AK front trunion hole locating & drilling jig

[SVT-40]

Having been frustrated with accurately locating and drilling the front trunion holes on AK type builds I came up with this simple homemade tool and procedure. It allows you to accurately locate and drill both sides of the front trunion holes at one time assuring proper alignment.

To use this method you need a drill press and a drill press cross vice. Along with the normal bits and accessories.

The "tool" consists of a 7/8" x 6" bolt and nut. In addition you need a piece of scrap steel at least 6" long and approx 3/4" x 3/8". The steel needs to be able to fit on top of the trunion between the "ears" on each side.



To make the tool you first thread the nut onto the bolt and turn it most of the way down. Slide the AK trunion on to the threaded end of the bolt until hits the bullet guide. turn the nut until the nut is flush with the face of the trunion.

(If your going to use the tool on a 5.45 trunion you will need to remove a slight bit of metal from the threads of the bolt to allow the trunion to fit on it. Remove only what is necessary using a file or grinder. (I used a flat file.)



Snug the nut up to the face of the trunion and adjust the nut so you back off slightly from the bullet guide. Place the bolt with the trunion on it in your cross vice. Place the your steel piece above the trunion in the flat on the top of the trunion.



Tighten the vice jaws. You will notice when you do this it causes the flats on the bolt head and nut to align with the faces of the cross vice. The clamping pressure pushes the bolt up onto the top of the barrel recess in the trunion this squeezes the top portion of the trunion between the bolt and steel piece which is pushing back towards the top of the trunion.

The clamping pressure causes the trunion to be perfectly parallel and perpendicular to the drill press chuck.


You need to file one face of the nut slightly so you have a gap as indicated in the photo. If there is no gap the clamping pressure will only be on the nut, and not transferred to the trunion. Don't take to much off.



Once everything is nice and tight you need to drill holes in the bolt to allow the bit to pass through so you can drill holes in both sides of the receiver at once. Move the vice table until one of the trunion holes aligns with your drill press chuck. Take a small drill and drill a pilot hole into the bolt. Do the same with the other trunion hole. Loosen the assembly and remove the trunion. Drill the holes in the bolt large enough for drill bits to pass through. I think I used a 3/8" bit. It's really not important what size you use as long as the holes are large enough for the bit you use to drill the holes in your receiver. After drilling de-burr the holes in the bolt and your ready to use the tool. Obviously no clearance holes are needed for the two bottom rear front trunion holes.





Move the vice table until the bit aligns with the trunion hole I use a rod which is just large enough to pass through the trunion holes, as it's easier to see and use than a drill bit. Loosen the vice so you have the wiggle room to move thing around a bit until there is no interference with the drill bit. Run the bit through both sides of the hole. You may need to loosen the vice with the bit entirely through the trunion so there proper alignment. When the bit aligns with both side of the trunion tighten the vice fully.




Slide the receiver on and once you have it properly located clamp and drill. After drilling the first set of holes I put a long rivet through the first set of drilled holes to help keep everything aligned while I drill the other holes.

You repeat the process for each set of holes.

Like a fool I was so excited when I first used this tool I didn't take any photos of actually drilling a receiver. But it worked slicker than snot!!

The entire cost of the bolt, nut and steel piece was less than $5.00 I got everything from a scrap/ surplus store which sells steel by the pound. (Cal Aero in Paramount)


If I had to do it over I think I would use a 5" bolt instead of the 6" as there would align better with the vice jaws.

I also checked and found it works just as well on 7.62 trunions as well as 5.45 trunions even though the hole for the barrel in the trunions are a different size.

I hope this helps all you AK builders out there!!

[SVT-40]

A few more photos of the Jig.

[jumbopanda]

What country is that trunnion from?

[SVT-40]

Bulgarian.

[motorhead]

circle 10=bulgy.

[scotthmt]

so youre saying drill the flat before you bent it?

[grammaton76]

Neat idea. Maybe, if you don't wanna sell them yourself, you should see if Curtis at ak-builder.com would consider making some (and perhaps paying you a bit for your design)?

[SVT-40]

scotthmt, No the completed receiver is slid onto the front trunion after you locate the holes and then it is drilled.

grammaton76, It's to simple to mass produce. If you can drill the two holes in the receiver you can make it yourself by drilling two larger holes in the bolt first

[grammaton76]

Well... you could mass produce it, and I could probably single-produce it... but most of Curtis's customer base are buying things which also fall into those categories anyway.

[ar15barrels]

When I did mine, I just located the trunion in the reciever, clamped it all together and center punched each hole really deep.
The center punch tore through the sheetmetal and centered itself in each trunion hole, leaving the reciever slightly countersunk into each hole.
Then I just drilled through the right size and riveted it all together.
Worked great.

[mason1800]

Good luck putting the sidefolder spring in!