shredding pistons on my M4, need help

[m102404]

I've run several guns on 9.6v batteries, both 2/3A and sub-c cells, with springs ranging from stock TM to M120. If you're set on semi and it's firing full-auto...dropping to a lower voltage battery won't help that.

It may help with shredding pistons...everything wants to move faster with higher voltages and anything slightly out of spec could/will lead to problems.

The only issue that I've had so far, that consistently leads to shredding pistons, is the aftermarket pistons/piston-head combos not being as long as the stock TM piston+head.

All my other pistons are in AEGs right now, but I've got one stock TM piston loose. From front face of the piston to the rear of the piston body, it measures 74mm.

A deepfire piston and head combo I have in another AEG measured about 72.5mm and a stock CA one measured about 73mm. The CA one seems to work ok in different builds (just finished putting together a test mechbox with it about 10 minutes ago) and the deepfire one works like a charm.

To build up the space, I've used a washer between the body and the piston head when needed. Before I used this little trick, I stripped the first few teeth off of 2 "short" pistons.

This may not be the cause of your stripping pistons, but it's worth taking a look at before your next build.

So far as firing full auto only in both modes...here's a couple of scenarios that I ran into...
1. The side plate screw on a G36 mechbox was pushing too far into the mechbox and jamming the switch assembly into the cut-off lever jamming it. Trimming down the screw fixed that.
2. The cut-off lever end (the end in the mechbox that flicks the trigger switch block) was too long and hitting the switch assembly chasis...a tiny bit of filing sorted that out.
3. The screw that screws down on the cut-off lever was too tight and would grip the lever in the up position...the little return spring was not strong enough to push it back. Loosening the screw just the tiniest bit was enough to free it up.

If you resoldered the switch assembly wrong you'll get full auto all the time (without even pulling the trigger!!)......just a dumb mistake I made once...

Don't know if the switch plates are the same for the MP5 and M4 series...

PM me if I can be of any more help.

[swatt13]

for the full auto issue, that "cantalever" that sits inside under your trigger contact assy (metal piece right infront underneath the sector gear, actuated by fire selector contact plate) prolly wasnt installed right. i had that happen before to me.

the piston issue, thats a tuffy, id consider the shimming or maybe gearing, battery and motor ratio(combo) or even all of them together. in that situation id just start from scratch and re-rebuild the mechbox. inspect the gears. reshim, reinstall and observe the meshing/ operation of the gears and piston. ensure your piston is running along the guide channels, and gears arent seated cockeyed.

how did the piston wear? only one side of the teeth were stripped off, just the back teeth, front teeth?

[Skruface]

Originally Posted by swatt13

how did the piston wear? only one side of the teeth were stripped off, just the back teeth, front teeth?

+1. This is important to know.

This issue could also be related to having a bearing on both the piston and spring guide. With an overly-long spring, the bearings on both ends may not allow the piston to compress all the way before the gears come around. Backwards-moving piston meeting forwards-moving gears = bad.

This may be the problem if you are just tearing off the last 2-3 teeth (the ones closest to the rear of the gun); you can simply shave down the 2nd last tooth of the piston (NOT the thick tooth right at the back of the piston; the little one just in front of it) and this will eliminate the chances of the gears engaging while the piston is still moving rearward.

I've done this on some super-high-ROF setups I built this summer and it works great. I've not lost a piston yet.

[BKHKMP5]

The first 5 teeth are completely shaved off, the next 5 teeth are 1/2-1/4 shaved off, and the last 4 teeth are completely fine, the final steel tooth is fine as well. This is probably a ROF issue with the 9.6V battery right? It seems to be the opposite of what you're describing.

thanks,

Bry

[DarkAngel]

Im not sure about the piston shredding :/ I was just told about this today on msn, however i already suspected the selector lever (what you called the cantalever) was the cause of the semi/full auto problem. I had explained that to him already.

Origionally he had given me a broken down CA MP5 Gearbox (also v2) and asked me to repair it. (the wiring kit had melted from a short because he had taken the old fuse out and put a higher amp fuse and destroyed the battery he was using. The Teeth on his piston was stripped as well back then.

At the time he was using the CA gearbox with the guarder sp120 full upgrade kit (with a very heavy spring). When i had gotten it, he had later brought to me a bunch of parts (ie a new systema wiring kit, new shims, new piston, bearings, KA v2 Gearbox, anti reversal latch etc) and I had shimmed it and installed it. I had tested the shimming and there was no play in the shimming (but enough that it wasnt tight) and the gears moved smoothly. I also double checked the piston guide to make sure that it wasnt pinching the guide on the piston and the whole gearbox seems to cycle manually quite well. I did not have tamia plug battery on me so after assembling it i asked him to go home, plug in his battery and cycle the gearbox a few times. He must have misunderstood me because he assembled the whole gun (Instead of just the gearbox and grip) and full autoed it for a bit (which i told him dry firing was bad for the gun).

Over the phone he told me that the gearbox worked fine, and he fired it again for a short burst so i could hear the gun cycle on the phone and hear the rough ROF of it. The only issues he had two days ago when he called me to say it was fine, was the semi auto/full auto thing (which i explained was most likely the selector lever) and which I had asked him to bring it back to me yesterday morning (when i was near his location) but i have not seen the gun yet since he said it was cycling and working properly.

As for the piston, it is possible that since the stock CA piston head was put in temporarily (because he had lost the bearing washer for the Guarder one, making it impossible to install the piston head on the piston), i think maybe its possible that the CA piston (though very similar in depth, may have a wider diameter (this is my guess since i havent seen the gun since the piston stripped) this may have cause the cylinder kit to compress at a very high rate (without the time for the air to escape through the small nozzel opening) causing the piston to act like it was hitting a wall when it reached maximum compression (could someone confirm my theory?). Putting excessive tension on the piston, making it hard to move at the higher ROF due to the 9.6v battery. \

That piston was stripped in less that one day (i assume of course he wasnt dry firing it all night) Pistons dont wear normally that fast. (and yes i also confirmed and checked that the gear teeth and the piston teeth were snug and spaced correctly) my other reasoning behind this is that, ive noticed with guarder cylinder kits (which i have as well) the guarder piston doesnt not have a solid seal on the cylinder walls. There is a minut space between the rubber and the cylinder (i first thought this may have been a factory defect) but it was the same way with BKHKMP5's kit. The stock CA piston had a better seal. From these observations without seeing the actual damage to the piston teeth or the gearbox itself, im assuming that this might be the problem.

sorry for the long read :#

[ILLusion]

That space between cylinder head o-ring and cylinder wall is 100% normal and fully expected from any ventilated piston head. It's a performance benefit for the piston drawback. The space gets closed when the piston is on its compression stroke - the incoming air will expand the o-ring to touch the wall.

Could Bryan have been dry-firing testing with his finger over the air seal nozzle? Doing that will create a compression blockage that can, in fact, cause the piston to stop in mid-stroke or slow down the return enough for the sector gear to rake the tooth rack before it's returned to battery.

[m102404]

I was just sorting through some old parts last night and got to the misc assortment of "crap" springs that I've accumulated over time.

This might just be my bad luck and the assortment/combination of parts that I have on hand, but I've found in the past, if you're setup with a bearing spring guide (he said he was using a guarder one, and I'm not sure if the make one without a bearing) and that if the spring diameter is a bit large for the thrust bearing, the spring could expand over the thrust bearing, causing the rear of the piston to get jammed almost at it's rear-most position. I've had this happen twice (not sure if it happened with two different spring guides but definitely with two different springs...which are now in the "crap" bin), and both time several of the front piston teeth suffered damage.

NOTE: I get the Front/Rear mixed up...by front teeth, I mean the ones closest to the piston head.

In both instances, I swapped to springs that seemed to fit better and none of those builds have had any issues since. On another note, both of those "crap" springs work fine with a stock spring guide that has nothing more than the little washer that they normally come with.

At least for when it happened to me, the piston was good and jammed tight on the spring/guide and stayed that way even when I opened the mechbox (NOTE: if this happens to you, point the guide away from you when separating them...but not if away from you happens to point at the TV screen...missed it by 2cm!), so it was easy to tell what happened.

Best of luck with the fix!