Shimming new gears issue - Is this normal?

[ao_tenshi]

Hey ASC!

I have been trying to install a new set of gears (SHS 13:1) into my G&P M4, and have been having issues at the shimming stage.

Following numerous guides and tutorials on shimming religiously, I FEEL that I have done near perfect shim jobs. Even tested the gear movements with the gearbox screwed back together. However, the PROBLEM comes when i put the gun back together to test fire. Here are the symptoms I encounter:

1) Semi-auto firing - gun will seize during semi-fire (ok in full-auto)
2) Loud gear mechanical sound, I dont think its grinding as I've checked the gears after

My conclusion is that the gears are shimmed too tightly creating excess stress and friction.

The issue Im facing now is if i loosen the shim to a point where the gun doesnt seize in semi, then the play/movement of each gear axle is extremely large. Is this a normal thing or is my gearbox not built to spec? The other thing i notice is my selector plate is pushing the bearings as well...

Maybe folks with lots of shimming experience can shed some light on this.

Thanks in advance!

[Hectic]

Lot of trial and error.
when you are testing are you putting some screws back into the gearbox (like 4 corners basically) then checking the spin and play? I did this before got em shimmed perfect then after screwing it down its too tight.
Also, what motor, battery and how heavy of a spring? Most stock motors won't pull a 350-400fps spring on 13:1 gears, even with a big ass 11.1 50c lipo.
depending where in Toronto you are located we could perhaps hook up and sort out the issue together.

[ao_tenshi]

Im in Markham area. Currently running stock G&P M120 Motor, SHS M90 spring, and 7.4v Lipo.

Yes, I checked the amount of play by screwing back 4-5 screws. I guess once the shell and grip goes on, they will all exert pressure on the bearings, which makes the typical shimming approach in the gearbox not valid anymore?

Originally Posted by Hectic

Lot of trial and error.
when you are testing are you putting some screws back into the gearbox (like 4 corners basically) then checking the spin and play? I did this before got em shimmed perfect then after screwing it down its too tight.
Also, what motor, battery and how heavy of a spring? Most stock motors won't pull a 350-400fps spring on 13:1 gears, even with a big ass 11.1 50c lipo.
depending where in Toronto you are located we could perhaps hook up and sort out the issue together.

[Fenrir92]

Did you shim your gears with your selector plate taken off? If so, try shimming the gears with the selector plate installed. The G&P bearings stick out quite a bit out of the gearbox causing to push against the selector plate.

I had to use less shims on the sector gear due to the selector plate pushing against the bearings

[Sequential]

you shim them near perfect, then you downsize in shim after tightening the gearbox. it happens. just a bunch of trial and error you will eventually get it all together.
upon tightening the gearbox the gears become to stiff hence why you are getting the seizing up.

the shs 13:1 gears are loud, not to worry.

[ao_tenshi]

Thanks for the good suggestions guys. I will try re-shimming with selector plate on and do some trial and error on the amount of shims to use.

Are there any tips on how to realize I've hit the sweet spot with the shims?

[foxtrot-one7]

Originally Posted by ao_tenshi

G&P M4,
1) Semi-auto firing - gun will seize during semi-fire (ok in full-auto)
2) Loud gear mechanical sound, I dont think its grinding as I've checked the gears after

It may not be the shimming, but could be the piston and the sector gear. I'm speaking from experience when I say, check to see that the amount of teeth on the piston matches that of the sector gear.

[lurkingknight]

check to see if the grip is pinching the bushing/bearing on the bevel. 8 and 9mm gearboxes have more issues with grips but it's not uncommon to see a grip interfering with the bevel gear on any gearbox. You'll see a little pocket in the top of the grip that is supposed to accomodate the gear axle. you may have to dremel that wider/deeper.


Also g&p 120 motor... lol.

[Styrak]

Also make sure to check the shimming/spacing on motor gear to pinion gear. That can create a lot of headaches and noise.

Better to have that more loose than more tight. If your motor is pushing the pinion gear up (test with the motor in the grip and place the grip tightly on the one half of the gearbox shell) you have issues and need to fix it.

[Hectic]

Yeah I'd say it's a combo of the motor and perhaps a lil tight on shining also have you corrected aoe? You will want to do that for sure, stick on a sorbo pad and shave the second (first smaller tooth at the rear of the piston) and perhaps some of the third so the sector will pick up the piston first tooth to first tooth without any problems like catching up on the other teeth along the way. Bearing are more forgiving on tight shimming, are you using the standard shims or the smaller diameter ones, if the shims are the wide ones and the bearings are say 6mm you may be just running on the shell rather than the bearing depending on how deep the bearings are seated into the shell.

[Spike]

Is it a stock G&P gearbox shell? They have that stupid "reinforcement" material in the back that causes the shell to rub on SHS gears.

[cetane]

Originally Posted by Spike

Is it a stock G&P gearbox shell? They have that stupid "reinforcement" material in the back that causes the shell to rub on SHS gears.

KA shells too have the extra stuff that rubs 13:1's

[ao_tenshi]

Thanks for the suggestions. I've checked:
- sector gear and piston teeth match exactly so that is ok
- I do see the grip pinch on the bevel bearing a bit, will see if I have to dremel out, this could be a big factor
- this is a stock G&P shell, but did not see where the gears rub the shell... which area did you find will rub on the shs13:1 gears?
- will do AOE on this gun as soon as the sorbo pad comes in

[pestobanana]

Did you start your shimming from the bevel gear?

[ThunderCactus]

Sounds like you're either shimming your gears into each other so they're rubbing.
The loud grinding could be the gears rubbing together, or it could be the pinion gear is engaging the bevel at the wrong spot. Use the pinion-bevel method of shimming just to make sure your bevel gear falls within the range of the pinion.