Gearbox (piston) Seizure

[ao_tenshi]

Hey guys,

I thought I've got a solid build finally but looks like I'm back for more help again.

Symptoms:
- piston is seizing in semi (especially with lower voltage battery)
- about 1 or 2 shots out of 10 will shoot very weak, fall off after about 20ft
- minor but obvious stripping of the first few teeth on the piston
- in full-auto once a while I notice a screeching sound
- occasionally fired in full-auto on semi mode

Current Build:
G&P base gun
SHS HT Motor
SHS 13:1 gears shimmed
SHS Piston (half metal) AOE corrected
D40 Sorbo
16AWG Wiring
Spring: tried both M90 and M120 with same issue. Piston stripping occurred with M120 spring.
Battery: tried both 7.4V 20C and 11.1V 20C

Not sure if this is pre-engagement and/or over spin or something else. My initial thoughts are maybe I should short-stroke? Hoping to get some idea of the issue and how to resolve it from the gurus on ASC.

Thanks in advance!

[RainyEyes]

If the AOE was corrected, I don't believe the piston should be stripping at all. Listing your symptoms:
high-pitches screeching sound (on full-auto it is usually normal, not on semi, but depends on the level of sound)
stripping piston teeth
gearbox seizing
low voltage battery

All signs point to improperly shimmed gears/piston AOE.

Gears should make maximum contact with each other and spin freely with no load. Piston should be at about a 90 degrees angle of engagement with the sector gear. Sometimes, the 2nd tooth needs to be filed down if it gets in the way of the 1st tooth.

Did you personally work on this build, or was it another technician?

[ao_tenshi]

I worked on this build myself. So I wasnt sure if this could also be due to pre-engagement. I will check the AOE again as well.

[pestobanana]

Piston seizing, in what way?
Shots lobbing would likely be a nozzle-hop up alignment issue.
If the wear is on the few teeth next to the pickup tooth tou are getting pre engagement due to your previously stated feeding issues.
Screeching sound? Either your shimming is bad or your piston is getting fucked.
Occasional full auto in semi means your cutoff-trigger interface is not set up properly.

13:1 neo motor setups aren't just "slap them together" setups. Take your stuff to someone that knows what he/she is doing.

[EOD Steve]

I would test the whole system on semi only and address all of your issues there BEFORE moving to full auto as to avoid exacerbating them with rapid cycling which could not only introduce its own set of issues but introduce weaknesses in the system that compound the problem.

Seizing on semi but not auto could be due to worn or improper cut-off lever and/or mating between the cut-off lever and the trigger trolley. Swap out with replacement units one at a time until you get a matching set. Rabbit had a similar issue some years back that took a bit of time to diagnose.

1-2 shots weak - Check your piston for any abnormal wear (in this case PE) or premature release

Intermittent screeching sound - possibly due to improper shimming at the bottom of the sector causing contact with the spur gear, or some other combination of gears. OR your bearings are blown/misaligned and your gear axles are no longer parallel.

Full-auto in semi - check your selector plate tolerances - G&P is notorious for this and you may have to replace either your selector disc or the plate or both.

G&Ps... sigh.

[ao_tenshi]

Thanks for the suggestions will try to isolate the issues one by one and report back.

If its PE, what would be the ideal way to fix the issue? Only constraint is need to keep the FPS under 380 as this build is primarily for indoor use.

[EOD Steve]

Short stroke.
Or optimize piston travel via lower mass, free-travelling piston assembly.