Sorry for the necro but I recently did Pesto's mod on my VFC scar-h using these instructions and thought I could contribute some pictures!
1) So here's a comparison between the stock, super uncomfy grip from VFC, versus a Magpul PTS miad grip.
As you can see, there's a raised base in the grip to compensate for the gearbox that sits higher in the receiver. To make a regular m4 grip fit we're going to have to mod our grip to bridge that gap .
2) First, you're going to need longer screws for the grip. Stock screws are around 10mm long, you're going to need m3x16mm screws. (Amazon has packs of these)
3) Next, I had some plastic sheets lying around, and decided to make a shim to properly tighten the grip to the gearbox. (Easier to properly shim the bevel gear and adjust the motor height after. For the Magpul PTS grip, my shim had to be around 5.8mm thick.
PS: Yes you should check the bevel gear and motor pinion gear meshing when changing the pistol grip.
4) Since the motor is sitting higher, the next 3 steps are gonna be to compensate for this.
5) You're gonna need to widden motor axle hole to about 11mm. Like Pestobanana said, make sure it's centered, or else it could affect the pinion gear mesh.
6) After that, you're gonna have to ditch the spring and replace with o-rings.
The orings that worked for my settup were 11mm ID orings with a thickness of about 1 to 1.5mm. (Taken from a Lezyne bike pump rebuild kit at a bike shop)
I only needed one.
7) Last, you're gonna need a shim on the motor height adjustment screw. I just superglued a thick m5 washer on mine (from a bike disc brake caliper mounting bolt). But pretty much anything will work for this.
8) Finally, just assemble everything (Shim first through the wires, then the pistol grip)
9) And VoilĂ*! No more uncomfy grip on your VFC scar!
