in the resting postion it can pick up the piston without any contact of the second tooth.

these are more extreme examples, some times just removing the bearing is enough.

[Image: IMGP1639.jpg]

i prefer to leave the rails intact when i do the mod.

Unless im running a dmr setup, any high rof gun i make will get this mod. its just better for the long term life of the piston.
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#8
I never saw Richard's response till now!! Shows you how much attention I spend on my own forums :p.

What Richard is saying is interesting! It makes sense but I was under the impression that AOE has a higher playing role than piston weight? In AirsoftMechanics, HS5 says that's possible to run a 40rps with full teeth but no mention of longevity.

So Richard, can I please clarify?

Bad AOE: Engagement tooth breaks?
Heavy Piston: stripped teeth?
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#9
Missed it too, but it surely makes sense, good to know!
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#10
(06-08-2010, 03:24 PM)Q Wrote: I never saw Richard's response till now!! Shows you how much attention I spend on my own forums :p.

What Richard is saying is interesting! It makes sense but I was under the impression that AOE has a higher playing role than piston weight? In AirsoftMechanics, HS5 says that's possible to run a 40rps with full teeth but no mention of longevity.

So Richard, can I please clarify?

Bad AOE: Engagement tooth breaks?
Heavy Piston: stripped teeth?

Yeah thats pretty much it, from the damaged cuased you can spot whats happening.

Premature engadgement
Aoe

being too separate things to watch for,

AOE is more of an issue when first fittng a piston and head combo, becuase all parts are different sizes. Some can be whole mm's shorter or longer.

Premature engadment is the killer, which is why people recomend reducing the height of the second tooth, usually for most setups lowing the tooth too half height is enough to avoid any problems, when wear starts appearing on tooths 2, 3 ,4 its definatly not back in time, the gearsets already come round and is trying the pick up the piston sooner and eating teeth. So many things can affect the speed at which the piston returns, spring rate, piston weight, whether it has bearings or not on the piston head, gearbox casing, how tight the gearbox is, whether the rails are smooth.

Its easy to increase the speed of the gears now, especially with high powered lipos, but theres still no way of knowing whether the pistons in a resting postion when the gears come round again looking for that first tooth.

Since a stronger spring is out of the question for you guys, youll have to look at polishing the casing/piston runners, lightening the piston, maybe a little light oil on the runners after you check for smoothness of the piston.

You could even go as far as vented cylinders, or undersized o rings all of which increase the piston return time, less air in the cylinder = less reistence.

Theres lots of options simply removing a little material from the unused top side of the piston is the easyest.

at lot of the guys on ASM are runngin m100 + springs, so even at a higher rof its still returning quickly.
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