VFC Scar H gearbox lockup issue.

[Varjeal]

Greetings.

Background:

Recently, my VFC scar H stopped feeding suddenly. Upon disassembly I found the usual suspect, a broken tappet plate arm. Since the piston was worn as well I decided to give the gearbox a good servicing.

I cleaned, shimmed (got rid of the self-shimming springs and did the pinion to bevel method), rewired with teflon wires, replaced the partially stripped selector plate (my fault on that one) replaced the tappat plate, and replaced the piston with a Madbull green full metal tooth piston and aluminum piston head. (have used it in the past in other guns with no issues.) AOE was good so I went ahead and put it all together.

Problem:

The gearbox now locks up with the piston all the way to the rear on the very first pull o the trigger. I did notice the spring guide bearings are very thick, and I'm wondering if the current piston is long enough where it is hitting that guide and the sector gear is unable to make its full turn. (can't really see whats happening when its stuck like that.) Also noticed the last tooth of the sector gear is thicker than all the rest...I'm assuming its designed that way because of the load???

After removing the motor I can use a small screwdriver to retract the ARL and it zips back fine.

I've tried viewing the cycle with gearbox open and spring out manually turning the gears and it seems to get very close but clear....maybe too close?

Ideas? Suggestions? Thanks in advance.

[lurkingknight]

with just the gearbox shell and the piston/cylinder assembly in... does the piston slide up and down the full length freely?

[Varjeal]

Yup. Checked.

[Varjeal]

After noting where it locks up, I've now re-opened the gear box and, minus spring, rotated the gears until it gets to approximately the same location. It locks up with the last two teeth of the sector still engaging the piston, but everything still seems pretty smooth...puzzling.

Any other suggestions or things I should look for?

[Stealth]

So with ONLY the gears in the mechbox closed, do the gears spin freely?

[Varjeal]

Yes they do. Very little side to side play and spin probably 4 times before slowing significantly.

[MaciekA]

Assuming your motor/power source/etc doesn't suck balls, and assuming your shimming is okay, there could be another source of binding that you're just not aware of yet.

With those assumptions out of the way, some oddball sources of binding include:

- The piston itself against the gearbox rails. Sometimes pistons have bad runs and the last or first guide rail on the piston is very very very slightly off-angle. If you don't have a strong motor this can lock your gearbox. If you have a very strong motor and 100A+ of power to give, this can produce bizarrely slow cycling or low fps.
- The piston binding against the sector gear, and for some of the following reasons: rack positioning, really bad AoE, teeth spaced apart wrong, fundamental incompatibilities between a set of gears and a piston (common), etc), large-size shims which once you have everything together bind against gears, etc.
- The piston binding against the sector gear for the reason that the sector gear is shimmed too high (i.e. too far away from the cutoff lever).
- The sector gear binding against the tappet plate for similar reasons to above
- The sector gear binding against the tappet plate due to the sector delayer allowing the tappet to migrate between the delayer cam and the gearbox.

Hoo boy, I could go on...

Given that your tappet plate broke, it's entirely possible your sector gear is too high for that tappet plate. An easy test involves taking out your compression parts but leaving in the tappet + gears + tappet spring and just making sure the gears can turn a full cycle without getting hard-bound, and also ensuring the tappet moves freely. Try various combinations of gently guiding the tappet plate inwards while cycling your gears with a tool and leaving the gears alone while pumping the tappet in and out.

Also, with your cylinder+head in the gearbox, but without the spring and without the spring guide, try putting in your piston and closing up the gearbox and letting it fall up and down the path of its travel as you flip the gearbox up and down. Here's a video of me doing the same test for a piston Stealth had me test:

New SHS piston batch fitment testing - YouTube

For the above test in the video, make sure you have your piston head + o-ring assembled on the piston, that's the only way to know for sure that your piston rails are simulating real world conditions of alignment.

For ALL the above tests, make sure your gearbox is fully closed and tight.

Keep testing and posting here and eventually we'll narrow it down. This usually comes down to ONE little annoying part.

[Varjeal]

Originally Posted by MaciekA

Assuming your motor/power source/etc doesn't suck balls, and assuming your shimming is okay, there could be another source of binding that you're just not aware of yet.

With those assumptions out of the way, some oddball sources of binding include:

- The piston itself against the gearbox rails. Sometimes pistons have bad runs and the last or first guide rail on the piston is very very very slightly off-angle. If you don't have a strong motor this can lock your gearbox. If you have a very strong motor and 100A+ of power to give, this can produce bizarrely slow cycling or low fps.
- The piston binding against the sector gear, and for some of the following reasons: rack positioning, really bad AoE, teeth spaced apart wrong, fundamental incompatibilities between a set of gears and a piston (common), etc), large-size shims which once you have everything together bind against gears, etc.
- The piston binding against the sector gear for the reason that the sector gear is shimmed too high (i.e. too far away from the cutoff lever).
- The sector gear binding against the tappet plate for similar reasons to above
- The sector gear binding against the tappet plate due to the sector delayer allowing the tappet to migrate between the delayer cam and the gearbox.

Hoo boy, I could go on...

Given that your tappet plate broke, it's entirely possible your sector gear is too high for that tappet plate. An easy test involves taking out your compression parts but leaving in the tappet + gears + tappet spring and just making sure the gears can turn a full cycle without getting hard-bound, and also ensuring the tappet moves freely. Try various combinations of gently guiding the tappet plate inwards while cycling your gears with a tool and leaving the gears alone while pumping the tappet in and out.

Also, with your cylinder+head in the gearbox, but without the spring and without the spring guide, try putting in your piston and closing up the gearbox and letting it fall up and down the path of its travel as you flip the gearbox up and down. Here's a video of me doing the same test for a piston Stealth had me test:

New SHS piston batch fitment testing - YouTube

For the above test in the video, make sure you have your piston head + o-ring assembled on the piston, that's the only way to know for sure that your piston rails are simulating real world conditions of alignment.

For ALL the above tests, make sure your gearbox is fully closed and tight.

Keep testing and posting here and eventually we'll narrow it down. This usually comes down to ONE little annoying part.

1. Power source, etc. check. I'm using a fully charged battery and a proven motor.
2. Shimming. 100% sure shimming is good. I take a long time to shim a mechbox, and I test each gear seperately and paired with the others with the mechbox tightened up inbetween, then all three at the very last. As mentioned previously at least 4 full revolutions of free spinning before lubrication.
3. Piston. Any testing to do with the piston I'm willing to look at as its the one part I don't have a spare of (next to the cylinder head and nozzle) to try swapping out at the moment. Its a madbull green full metal tooth piston but the side rails are not one continuous rail but broke up into sections, so your concerns are legitimate and I'll be performing the tests you mentioned.
4. AOE looks proper as does the teeth meshing, the original piston did have a shredded 4th tooth in it however.

I'm going to keep this thread updated with my progress. Thanks all for the assistance thus far.

[Varjeal]

Success!!...well..sorta...the piston is indeed the issue. The "rails" on the piston are simply too tall. Had to back out the screws about 3-4 full turns before the piston would drop down like the one in your vid....question is now whether to order a new piston or try modifying this one.... Anyone with a VFC Scar have a preference? I'm looking to squeak close to 400fps and a standard ROF, nothing crazy.

Sorry lurking, I misunderstood your assessment you were correct right off the bat.

Thanks guys...another lesson learned. At least this one didn't cost me, lol.

[MaciekA]

The SHS blue or the Lonex pistons should work fine in your VFC mechbox.

I'm surprised the Madbull piston doesn't work though... Good to know.

[Stealth]

Originally Posted by MaciekA

The SHS blue or the Lonex pistons should work fine in your VFC mechbox.

I'm surprised the Madbull piston doesn't work though... Good to know.

Lonex yes.
SHS 15 teeth - not completely. VFC rails seem to be fatter than others.
SHS 14 teeth - doesn't budge.

[MaciekA]

Originally Posted by Stealth

Lonex yes.
SHS 15 teeth - not completely. VFC rails seem to be fatter than others.
SHS 14 teeth - doesn't budge.

Weird. I need to go review which piston is in my VFC gearbox, I'm almost certain it's an SHS blue. It would be one I acquired a while back though.

[Varjeal]

Well thats good to know before I order, lol. I should probably upgrade the cylinder head as well...any recommendations for that? Kinda stuck with the stock air nozzle as its proprietary (ie. looong) unless ya know of someone who makes those too other than vfc.

[Stealth]

Originally Posted by MaciekA

Weird. I need to go review which piston is in my VFC gearbox, I'm almost certain it's an SHS blue. It would be one I acquired a while back though.

That's the case on my VFC Hk416 anyway. Maybe they recently made a change? More likely SHS made a change... Sigh

SHS 15 teeth seem to work fine in Marui and Lonex boxes though.

[MaciekA]

Originally Posted by Stealth

That's the case on my VFC Hk416 anyway. Maybe they recently made a change? More likely SHS made a change... Sigh

SHS 15 teeth seem to work fine in Marui and Lonex boxes though.

Also G&G, G&P and a couple ACM gearboxes