In my previous article I checked out some airsoft parts from Russian Airsoft parts manufacturer, Combat Union. Overall I really appreciated the attention to detail reflected in their work.  Although there were some fitment issues, I really liked the look and set out to prep them for installation.

Smooth Travel

The first piece I was eager to adjust was the piston body.  Originally I was having trouble getting the piston to move along the gearbox rails freely.  My solution to this was to buy a tiny file, mod it to fit the gap on the piston body, and remove tiny bits of material until they ran along smoothly.

Filing airsoft piston body.
Filing piston body tabs to fit.

Weight Reduction

I like to keep the bearings off of the piston heads.  The extra compression they provide translates to a bit more muzzle velocity, but I prefer to keep the piston assembly as light as I can which necessitated their removal.

Bearings not included
Bearing removal on piston assembly.

Feeding Trouble

The cylinder head unexpectedly gave me the most trouble.  There were three issues I needed to deal with here which ultimately prevented me from using it altogether.  For starters, there was a possibility of it rotating in the gearbox causing drag on the tappet plate.

Cylinder head rotation
The cylinder head rotating off-center can cause tappet plate drag.

In this case, there’s not much to do to get around this.  Once the tappet plate is installed, it will act as a post that centers the cylinder head in the right orientation.

Next, the cylinder head’s nozzle tube unexpectedly had a lot of drag towards the deeper end.  Both the cylinder rotating and drag on the Air Seal Nozzle, or ASN,  are problems that could cause feeding issues in high rate of fire builds.  This is why you need to minimize sources of drag as much as possible.

To address this I would use some really fine sand paper to take off some material from the high spots of the cylinder head nozzle.  You have to be really careful though, as excessive sanding might damage the seal between the ASN and the nozzle on the cylinder head.

Prevent Cracking

On a typical v2 gearbox there is about one millimeter of rotational play in either direction due to a slightly loose fit.  The ICS gearbox has smaller pegs to retain the cylinder head.

Cylinder and gb comparison
Holes are bigger on the cylinder head than the gearbox pegs.

The small play experienced by a standard v2 gearbox is accentuated by the added void in the ICS version and promotes an inconsistent delivery of impact forces.

Loose cylinder head
Cylinder head to gearbox gap will direct greater force on the retaining pegs.

If we really wanted, we could fill the gaps with some sort of filler in the form of epoxy, silicone or gasket sealant, and maybe even clay.  However, this DIY approach can make it difficult to service the AEG repeatedly because you’d have to redo the process when swapping out a new cylinder head.

Stock ICS cylinder head on the left compared to the Combat Union offering on the right.

This isn’t a problem with Combat Union’s part and is more of ICS’ design being non standard.  Newer versions of ICS’s split gearbox has updated the tabs to better conform to the standards and added bigger tabs.  However I do not have any on hand and for this reason I will have to deploy the stock or other alternative cylinder head in the meantime.

ASN O-Ring Re-Seat

The o-ring in the Combat Union ASN seemed like it could make a better seal so I used a pick and took it out.  Initially I thought that the o-ring would be too wide, but for some reason the fit was better after re-seating it.  I recommend you do this when you first get yours.

O-ring-less stock air seal nozzle on the left.

In comparison, ICS’ stock air seal nozzle had no o-ring so this was certainly going to improve compression.  Better compression equates to better shot to shot consistency and reduction in power loss.

The rubber o-ring was a little dry though.  A bit of silicone grease would help here.

Spring Guide Fitment

The Combat Union spring guide has a more than adequate design. The bearing will add extra compression on whatever spring we decide to install and we’ll get a little boost in muzzle velocity.

Again this design is made with a standard version 2 gearbox shell in mind. A standard v2 gearbox has deep holes for the spring guide’s anti-rotation tabs to seat into.

Version 2 and version 3 spring guide anti rotation tabs compared. We are using the narrower v2.

However, the ICS v2 upper gearbox shell has shallow slots and you won’t be able to close the gearbox without minor modification to the length of those tabs.  All you have to do is shave just enough until the gearbox shuts close.

Next, I was able to trace the gap on the outer shaft to the portion of the screw head where the threads end.  It looks tapered preventing it from keeping the outer shaft from sitting flush.  Shaving down half a millimeter here would allow the outer shaft to seat lower to close the gap.

The section towards the screw head tapers out preventing the head from seating deeper into the shaft.


In the end we used four out of the five components in the Combat Union kit.  The only part that was set aside was the cylinder head.  This wasn’t due to the fault of the item or manufacturer and ultimately came down to a problem of compatibility with the parts I currently had on hand.

The cylinder head from the Combat Union kit is left out because ICS does not completely conform to standard v2 gearbox dimensions.

I’m sure that the cylinder head will find its way into a different build.  On the other hand the rest of the airsoft parts only required minor modifications to fit on ICS’ version 2 gearbox upper.  Overall I really like the completed look.  It may be mostly a cosmetic upgrade, an internal one at that, but if you’re confident in your skills to make AEGs run like clockwork and are interested to install robust Russian airsoft parts, I suggest you give Combat Union a try.

– J4

Published by J4

I'm Julius the tinker! I'm the owner of the Hobby & Tech Warrior website. I'm a website developer noob, gamer, hobbyist, tinker, and dreamer. I aspire to be a digital nomad.

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