It's a common problem with M14s used as M1 Garands that they lose Semi-auto.
This is actually because the cut-off plate wears down where it contacts the sector gear inside the gearbox.
Luckily though Guarder do produce a replacement part made of steel that should outlast the original alloy part. Also luckily you don't actually need to go inside the gearbox to fit it!
I bought my replacement cut-off lever from Wolf Armouries, but other airsoft retailers do have them in their catalogue, they just weren't in stock when I ordered.
Here is the original part (left) and the guarder replacement (right). You can see where the original has worn away, encircled in green.
The job of fitting the part is quite quick, just two annoying little springs to worry about.
First separate the gearbox from the stock and then turn to the left hand side of the gearbox. There are 4 screws and 2 springs that interest us.
First carefully remove the springs with a pair of needle nose pliers. Ones with serrated jaws will help with this.
Then remove the 4 screws.
Note that three of the screws are the same length. The shorter one is from the top right hole. The springs are pretty much the same as each other and are interchangeable.
Now the plate that they all held down can be removed.
You can see the old cut-off lever in place. Replace that and put everything back as it was. You should find that semi-auto has returned
Be careful when putting the springs on, they have an annoying habit of pinging off and flying around the room. Don't say you haven't been warned!
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Whilst I was messing with the gun, my selector switch fell off. I tried to glue it back together, but had a phone call whilst doing so and it's now just a munge of metal and superglue.
I did panic for a bit. But then decided that since the default position for the switch is in fact semi-auto this isn't too much of a problem. I have hence decided to go without a switch and fill in the gap in the stock as well. This is quite pleasing as the gun looks more like a garand than ever before!
I know that semi-auto can jam occasionally and the only way to free it up is to switch to full. Luckily there is just enough room in the gap between stock and receiver to get a screwdriver, knife or bayonet in and push the selector lever to full-auto temporarily. I hope I don't have to do this too often. If I do them I'll have to have the gearbox apart and look at the gear timing a bit.
Nice guide there Rich. However,"Gear timing" (which is a myth) has naff all to do with gearboxes locking up on semi-auto. It's to do with the way that the semi-auto latch is supposed to flip the sliding part of the contact switch to physically open the contacts - basically, it doesn't reset it to quite the right position.
The best way around it (which STAR do on their semi-only SLR's) is to bridge the wiring through a hidden micoswitch. Gearbox locks up, then just press the microswitch to full auto it for a moment to sort the problem. Also means you can get rid of the fire selector off the right hand side of the receiver 
When we were a Kingdom it was run by a King
When we were an Empire it was run by an Empress
Now we're a country we're run by a..........
Nice guide there Rich. However,"Gear timing" (which is a myth) has naff all to do with gearboxes locking up on semi-auto. It's to do with the way that the semi-auto latch is supposed to flip the sliding part of the contact switch to physically open the contacts - basically, it doesn't reset it to quite the right position.
Aha... cool... that saves me worrying about opening the gearbox.
The best way around it (which STAR do on their semi-only SLR's) is to bridge the wiring through a hidden micoswitch. Gearbox locks up, then just press the microswitch to full auto it for a moment to sort the problem.
Hmmm... good plan. I wonder where I'll put it. Under the fake bolt sounds like a good place. Or somewhere in the trigger housing.
Also means you can get rid of the fire selector off the right hand side of the receiver
Which has already been done... see the pic at the end.
I didn't mention how to get the stock off as it's in the TM manual.
Also its a bit more complicated on mine as it's a garand conversion. There's an extra screw.
And the stock has been cut down
Exactly... I didn't want to get into all that.
