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View Full Version : Setting the Cetme/HK cocking tube gap



holescreek
11-26-2016, 12:38 AM
I will be doing this in two parts because I didn't get it all done tonight so bear with me. The gun I'm using for this is me retro PSG1 Cetme clone so it will look a little different, but the same steps apply.

In this part of the build you will have just welded your trunnion into the receiver and already have the barrel pressed and pinned and have a known good bolt gap (spec = .004"~.020"). You also will have already drilled your 3/16" holes in the top front of the receiver to use for plug welding the cocking tube in place.

The cocking tube gap is extremely important. Your cocking handle pivots on a pin and acts as a fulcrum or lever to unlock the rollers in the bolt head. If your cocking tube gap is too large, you will not be unable to unlock the bolt to cock the rifle. If your gap is too small, the cocking tube on your bolt carrier will literally slam into the cocking handle support and beat the rifle to pieces. On top of that, a short cocking tube gap will allow the carrier to rest on the support and give you a false bolt gap reading. There are thousands of Century Cetmes with this condition.

There are several ideas out there as to what the cocking tube gap should be. At the very least, the gap between the bolt carrier tube and the cocking piece should be at least as large as your bolt gap. As the bolt gap deteriorates due to worn components and starts to close, so does the c-tube gap. Some builders use the bolt gap size, and some double it on the carrier gap. Keep in mind that the larger you make it, the harder it will be to unlock the bolt.

I start with a piece of shim material of the thickness I want my gap to be. I cut the material to fit over the end of my carrier tube and use double sided tape to hold it in place. Note at this point that the assembled bolt carrier is already in the rifle and locked into the trunnion.:

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Insert the cocking tube support and cocking handle into the cocking tube. I fully assemble it into the tube and slide it all the way forward against the stop. Then slip the assembled c-tube over the carrier extension. In this photo you can see the shim and the cocking handle support. On a Cetme it will go up inside the support, on an HK it will stop against the back of the support:

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At this point the whole c-tube assembly should be resting on the shim. Now is a good time to put the forearm on and check for alignment of the groove. Note the gap between the receiver and c-tube. This is unimportant, just as long as you have one. :

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You also want to check access to the access holes for the cocking handle pin. A little too much rotation will become a real problem if you can't get the pin out later.

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holescreek
11-26-2016, 12:39 AM
Once I get the rotation where I want it, I use a marker to make some alignment marks for reference.

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I stand my receiver on it's back end on the welding bench and let gravity hold the c-tube assembly against the shim on the carrier tube. If my barrel will be using a triple frame I slide it on and use it to align the front of the cocking tube to hold it in place while I put a few tack welds in the gap between the carrier and receiver:

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It is extremely important that you keep the heat down on these tack welds. These are just to keep the tube aligned and in the proper rotation. The last thing you want is burn through before you get your heat sink in place and while your carrier is inside.

Once the c-tube is tacked in place, remove the bolt carrier. I caution you against using the cocking handle for carrier removal. Your tube is just tacked and may change position if you pull back on the handle. I use a small flat blade screwdriver pushed between the bolt head and carrier to unlock the rollers and remove the carrier.

I use a solid copper heat sink, roughly .705" OD x about 3" long. It doesn't matter if you use aluminum or even copper tubing repair sleeves from the hardware store as long as they can keep the weld heat from bubbling metal into the inside of the C-tube.

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Insert the heat sink into the c-tube:

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Now you should be ready to plug weld the c-tube extension to the receiver through the 3/16" holes, then weld the gap between the c-tube and receiver.

holescreek
11-26-2016, 12:40 AM
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Since I tacked the gap already, I start with the plug welds. I plug weld one side then roll the receiver over and do the opposite side. I keep going from side to side in order to keep the cocking tube from drawing to one side or the other. The metal will follow the heat. If you plug weld everything on one side at once, the tube will sometimes pull to that side. Once the plugs are done, start filling the seam between the tube and receiver.

If you are a good welder, you are probably done now.

I do my best welding with a 4" angle grinder. I start knocking them down to size:

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Then switch to the air grinder with rollock discs:

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Then drop down to swiss files and sandpaper:

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I will do more finishing on the receiver after the rest of the parts are welded in place.

This is the end of this tutorial. Thanks for playing along!

Oh yeah, don't forget to take the shim off of the end of the bolt carrier!

21HK
11-26-2016, 01:07 PM
...I do my best welding with a 4" angle grinder.


Me too! that's why I let someone else do my welding : )

Thanks for the resurgence of these tutorials.

cpitylak
09-06-2017, 06:38 PM
This helped out a ton, thank you!