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Hi all, currently building a faster trigger response/ higher rof m4. I'm using a 7.4vlipo with , double airseal nozzle, sealed cylinder head( compression is very consistent) , 13:1 gears, high speed motor, rewired to deans, madbull python 6.03 barrel, standard green g&g bucking and lonex hop. Gun works a dream in semi, can spam the trigger with no problems at all. Consistently 330/332fps on 0.2g. gun is shimmed lovely, gun cycles very smooth and quiet, problem is that when I fire in full auto it sounds like it's double fedding and screeching(hard to describe) and as soon as I go back to semi it's fine. Chronoed rps and was 32 (if that's needed to help diagnose) 

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2 hours ago, Pewpewpetie said:

Hi all, currently building a faster trigger response/ higher rof m4. I'm using a 7.4vlipo with , double airseal nozzle, sealed cylinder head( compression is very consistent) , 13:1 gears, high speed motor, rewired to deans, madbull python 6.03 barrel, standard green g&g bucking and lonex hop. Gun works a dream in semi, can spam the trigger with no problems at all. Consistently 330/332fps on 0.2g. gun is shimmed lovely, gun cycles very smooth and quiet, problem is that when I fire in full auto it sounds like it's double fedding and screeching(hard to describe) and as soon as I go back to semi it's fine. Chronoed rps and was 32 (if that's needed to help diagnose) 

Almost like the piston isn't being picked up every cycoe in full auto

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PME for days.

I had a very similar issue with my specna 933 (C-02), I solved by short stroking the sector (by two) and slapping a longer spring (SHS M90).

On its full stroke configuration the gun cycled fine when dry fired, but as soon as I got a bb in the chamber, it would PME like a mofo. I identified the issue in the spring being too short (specna loves stubby springs, mine was cut down as well to stay below 1J)...

 

Now that I think about it, my C-08 did the same, solved without realizing when I knocked off one tooth off the sector and installed the same SHS M90 spring :D

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8 minutes ago, Skara said:

PME for days.

I had a very similar issue with my specna 933 (C-02), I solved by short stroking the sector (by two) and slapping a longer spring (SHS M90).

On its full stroke configuration the gun cycled fine when dry fired, but as soon as I got a bb in the chamber, it would PME like a mofo. I identified the issue in the spring being too short (specna loves stubby springs, mine was cut down as well to stay below 1J)...

 

Now that I think about it, my C-08 did the same, solved without realizing when I knocked off one tooth off the sector and installed the same SHS M90 spring :D

So take a tooth off of the pick up side of sector?  They are already 13:1 gears.

10 minutes ago, Skara said:

PME for days.

I had a very similar issue with my specna 933 (C-02), I solved by short stroking the sector (by two) and slapping a longer spring (SHS M90).

On its full stroke configuration the gun cycled fine when dry fired, but as soon as I got a bb in the chamber, it would PME like a mofo. I identified the issue in the spring being too short (specna loves stubby springs, mine was cut down as well to stay below 1J)...

 

Now that I think about it, my C-08 did the same, solved without realizing when I knocked off one tooth off the sector and installed the same SHS M90 spring :D

Also in the video you can see when on full auto the piston stops moving for while whilst the gears continue to turn. Is that caused by pme

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7 minutes ago, Pewpewpetie said:

So take a tooth off of the pick up side of sector?  They are already 13:1 gears.

Also in the video you can see when on full auto the piston stops moving for while whilst the gears continue to turn. Is that caused by pme

I took them off the pickup side as those have the least impact on cutoff/tappet timing. 13:1 gears have 16 teeth like all the other regular single sector gears, it's just the spur/sector "connecting" teeth that are different from 18:1s 

 

Now that i think about it, your piston rack might be a bit fucked, but you're going to take the gearbox apart anyway so you can check for damage on there too.

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5 minutes ago, Skara said:

I took them off the pickup side as those have the least impact on cutoff/tappet timing. 13:1 gears have 16 teeth like all the other regular single sector gears, it's just the spur/sector "connecting" teeth that are different from 18:1s 

 

Now that i think about it, your piston rack might be a bit fucked, but you're going to take the gearbox apart anyway so you can check for damage on there too.

I'll have a look when I strip it. Many thanks. If the piston is fine then what do you think the problem could be. Almost as if the piston is slipping on sector gear

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The only other options would be the sector sitting way too high so it only catches on a minimal portion of the rack, or the piston guides being chewed to all fuck so when the sector grabs it, it is lifted. But the latter seems unlikely as from the video the piston body seems intact 

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5 minutes ago, Skara said:

The only other options would be the sector sitting way too high so it only catches on a minimal portion of the rack, or the piston guides being chewed to all fuck so when the sector grabs it, it is lifted. But the latter seems unlikely as from the video the piston body seems intact 

When I split the gearbox tomorrow evening I'll upload some pics. Really appreciate your help.

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2 hours ago, Skara said:

The only other options would be the sector sitting way too high so it only catches on a minimal portion of the rack, or the piston guides being chewed to all fuck so when the sector grabs it, it is lifted. But the latter seems unlikely as from the video the piston body seems intact 

 

IMG_20210503_211903.jpg

IMG_20210503_212215.jpg

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Posted (edited)

Crushed piston rear end. PME failure. You can see how the rack has twisted up and out of the way allowing the sector to gouge the rack. You were lucky it didn't destroy anything else, but I wouldn't have any faith in that gear set not having cracks in the teeth.

I'm surprised it was running at all with all that lube. For less than £3 you can get a nice soft moly grease for gears... At a push I'd use it on the pistol rings as well. It's far better than nothing. I've been using it on my own gears for a pretty long time now. Use on metal/metal contact areas if you have a silicone based grease as well for the plastic/plastic plastic/metal areas.

https://www.ebay.co.uk/itm/391721324225?epid=1548017327&hash=item5b34690ec1:g:bj4AAOSwsnpe6QpA

 

If you are feeling flush then I use this on my plastic/plastic and plastic/metal contact points. Pistons - O-rings - Selector plates ect.

https://www.ebay.co.uk/itm/391399159157?hash=item5b21353575:g:fDUAAOSw6fBclQTY

Edited by Iceni
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8 hours ago, Iceni said:

Crushed piston rear end. PME failure. You can see how the rack has twisted up and out of the way allowing the sector to gouge the rack. You were lucky it didn't destroy anything else, but I wouldn't have any faith in that gear set not having cracks in the teeth.

I'm surprised it was running at all with all that lube. For less than £3 you can get a nice soft moly grease for gears... At a push I'd use it on the pistol rings as well. It's far better than nothing. I've been using it on my own gears for a pretty long time now. Use on metal/metal contact areas if you have a silicone based grease as well for the plastic/plastic plastic/metal areas.

https://www.ebay.co.uk/itm/391721324225?epid=1548017327&hash=item5b34690ec1:g:bj4AAOSwsnpe6QpA

 

If you are feeling flush then I use this on my plastic/plastic and plastic/metal contact points. Pistons - O-rings - Selector plates ect.

https://www.ebay.co.uk/itm/391399159157?hash=item5b21353575:g:fDUAAOSw6fBclQTY

Thanks for replying, what do I do in this situation? 

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8 hours ago, Iceni said:

Crushed piston rear end. PME failure. You can see how the rack has twisted up and out of the way allowing the sector to gouge the rack. You were lucky it didn't destroy anything else, but I wouldn't have any faith in that gear set not having cracks in the teeth.

I'm surprised it was running at all with all that lube. For less than £3 you can get a nice soft moly grease for gears... At a push I'd use it on the pistol rings as well. It's far better than nothing. I've been using it on my own gears for a pretty long time now. Use on metal/metal contact areas if you have a silicone based grease as well for the plastic/plastic plastic/metal areas.

https://www.ebay.co.uk/itm/391721324225?epid=1548017327&hash=item5b34690ec1:g:bj4AAOSwsnpe6QpA

 

If you are feeling flush then I use this on my plastic/plastic and plastic/metal contact points. Pistons - O-rings - Selector plates ect.

https://www.ebay.co.uk/itm/391399159157?hash=item5b21353575:g:fDUAAOSw6fBclQTY

I do have grease but cleaned it for the photo. I use a dab of silicone on all plastic to plastic parts and the gun grease on metal on metal. Would you suggest get new piston and short stroke it then?

IMG_20210504_075426.jpg

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20 hours ago, Skara said:

The only other options would be the sector sitting way too high so it only catches on a minimal portion of the rack, or the piston guides being chewed to all fuck so when the sector grabs it, it is lifted. But the latter seems unlikely as from the video the piston body seems intact 

I think the sector was sitting a little high. Also this screw on the sector was loose and the piston is broken on the inside

IMG_20210504_144514.jpg

IMG_20210504_145041.jpg

IMG_20210504_144535.jpg

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Grab something like a CYMA 15 tooth or Rocket 14 tooth to replace.  You might want to correct AOE with some spacers, it's not 100% necessary but it's something I would do personally.  I'd also epoxy down the rack, again that's just me.   Thread-lock that gear screw and the piston head screw just to be on the safe side.

 

The height of pistons actually vary slightly, generally around 0.5mm.  From the looks of your piston, it's similar to the E&C design which is one of the larger bodies.  It's about 0.6mm taller than the XT / Rocket pistons I've got

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Consider yourself lucky, a 15 FULL teeth piston at 32 RPS should've exploded 😮

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32 minutes ago, ak2m4 said:

Grab something like a CYMA 15 tooth or Rocket 14 tooth to replace.  You might want to correct AOE with some spacers, it's not 100% necessary but it's something I would do personally.  I'd also epoxy down the rack, again that's just me.   Thread-lock that gear screw and the piston head screw just to be on the safe side.

 

The height of pistons actually vary slightly, generally around 0.5mm.  From the looks of your piston, it's similar to the E&C design which is one of the larger bodies.  It's about 0.6mm taller than the XT / Rocket pistons I've got

Thanks alot for your input. You have an eBay shop don't you? The name rings a bell. Going to order a 14 tooth piston, some new 13:1. (just to be safe). When adjusting aoe is it better to use sorbo pads or washers between piston and piston head?

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better to use the website buddy as it's much cheaper on there.  instead of the sorbo pads go for a £2 set of AOE washers, much easier to fit.  as i said you may not need to fit them, there's a lot of debate at the moment etc.

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4 minutes ago, ak2m4 said:

instead of the sorbo pads go for a £2 set of AOE washers, much easier to fit.

They also provide a much more precise and constant spacing effect, sorbothane works well on bolt actions but the fact that it compresses and expands (over time it stays compressed btw) makes it crap for AEGs.

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1 hour ago, Skara said:

They also provide a much more precise and constant spacing effect, sorbothane works well on bolt actions but the fact that it compresses and expands (over time it stays compressed btw) makes it crap for AEGs

Makes sense to me. On payday I'll have to get an order on and hope it helps. Many thanks guys

1 hour ago, ak2m4 said:

better to use the website buddy as it's much cheaper on there.  instead of the sorbo pads go for a £2 set of AOE washers, much easier to fit.  as i said you may not need to fit them, there's a lot of debate at the moment etc.

I'm guessing the way the pic up tooth must hit the piston rack the aoe must have been off.

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On 04/05/2021 at 16:23, ak2m4 said:

better to use the website buddy as it's much cheaper on there.  instead of the sorbo pads go for a £2 set of AOE washers, much easier to fit.  as i said you may not need to fit them, there's a lot of debate at the moment etc.

Hi again so I got hold of a rockit Swiss cheese piston, short stroked some old 16:1 gears by 3 teeth and same on piston rack. Gun fires lovely now, no jamming feeds fine, sounds lovely. Only problem is, is the high speed motor is now getting hot with a 7.4 lipo. 

On 04/05/2021 at 16:23, ak2m4 said:

better to use the website buddy as it's much cheaper on there.  instead of the sorbo pads go for a £2 set of AOE washers, much easier to fit.  as i said you may not need to fit them, there's a lot of debate at the moment etc.

Also corrected aoe .

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Also I have tried the bevel to pinion method of shimming and still motor gets hot very quickly. I'm 99.9% that the shimming is perfect, no grinding and gears spin freely once gearbox is screwed together( very well indeed). My guess is the stock wiring or the motor itself. The motor is an unbranded but specified 16tpa. I will be upgrading the stock wiring to 16awg payday.

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