G&P Mk23 Stoner 63 PolarStar Fusion Engine Conversion

UPDATE: With the new gen3 V2 Fusion Engine, the conversion of the Stoner 63 has changed a bit. The information below pertains to the gen1 and gen2 V2 Engines. Here’s some updated information for the Stoner 63 with a gen3 V2 Fusion Engine.

I wanted to share some info and pictures on my recent G&P Mk23 Stoner 63 conversion to use the Polar Star Fusion Engine. I’ve also made some other customizations to the Stoner that I’ll describe below.

Here are some pictures of the completed Stoner 63 conversion. I’ve added a quick release bi-pod, 3 point sling, and 110mm foam-filled suppressor.


Other modifications I have made include an easy access BB filling cover. The stock way to fill the box mag was painful to do in the field. This modification makes it easy to pour more BBs into the mag, and it also allows me to add an extra capacity tube to make it so the box mag is now more in the 2000+ range capacity.

Yes, that’s a spice bottle cap added to the box mag! Attached using a Velcro strap.

Here’s the extra capacity tube (another good use for a seasoned salt bottle). I weighed and counted the number of bbs the spice bottle adds: 1350 bbs, and the capacity of the box mag is 1225, so the total with the extra capacity tube is over 2500 bbs. An additional note to the spice bottle mod, BBs can shatter the bottle plastic, so it’s a good idea to reinforce it with some duct tape. I had a BB put a hole in the bottle and the shards of plastic got into the winding mechanism and jammed it up. so now I have black duct tape around the bottle to hold the plastic better.

I’ve also reenforced the Stoner 63’s stock. I read that other folks had some problems with it breaking at the neck, so I added two metal strips (flattened 5″ L brackets), bolted them to the metal mount, and bolted it to the sides of the stock. I then Bondo’d up the stock, sanded and painted it flat black. Strong as rock now, should never break, and you can’t even tell it has been modded.



So on to the Polar Star Fusion Engine (FE) conversion. There are several customizations/modifications that have to be made to make the Fusion Engine work in the Stoner. First, the trigger needs to be replaced. The Stoner uses a unique safety mechanism, so the trigger from the Stoner gear box needs to be pulled out and replace the stock trigger that comes with the Fusion Engine. Also, the stock safety mechanism in the FE should be removed completely.

New trigger put in FE (the picture has the FCU wiring to the front, but it’s to the back for the Stoner):

The next mod, which is the most complex, is the selector plate on the FE does not work with the Stoner 63. You have to use the selector place from the Stoner gearbox. Here are pictures showing the two plates, and the mods made to use the Stoner plate on the FE. I used the FE aluminum selector plate as a template by clamping it to the stock Stoner one, with vise-grips. Then I used a Dremel to make the slot (not my best work), and a drill for the left hole. Also note, to remove the selector plate from the stock Stoner gearbox, you will need to remove all the gears and bearings on that side, so the selector plate can slide out, the bearings protrude up into cutouts on the selector plate.


Using a dremel tool, I cut out the same holes on the Stoner plate to match the FE plate. The guide slot and the hole on the far left are needed.

To have the new modified plate work, a guide rail on the FE box will need to be removed. I used a dremel wheel to grind the guide rail off. It blocks the modified selector plate.

Here’s the new plate mounted:

The back bolt that mounts the gear box/FE to the Stoner body is really long. It needs to be cut/ground down by about 1/2″ to allow it to fit with the FE:

Next, the hop-up brace that is mounted to the front of the gear box needs to be modified. It has two tabs on the inside that fit into slots on the Stoner gearbox. To make this piece fit the FE properly, these tabs need to be cut off (use a blade or dremel tool):

To allow the mag feeding to work, now that the trigger is isolated from the battery, I took power from the FCU battery (I use a 7.4v Lipo) and route it to the box mag using the previous power wires. I also moved the priming switch from the box mag to the hand guard. Here’s a picture of the FCU and battery connections in the stock. I use JST connectors to make it easy to disconnect the FCU from the FE.

Hand guard priming switch:

I wired in a trigger feed (via reed relay) from the Fusion Engine circuit board. The trigger pulls now activate the winding electronics. It’s all powered by the 7.4 Lipo in the stock, so no need for an extra 9V battery.


I wired a connector to the trigger pins on the circuit board inside the FE, and I run it out the bottom of the Stoner and connect it to the box mag. I also had to put several layers of heat shrink tubing on the full auto switch post. This is the post that is connected to the selector plate and pushes against the button to enable full auto mode. You have to test and tweak that to make sure the full auto switch on the Stoner body pushes the selector plate over enough to engage the full auto micro button in the Fusion Engine. For more detailed information on the box mag wiring, see this post.


I added a small piece of rubber to the trigger (using superglue) to reduce the trigger travel.

The Stoner 63 is an AWESOME gun, and the Fusion Engine is AMAZING. The Fusion Engine turns this gun into a devastating field support weapon.

– captainbingo