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

9891 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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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 tend to hold position better, though it's possible for there to be exceptions.
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The Physical Properties of Fluid Dynamics proves that the part that is important is only the cylinder head, cylinder and piston with piston head. Air takes the path of LEAST resistance. Air will NOT move down and back in a system that is an "open" system which is what an AEG is operating under.
Ummm. Except that I've already demonstrated on multiple AEGs that this isn't the case. One of the issues with your supposition is that when it fires, the BB causes resistance as long as it's in the barrel (which is why I'd argue that it's a closed system, but that's less relevant). This resistance is pretty shortlived, but it's there nonetheless. This is lessened in systems with properly fitted nozzles(honestly, not aware of any companies that fall in this category other than TM), but is still present.
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Cyma OEMs more than just AKs (I can't speak for the platinum series though).

Personally, I'd just rethread the original shell. It'll be easier in the long run.
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Daihatsu is owned by Toyota.. and the last Scion I jump-started sure looked like a Toyota under the hood.. So are you saying that a division of Cyma makes the MP5's?
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