Short stroking the sector gear without removing piston teeth or fitting a heavier spring

[Rogerborg]

Before anyone goes all Reddit Skylaar, here's what I'm starting with: an MP5K with a 110mm-ish barrel, a cheap plastic rack with (I think) one metal tooth at the end, and a 3/4 cylinder which is presumably significantly over-volumed and producing a significant amount of smack and crack noise (IIRC it was fitted as 1/4 as stock and I just flipped it end-for-end for funsies).  It's shooting bang on 1J with an M100 spring, and is used as a CQB primary or woodland secondary for close encounters. I'm looking for a little better response and ROF (it's already on a Big Dragon M140, thicc wiring and trigger protection / current flow mosfet, and can't fit any 11.1V battery that I'd actually want to use).

Now, here's what I'm thinking: if I leave everything as-is, and only remove 2 teeth or so from the pickup side of the sector gear, what could possibly go wrong?

From the point of view of the piston, it's not going get pulled any harder than it currently is, at any point in the cycle.  The gear won't reach the final metal tooth, but the plastic teeth won't experience any more stress than they currently are.  It'll just release earlier.

The energy loss should be minimal given the over-voluming. If anything, short stroking should get the volume down closer to where it should be, and BB energy will be a question of how fast the air is being pushed, rather than how much of it.

I'm thinking that the worse case is that comes out weedy, and I throw in an M110 or M120 until it eats the plastic rack, then I replace that as well.

Any glaring flaws in that plan?  Anyone care to predict the results in terms of energy or longevity?

 

[ak2m4]

Technically I think the plan is sound, should be less noise from the over-volume with a small increase in rof and trigger response.  Looking forward to seeing the results.

 

[Speedbird_666]

Before anyone goes all Reddit Skylaar, here's what I'm starting with: an MP5K with a 110mm-ish barrel, a cheap plastic rack with (I think) one metal tooth at the end, and a 3/4 cylinder which is presumably significantly over-volumed and producing a significant amount of smack and crack noise (IIRC it was fitted as 1/4 as stock and I just flipped it end-for-end for funsies).  It's shooting bang on 1J with an M100 spring, and is used as a CQB primary or woodland secondary for close encounters. I'm looking for a little better response and ROF (it's already on a Big Dragon M140, thicc wiring and trigger protection / current flow mosfet, and can't fit any 11.1V battery that I'd actually want to use).

Now, here's what I'm thinking: if I leave everything as-is, and only remove 2 teeth or so from the pickup side of the sector gear, what could possibly go wrong?

From the point of view of the piston, it's not going get pulled any harder than it currently is, at any point in the cycle.  The gear won't reach the final metal tooth, but the plastic teeth won't experience any more stress than they currently are.  It'll just release earlier.

The energy loss should be minimal given the over-voluming. If anything, short stroking should get the volume down closer to where it should be, and BB energy will be a question of how fast the air is being pushed, rather than how much of it.

I'm thinking that the worse case is that comes out weedy, and I throw in an M110 or M120 until it eats the plastic rack, then I replace that as well.

Any glaring flaws in that plan?  Anyone care to predict the results in terms of energy or longevity?

View attachment 91084

What's the worst that's gonna happen? A slight drop in FPS perhaps and a trashed piston? 

There will be a lot or stress on the last plastic tooth though when the final sector tooth releases it. For the cost of a full-metal racked piston I would replace at the point of short-stroking the sector gear. 

 

[Badgerlicious]

Could end up with the front tooth of the piston running into the remaining teeth on the sector gear. 

 

[Skara]

Could end up with the front tooth of the piston running into the remaining teeth on the sector gear. 

That is not possible unless you short stroke the piston but not the gears.

On all my guns I removed two teeth from the sector (pick up side) and left the racks intact, no PME, no smashed front piston teeth, nothing.

 

[Adolf Hamster]

whilst it is over-volumed, short stroking will still result in a drop in energy.

any piston travel before the compression stage will still be contributing to energy as the piston accelerates gaining kinetic energy which gets dumped once compression starts (essentially changing the pressure curve to one with a sharper ramp up)

as others have mentioned, having the release tooth being plastic rather than metal might cause wear/failure, but it's not the worst to change out aside from cleaning up the box if it goes.

keeping the remaining teeth on the piston is fine, tbh aside from the slight weight saving there isn't strictly any cause to remove them, although it feels neater having matching teeth.

 

[Rogerborg]

There will be a lot or stress on the last plastic tooth though when the final sector tooth releases it.

Ehhhh, will there though?  I'm trying to feel it out in my head.  What it seems like is that even if multiple teeth appear to be engaged at once that in imperfect reality most of the stress is going to be on one of them.  I can't see that there will be a lot more stress on the 3rd-last tooth - it's not going to get pulled any further or harder than before. If the argument is "ah, but the 2nd-to-last tooth was just about to pick up some of the stress", well, yes, but then that (also plastic) tooth was already receiving and surviving more stress anyway, before the gear and rack got to the final metal tooth.

I'm gambling that plastic racks aren't as fragile as we might imagine.

For the cost of a full-metal racked piston I would replace at the point of short-stroking the sector gear. 

I probably will do at some point, I just fancy sacrificing the existing piston on the alter of pseudo-science first.

On all my guns I removed two teeth from the sector (pick up side) and left the racks intact, no PME, no smashed front piston teeth, nothing.

... or maybe not sacrificing.

whilst it is over-volumed, short stroking will still result in a drop in energy.

Care to take a guess?  I'll wager just 0.05J from 2 teeth, because I'm a wild optimist.  I'll be re-greasing and fitting a BS910 24.1mm piston o-ring while I'm in there but will otherwise leave as-is (unless I don't).

 

[Adolf Hamster]

Care to take a guess?  I'll wager just 0.05J from 2 teeth, because I'm a wild optimist.  I'll be re-greasing and fitting a BS910 24.1mm piston o-ring while I'm in there but will otherwise leave as-is (unless I don't).

nope, that's what the box o' springs is for

 

[Rogerborg]

I'd have lost my wager because I tested at 0.95J hopping 0.28g before, and I'm down to a disappointing[*] 0.8J now.  The trigger response and ROF doesn't feel hugely better either from removing 2 teeth.  [Sad womp-womp sounds]

Although I suspect it was an error to swap the already well stretched and greased piston ring for a "better" one, and introduce another variable. I'm not completely happy with the air seal, and it was pretty variable for a while.

Well, I'll chuck more silicone up the nozzle, run it as a sniper secondary for a bit then replace all the things again and see how well the plastic rack stood up to it.

[*]For everyone except Tokyo Marui true believers.