View Full Version : Better sector gear mesh with the piston

04-19-2008, 10:11 AM
Sick of ripping off the back pick-up tooth of pistons in that typical "half-moon" failure? Fairly typical failure on high ROF and high-power guns like support weapons which I encountered often back in the TOP M249 with PGC mechbox days, which was not fun burning through pistons.....and most of the time it was always that same half-moon failure. Ever notice the nipped teeth on pistons from improper clearance from the sector gear rotating around and interfering?


It is directly related to how the sector gear engages the piston in typical stock position. Don't ask why they have it that way, I don't know. Perhaps it has to do with how companies just copied each other all these years, so it has carried over and is just the "standard". However, it is easy to tell that it isn't the optimal engagement for gears to make contact and causes unnecessary stress. Some of us have been doing this to guns for quite some time, and typical of mechboxes it is just one of those things to take for granted and not think about, especially since most people don't run support weapons. :doh:

Here's the typical engagement you'll see in a mechbox:


See how the sector gear tooth is hitting at an angle? Not good, as you want it to get a better parallel surface-to-surface mesh.

To do that, add washers or some sort of spacers between the pistonhead and piston. You'll need to trial and error it to get the better engagement, which you can see an example of farther below. You may need to use a longer pistonhead screw as a result, depending on the number of washers because pistons and pistonheads are not standard lengths in airsoft. Wheeee. You want the washers to be as close to the diameter of the piston body as you can get them without overlapping obviously so they don't catch on anything as well as wanting to spread out the force on the piston face from tightening the pistonhead screw down. Make sure the screw has enough threads to engage the pistonhead threads and always use loctite so it doesn't come loose.
There are two washers in this case to get the mesh desired:


That results in a better mesh as seen below (sorry, I need to take a better straight-on picture.....this mechbox is already assembled so I'll get pictures on the next one I do). You'll also see the shaved 2nd piston tooth so the sector gear can clear it. Hold the piston forward while you rotate the gear to make sure it clears those 2nd and 3rd teeth. Shave them down as needed.


Even if you don't use the spacer method, which is entirely fine since guns have run for long times like that (over 30,000 rounds with quality parts) on the typical low spring strengths and average ROFs most use on regular AEGs, be sure to always check the sector gear interference with that 2nd piston tooth. You'll often see that tooth and the 3rd one nipped and worn down on the backside because of this. You should enjoy how seemingly low-regarded pistons will hold up.

For a longer, more complete read on the same subject:

04-19-2008, 01:44 PM
Interesting read, Wolv. Do you know if this issue applies only to the V2 mechbox, or does it apply to all mechboxes?

04-19-2008, 03:30 PM
It needs to be checked on each and will vary based on what parts are being used. Parts from different brands are slightly different even if they are intended to do the same thing.

04-19-2008, 09:00 PM
This a lesson Im learning the hard way. It seems as been stated everyones copied each other but along the way some parts have varied over thier tolerances from one company to the next and so stuff marked Marui verision this and that compatible etc... might not actually be all that compatible.

Thanks for these observations and sharing the knowledge with the rest of us.

04-20-2008, 08:16 AM
This is especially good info given the fact that the only piston that I know of that won't give the "half moon" failure, the Prometheus hard piston, is becoming increasingly difficult to locate. WGC isn't stocking them anymore, and Redwolf has been out of stock for months now.

Maybe this will prolong the life of some of the less-expensive pistons out there.

04-20-2008, 07:14 PM
Ehobbyasia has the hard piston in stock. And this sort of thing applies to high FPS setups as well, it seems like its mostly meant for high ROF guns.

04-21-2008, 08:20 AM
I was about to add a comment on this subject but Shak mentioned it first. I have a Prometheus hard piston and it has added reinforcement by a small 'wall' along the sides of the piston teeth. I considered making this modification, but decided against it because it *looks* very solid to me, and I don't hang on the trigger much. I hope this doesn't come back and bite me somehow, but at the moment I didn't see need for this mod. Thanks AW for posting this, I'll consider this for the other regular pistons I have.

11-08-2008, 11:24 PM
Kudos to VFC, as their stock parts in their HK416 offering exhibits this concept. Check it out here (http://www.lonestarairsoft.com/forums/photopost/showphoto.php?photo=6149&cat=500).

11-09-2008, 08:10 PM
Same with their VFC AK74s. Opened mine up a while ago to that same pleasent suprise.