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Short Stroking & Swiss Cheesing – Part 1

Today we have a little update on a current staff project M4 build. Myself [karl] has nearly finished the project internally but today I have been playing with the internals some more.

This is not a 100% how to guide but a bit of information on what I have learnt from short stroking and swiss-cheesing plus the results.

So why bother short stroking and swiss-cheesing?

sector gear 1

Short stroking is basically removing teeth off both your sector gear and piston rack. Less teeth means your piston is released sooner resulting in a quicker single shot and usually noticable increase to your rate of fire.

As for swiss-cheesing, this removes weight from your piston allowing for it to move forward sightly faster. This usually gives you a couple of rounds per second extra. Different people swiss-cheese pistons to different levels depending on your confidence and also how strong the piston actually is.

swiss 3

So how is it all done? With a dremel! This is your best and fastest option.

First though a few warnings. When you short stroke you are essentially removing 10+fps per tooth you remove and it is advised usually not to take more than 4 teeth off. People usually just do 2. As you are loosing quite a bit of FPS you then need a bigger spring to compensate for your FPS loss. Usually you would fit an m110 or m120 to compensate.

With a bigger spring this means your whole gearbox is now under a load more stress. It is usually advised to also have a quality high torque motor, strong gears and a full metal rack piston or atleast 4 to 7 metal teeth on your piston.

You also seriously run the risk of a piston or gears shredding so it is highly important to use quality parts and to have your gearbox well shimmed!

So how did I short stroke?

I opted to take 4 teeth off the sector gear, remember take off the first 4 teeth that make contact with your piston.

sector gear 1

You then need to take off the last 4 teeth of the piston rack. Always take off the same amount of teeth off both components.

Just buff the gear lightly to make as clean as a finish as you can. With the SHS 13:1 gears I have used ensure the screws are also tight after as heating up the gear removing the teeth can undo the thread lock holding your gear together!

sector gear 2

As with the sector gear I removed the piston rack, placed in a vice and removed 4 teeth from the back.


And that is it! Try to keep the cutting as clean as you can, go over with a file and remember you can't add teeth back! So ideally mark before you start cutting.

all short stroked


Now onto swiss-cheesing.... I used the dremel once again to make the largest slot as shown below.

swiss 1


swiss 2

I then went and added more holes with a drill attachment.

swiss 3

Remember to cut and clean off excess plastic. I did this with a craft knife and also a normal AEG spring to cut out and clean out the excess plastic inside the piston.

Once all cleaned and assembled here is the final result.

swiss 4

So what's next? Simple! Put it all back together and go chrono your results. You may have to experiment with different springs to get your desired FPS or cut a coil or two off.

What's next for my build? The inside of the gearbox will be polished. I need to cut 1 coil off as I am getting just over 350fps with an m120 spring, pro-win hop, Mad Bull Blue rubber and standard inner barrel.

Part 2 of this blog will show a before and after result video too of the gun being fired and polishing guide once I buy more dremel bits.

Written by Karl, who has written a total of 202 posts.

Posted in Patrol Base News | No Comments

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