Piston damage

G

Gaza666

Members
Joined
Sep 15, 2018
Messages
7
Reaction score
0
Can anyone suggest some things to look at regarding the cause of damage to this piston please.

The gun started making grinding sounds intermittently on full auto but never on single shot.

It had an SHS ali 2 o-ring piston head and cylinder head, the AOE was adjusted and ok.

I was thinking PE due to the extra weight of the piston but even uprating the spring to an M120 and with a new SHS piston it still has the intermittent grinding noise on full auto with very slight damage to tooth 4 (I mean minuscule, certainly not enough for the noises)

Motor height and shimming is all done, the only thing i don't like is that the bearings are fairly loose in the gearbox halves.

Oh and it's a G&G Predator.

Thanks

View attachment 42616

View attachment 42617

 
PME to my eye. It's on the path to a stripped piston.

You probably have excessive wear on the piston rails as well. Allowing the piston to ride upwards.

You may also have problem bearings.

The solution in my mind would be.

New piston (full rack), Solid bushes, Replace the sector.

Reduce the ROF, Or shorten the stroke of the piston to correct the PME.



The reason you don't see it on semi is you have an AB mosfet with the ETU. I would hazard a guess that you upgraded the motor, but didn't correct the stroke, Running 11.1v  on 18:1 with a 30K+ motor. You can't drop the battery voltage because of the ETU, so the only options you have are putting the stock motor back in, or short stroking the box with a new piston and some tweaks.

The short stroke will need an even bigger spring to hit 350fps, so you will have to make some sacrifices to either ROF or FPS.

 

 
Last edited by a moderator:
Iceni said:
PME to my eye. It's on the path to a stripped piston.

You probably have excessive wear on the piston rails as well. Allowing the piston to ride upwards.

You may also have problem bearings.

The solution in my mind would be.

New piston (full rack), Solid bushes, Replace the sector.

Reduce the ROF, Or shorten the stroke of the piston to correct the PME.



The reason you don't see it on semi is you have an AB mosfet with the ETU. I would hazard a guess that you upgraded the motor, but didn't correct the stroke, Running 11.1v  on 18:1 with a 30K+ motor. You can't drop the battery voltage because of the ETU, so the only options you have are putting the stock motor back in, or short stroking the box with a new piston and some tweaks.

The short stroke will need an even bigger spring to hit 350fps, so you will have to make some sacrifices to either ROF or FPS.

 
Click to expand...
Click to shrink...
Thanks for the reply, I forgot to say that the gun is only 3 months old with around 4 games played, It's also the standard 25k motor. The only things changed originally before the piston started to strip were the piston head, cylinder head, oh and the nozzle so it shouldn't have been any higher ROF. This was to increase the air seal (original fps was at 260) and prevent piston head failure. I agree about the piston riding up /sector gear down though but apart from the crap fitting bearings which when in and shimmed don't really move, I can't see a reason for the poor engagement.

 
How tight is the piston in the cylinder?

260 is awfully low for a stock gun. Would suggest there were problems to begin with inside that gearbox.

What was the FPS with the M120, and did you need to cut the spring?

I would expect an M120 on a full cylinder with no air leaks to be producing 370-400+fps depending on the make. If you needed to run the M120 to get into the 300-350fps area then there are other problems.

Check the gearbox shells and the piston to ensure it's all smooth.

Check the piston in the cylinder, Do a drop test, Ensure there are no points where the piston is binding in the bore.


Once you get onto problems like this experience becomes more important than engineering skill. You have to know what to look for and feel. It might be prudent just to send the gearbox out and have a tech rebuild it with the damaged parts replaced.

 

 
Last edited by a moderator:
Iceni said:
How tight is the piston in the cylinder?

260 is awfully low for a stock gun. Would suggest there were problems to begin with inside that gearbox.

What was the FPS with the M120, and did you need to cut the spring?

I would expect an M120 on a full cylinder with no air leaks to be producing 370-400+fps depending on the make. If you needed to run the M120 to get into the 300-350fps area then there are other problems.

Check the gearbox shells and the piston to ensure it's all smooth.

Check the piston in the cylinder, Do a drop test, Ensure there are no points where the piston is binding in the bore.


Once you get onto problems like this experience becomes more important than engineering skill. You have to know what to look for and feel. It might be prudent just to send the gearbox out and have a tech rebuild it with the damaged parts replaced.

 
Click to expand...
Click to shrink...
Thanks for your help with this.

I agree, 260 seemed low but it actually fired very well. 

With the Brand new Guarder M120 the gun is at 340 fps, it's a 3/4 cylinder.

I did a drop test with the piston and the cylinder in place and it fell all the way, both ways. Obviously couldn't see how smooth it was so will repeat with just the piston in the gearbox and then in the cylinder.

I'm also going to check the mesh between the section gear and the piston as it seems to be skipping over the teeth.

The interesting thing I noticed but need to test more (it was very late) is that I didn't seem to get the noise without a magazine and bbs fitted. I presumed this could be down to the lack of back pressure from not having a bb sat at the nozzle, I'm sure it's not a magazine issue.

 
Some G&G GC16 guns came from the factory with shitty AOE. I was warned about it when I bought mine and even given an extended warranty on the piston

Some G&G GC16 guns came from the factory with shitty AOE. I was warned about it when I bought mine and even given an extended warranty on the piston

 
Gaza666 said:
The interesting thing I noticed but need to test more (it was very late) is that I didn't seem to get the noise without a magazine and bbs fitted. I presumed this could be down to the lack of back pressure from not having a bb sat at the nozzle, I'm sure it's not a magazine issue
Click to expand...
Click to shrink...
The only pressure the mag should be putting on the gearbox is the pressure of the bb's pushing up into the air nozzle in the fire position, and across into the hop when it feeds. Even when combined with the tappet spring it shouldn't be enough to pull anything out of line or cause grinding. It might be enough to show a bad shim job off at the pinion bevel engagement. And that can sound like a bag of spanners in the works.

 
Last edited by a moderator:
Ok, so I've spent all day on this gearbox and got to the point I was happy with it after lightening the piston to within 1.5g (I didn't feel comfortable taking any more off) of the original. I marked the teeth with a sharpie and ran it for long periods in full auto with absolutely no marks to the teeth whatsoever. I then put the box back into the lower receiver, attached the barrel and now its stripped itself. ?

If the hop was fouling the nozzle could it cause this? I'm nearly at a loss with it, it sounded so good when it was just the gearbox and motor.

View attachment 42675

 
Why would you even put that air nozzle in a gun without testing it in the hop first? There is so much going on in that gearbox, and it is so badly executed I don't think I could help and be sure of the outcome.

Basics have been ignored on that gearbox. Things like checking parts fit together. Added into that without knowing the basics you have then decided to modify parts.

For a 3-month-old gun, With 4 games on it, you have ruined it.

If someone else wants to help you then that's on them, but I'm out bud.  Take it to a tech, You are out of your depth.

 

 
Iceni said:
Why would you even put that air nozzle in a gun without testing it in the hop first? There is so much going on in that gearbox, and it is so badly executed I don't think I could help and be sure of the outcome.

Basics have been ignored on that gearbox. Things like checking parts fit together. Added into that without knowing the basics you have then decided to modify parts.

For a 3-month-old gun, With 4 games on it, you have ruined it.

If someone else wants to help you then that's on them, but I'm out bud.  Take it to a tech, You are out of your depth.

 
Click to expand...
Click to shrink...
Alright bud, take it easy.

I take on board what you're saying but it's not a nuclear reactor that is going to kill people if not done correctly. I'm tinkering with a toy gun and the worst case is I buy I new, ready built gearbox.

It was obviously my mistake to presume these parts would be built to a standard and fit without too much tuning and I accept that i should have checked everything first but this is a learning curve and nobody is born knowing this stuff.

 
Took me about two weeks to put my first gearbox back together and a lot of swearing. Sweating like a bomb defuser :lol:

 
Last edited by a moderator:
Got my first one back together eventually after many aborted attempts due to the spring escaping or the anti reversal latch popping out of place, sat there feeling all smug and very pleased with myself.........right up to the point I noticed a shim sitting on the desk ! ?

 
AEG gearbox is a simple thing so crack on with sorting it yourself.

The G&G boxes can be some of the worst though. Poor AOE as mentioned above is common but easy to correct. Make sure the shimming is correct plenty of vids on youtube to help, fit some decent bushes and make sure everything is greased well.

The ground piston could be AOE or you might have just put it back together badly. Looks like it was not on the runners properly and bound up maybe? Feed issues can cause it but less likely.

 
It turns out the piston was binding. Really odd though as on its own it was fine, in the cylinder it was fine, but put both into the gearbox halves and it bound up at the last 10mm of travel. No matter what I trimmed away it would not stop binding. Interestingly I had bought 3 or these SHS pistons (ebay) so I tried another. This one was fine, not even a hint of binding. So I tried the 3rd piston and this one, again bound up tight at the furthest point of travel.

Needless to say I returned the 2 faulty pistons for a refund but it goes to show that this stuff does not seem to be made well.

 
You must log in or register to reply here.
Back
Top