M4 gearbox lock pin.

[Iceni]

Looking at rebuilding a scrapped m4.

And I've come to the gearbox lock pin. It is missing from the parts I have available. This is the first m4 I have touched so the pin is new to me.

So the question is. Is the part essential I suspect it is a useful part that improves overall reliability. And do I need the official pin, or is a rolled pin cut to suit going to be enough. Extraction will not be a problem at a later date with a rolled pin. I  have a drift set and several screw type pin pushers.

I already have this kit at home so I can fashion one in a few mins if it going to do the job.

https://www.machinemart.co.uk/p/cht397-120-piece-roll-pin-assortment/

 

[Samurai]

Not essential. I wouldn't use rolled pin there because if you need to remove it with a hammer later, that could cause problems.

 

[Iceni]

Awesome TY. Wasn't sure if the holes were going to be uneven sizes across the casing for a taper pin, Or there was a lifting/moving problem with a box without one.

I'll leave it out for the time been then. The upper lower are the older G&G magnesium casts and I think I would struggle to replace them if they cracked.

 

[Hangtight]

If you find the air nozzle doesn't line up with the hop then having the pin in can help pull everything back into alignment, but usually just putting shims between the rear of the casing and the back of the gearbox will drop it or a gentle file of the back of the gearbox to raise it. The combination of the rear pin and the buffer tube screw do the job.

 

[Lozart]

Awesome TY. Wasn't sure if the holes were going to be uneven sizes across the casing for a taper pin, Or there was a lifting/moving problem with a box without one.

I'll leave it out for the time been then. The upper lower are the older G&G magnesium casts and I think I would struggle to replace them if they cracked.

Click to shrink...

It's usually a straight pin with a knurled ed to give a friction fit in one side of the receiver. If you look closely the two holes either side of the receiver are often very slightly different sizes for this reason. It's not an absolute essential part but you may find alignment with the nozzle to hop is a bit off without it.

They're cheap enough to get if you want them though (assuming nobody has a spare kicking about in their bits box you could blag) https://www.fire-support.co.uk/product/dytac-m4m16-aeg-metal-bodygearbox-trigger-pin-pack-of-3

https://www.fire-support.co.uk/product/guarder-steel-pins-for-marui-version-2-gearbox