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Piston short stroking is a well-known technique that helps increase ROF. The logic is simple: decrease piston
cycling time by removing teeth from the piston and the sector gear.
Advantage? Less stress on the gears, the motor, and the battery. Gain?
One sixteenth rps per every tooth removed. Loss? 15-20fps per tooth
removed.

 
Short stroking is done by removing teeth from the piston and the matching teeth from the sector gear.

 
First, the piston.

 
After removing the metal bar with the metal tooth, remove as many teeth
as you want. Three teeth, the minimum  accepted number, was removed from this particular piston.

Then cut out the center part of the piston underside to make way for the metal insert.
 

Next, cut out a channel along the sides of the cut-out middle so that
you can move the metal insert to butt against the intact tooth.
 

Next, cut the retaining right angle off the metal insert.
 

Last for the piston part is making a metal extension to cover the gap exposed by moving back the metal piston insert.
 

 
Be sure to super glue the metal insert and extension to the plastic channels.

 
Now for the sector gear.
 

Remove the corresponding sector gear teeth with a rotary tool or something suitable.
 

 

Smoothen the curve with a file and some sandpaper.
 

And you're done!
 

 

Be sure to check piston and gear teeth engagement.
 

As additional measures, you can also lighten your piston, polish the rails on the mechbox body where the piston rails ride, polish the internal surface of the piston, and bevel the piston mouth.

If the brand of the piston you use for the upgrade is different from the brand of the gearset and/or the piston head, you may encounter teeth engagement issues. Piston head shims/spacers of appropriate thickness/number may then be used to get proper piston teeth to spur gear teeth engagement.  This is particularly  important for the first or catch teeth of the piston.

In high ROF applications, it is also usual practice to remove all or part of the second and third tooth to prevent premature teeth engagement.

 
Many thanks to Soulman of ACMdotCom fot the pictures.