SHS Piston and ACM Piston head incompatibility

Just a quick post that may be of use to someone who is going to repair an airsoft L85.

I recently needed to replace the piston and piston head on  an airsoft SA80 (an Army Armaments L85 to be exact) due to damage to the existing set. The teeth were stripped on the piston and there was a chunk taken out of the piston head.


The parts I chose to replace them with were an SHS 19 metal tooth piston and an ACM bearing piston head. The Piston was chosen as it was the cheapest one I could get that was suitable for the L85. A standard 15tooth piston will not work without a spacer as the gearbox design requires a longer piston. The piston head was chosen, as the cheapest decent part available.

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The piston head had a bearing set that allows it rotate as needed. This reduces the possibility of the piston going off center and therefore reduces risk of piston/gearset damage/failure. The bearing set goes on the inside of the piston and connects to the piston head on the outside via a screw.

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There was an issue on assembly though. The screw that is designed to go through the piston to hold the piston head in place, was too short. It would work on other pistons, but the SHS 19 tooth piston is thicker at the top and so the screw is just a little short. I was able to replace it with an M3 x 10MM which worked perfectly. The original screw had a countersink head but the thickness of the piston head was such that it accommodated the standard head on the replacement.

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Success! The assembly went back into the gearbox with no issue and has been reliable during testing and game time. Silicone grease was added to the oring before putting in – helps with airseal by ensuring the oring doesn’t dry out. Abbey Lt2 (silicone grease with added molybdenum for heat resistance) to the piston teeth.

It is important to use silicone based grease as petroleum based oils/grease will “eat” the rubber seals. Keep an eye on the components of the lubricant as sometimes they will mix in some oil based lubricant. I personally avoid the aerosol sprays as you can get better placement with the raw oil/grease and makes certain that you are not getting any extra ingredient you didn’t want.


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