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My First Time Opening a Gearbox (VFC avalon gen 2)

9890 Views 49 Replies 8 Participants Last post by  Guges Mk3
Howdy. I'm brand new here and thought I should share my latest repair experience. I will attach photos with some commentary/questions in the replies. I will continue to update this thread as I continue work on my RIF.
This gun was having semi-auto lock-up issues since brand new, not terribly common, but usually at least once per day of play. Finally felt confident enough to open up the GB and found a tooth broken off on the spur gear. I decided SHS 16:1 gears (same ratio as stock) would serve my needs just fine and be usable for a ~400 fps build later on.
During shimming, I broke a screw off in the GB shell and failed to extract it, so I will revisit that some other time (drill and re-tap threads).
I bought a new Lonex GB, which I found does not drop right into the avalon body, it seems to get hung up where the motor grip slides over the GB shell, towards where the trigger would be when fully assembled, as well as towards the back near where the wires pass through to the buffer tube.
Also had to get a new spring guide since the avalon has a quick change design that cannot be used in a standard gearbox.
Thank you for reading and I definitely wouldn't mind if anyone offered some insights on what I'm working with XD.
Air gun Trigger Machine gun Sleeve Wood

Update: I've added a summary of the work I've done to this RIF in the replies. Please be sure to read through the rest of the thread or do some research before asking questions here! Here is the summary:
Well then, since that's settled, here's the summary of the effective job completed on my VFC Avalon MK18 as I understand it (not in any particular order):

Problem: Broken spur gear tooth
Solution: Replace with SHS 16:1 (stock ratio) CNC steel gears, and shim
Observations: This gear broke VERY soon in the gun's life, perhaps a lemon. I accidentally messed with motor height before I knew what I was doing. The top of bevel gear was scraping on the inside of the GB shell at some point. No other evidence to suggest fatal motor height issue.

: Trigger locking up on semi-auto, feels as if on safe. This problem was present since brand new OOTB. Eventually progressed to firing full-auto in semi.
Solution: New, higher quality trigger switch assembly. Unknown brand. Black in color, unknown material, good quality casting/machining. Labeled SHS from my local shop, but I've only seen red V2 units from SHS and they look of lower quality than this one. To maintain the original VFC MOSFET, I de-soldered everything from it, unscrewed the plate holding the trigger contacts in the unit, then carefully separated the MOSFET from the contact assembly with a small flathead screwdriver. Once removed, I VERY CAREFULLY sanded off the old, dried superglue*, placed the trigger contact unit in the gearbox, and got an idea of how I wanted the MOSFET to sit. I put the small body pin in place that would normally be there once the gun is fully assembled. I then proceeded to apply super glue quite sparingly, spreading it around the contact assembly surface without getting too close to the edges in order to avoid squeezing it out into places I didn't want the glue, then pressed the MOSFET into place and held it there with good, even force for about 2 minutes. Re-soldered it back together. Good as new.
Observations: It would be a very good idea to take pictures of anything wiring related to be sure you put everything back the way it should go.
*Be VERY careful when doing this, as you may sand away silicon and expose the leads in the PCB. Not good. If you mess this up, you could still cover it with super glue, but you may have added some resistance to the circuit if you removed any of the electrical lead.

Here's a secondary part of the job that was due to my own errors and/or having used a different gearbox shell:

Problem: Broken screw stuck inside VFC "ECS" Gearbox shell
Solution: Replaced with Lonex Gearbox shell -- 8mm ball bearings and proprietary tappet plate included. Failed to extract screw from VFC shell.
Observations: VFC cylinder has a tough time fitting in the Lonex shell, as does the trigger contact unit. You really have to muscle these pieces in there for them to work. I do not recommend using these trigger contacts in this scenario. I suspect if I went with another VFC gearbox, there would have been significantly less headache with this job. VFC seems to like not doing things to Tokyo Marui spec, or perhaps their tolerances/QC are ****, or my particular unit just happened to get the **** end of the tolerances on a bunch of parts. It's truly a mystery to me. I wish I had just gotten a complete Lonex gearbox, or a VFC ECS shell.

: Low FPS (~150 FPS on Lonex SP100 spring with either .2g or .28g BBs -- got mixed up in testing) -- nozzle not moving during test fire, unable to be pushed back or pulled forward.
Solution: Used a Dremel to cut down the sides of the tappet until it had enough space to move the way it should. I ended up removing about 1mm on each side, nearly flush with the recessed portion of the tappet plate that Lonex cuts out in order to fit around the extra reinforcing material inside their gearboxes.
Observations: Very strange to me that this tappet plate did not glide freely in the gearbox it was designed to work in. In fact, it didn't move at all when assembled, it would get pulled back by the sector gear, then stay seized in place by the gearbox.

Problem: Feeding issue
Solution: WIP
Observations: Initially, it was feeding ~8/10 shots at the beginning of my latest game day (Sunday, June 28), by the end it only fed ~1/2, sometimes double feeding.
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Lonex GB shell fitment with VFC avalon lower receiver
Might want to pin both the rear takedown and gearbox/trigger to see fitment without the gearbox moving around freely in the receiver, especially if you are comparing grip fitment

Lonex GB came with this set of hardware. 3x flat top long, 1x button top long, 4x button top short. I think there's an error here from evike or lonex because there is only one screwhole with a conical shape that matches the flat top screws. Also, I'm not entirely sure why but 5x locking washers? Are these necessary? Why 5?
Mine came with strange hardware too, I doubt it matters but you can always get a proper set from the hardware store. I got screws in stainless to match, don't forget to loctite them in

I picked up some other various parts to try out. The gold colored aluminum piston head was $4.50 on and it seems to fit similarly in my cylinder as the stock plastic one. It was branded as "SCG" on the website. I assume the smoothed-out shape going into the nozzle can at least attribute to some better airflow. Can this produce any noticeable differences?
I don't know if it makes a difference but it certainly appears it will in theory. I countersink the cylinder head openings in my higher end builds.

I also got a madbull m100 spring to drop the power to meet regulations for the local indoor field. The coloring on the spring flaked everywhere inside the gearbox, so I'm not a fan of that. Otherwise, the quality of the spring seems fine. I ordered a lonex sp100 spring to compare.
I recommend also getting a Prometheus and/or Guarder spring to compare. I have not used Madbull and the one Lonex spring I bought I think is off spec or mislabeled

Any insights as to how well rubbers can function based on rotary/traditional wheel style hop-up dials? I thought I heard somewhere that rotary dials have a harder time staying in place, so stiffer rubbers can push them out of place easier?
Rotary style usually has notches that the rubber will hold to. In my experience traditional dial hop units have more of a tendency to drift as they rely only on friction.
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I wasn't able to pin the lonex gearbox into the vfc lower, that was the point of the photo. I had to grind material away in order to get it to fit.
Looking back at the photos now that makes sense. I would have left the motor grip off to better show the alignment of the shell.

Here's a video demonstration of the issues I'm having with semi-auto.
It sounds like your anti-reversal latch is missing or not engaging properly (spring slipped off maybe). The safety lock might be due to the alignment of the safety mechanism being off, make sure it's not binding on anything and the two parts are keyed together properly. All of the safeties I've seen are fairly soft so they can rotate internally which throws off the angle between the selector plate and the trigger stop.
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Ooh yeah that's not good. I would inspect the gearbox carefully around the ARL socket areas to make sure they aren't damaged as well.

ARL brand I've never noticed to make a difference, they aren't supposed to hold a heavy weight on their ratchet after all.

The cutoff lever isn't supposed to stop the gears from spinning, it's supposed to kick off the trigger contact trolley from the trigger itself and thus disconnecting power.
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It's possible that the trigger components are out of spec, it's also possible that the trigger contact assembly is mounted off spec as well. I'd loosen the screw holding it down and see how much play in all directions it had compared to how it was assembled. I've had a gearbox give me nasty problems with trigger tolerances, tried mixing and matching a whole bunch of brands of parts in order to get it to work.

I've also incorrectly installed the cutoff lever before, it was not interacting with the trigger trolley correctly. Externally, the cutoff lever would not disengage in full-auto correctly - like it would stop moving a couple mm early. I remember this clearly since the symptoms were just like yours, full-auto in semi. Maybe you can try confirming proper cutoff operation with the gearbox open and installing just the cutoff lever/selector/spring, trigger contact assembly, and trigger/spring.
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I shot a quick vid for function testing the cutoff lever without too many components installed, does your gun pass this? Notably try doing semi/full auto selector plate engagement with the cutoff lever without the gears in, trigger should pull cleanly on both modes. If the cutoff levers are sticking halfway that can affect trolley movement, you might need to shim the lever upwards
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I felt those systems can be quite fragile, relying on being held in place with only one screw. I was having visions of the unit jiggling in the gearbox and causing unintentional discharge, or getting wet or something and being out $150.
While it's true there can be mechanical clearance issues (sector gear, trigger) that lead to damage, I think it's highly unlikely. Sensible greasing, and not dunking your AEG in water or playing in rain will mitigate liquid-based damage as I think majority of us all do with any AEG. The screw is mainly there to lock in X-Y axis movement, the top portion of the Titan has a compression fit with foam to the top of the gearbox to prevent Z-axis movement, I have never had issues with mine moving around while in use. The wires are also extremely stiff and I would assume help with keeping the ETU itself immobile in the shell.

Frustrating problems. The cutoff lever is the biggest problem in my opinion with airsoft gearbox designs and I'm glad the computerized systems these days are mitigating how essential it is for operation. It's way too sensitive and sometimes temperamental for how important it is.
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I'd recommend fixing the alignment rather than trying a fix that doesn't solve the underlying issue.

The grip looks tight on the replica, you should probably triple check to make sure that tightening down the grip doesn't alter the alignment due to how it fits in the body/gearbox. Shave down the motor grip where it contacts the lower receiver, ideally you want a barely visible space between the two.
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