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Receiver Flat Instructions

AK-Builder Receiver Flat Instructions

Material 4130 Chromemoly Annealed Steel on rails and flat

These flats were designed to be bent and assembled like a manufacturer was trying to make a receiver. With that in mind the problems associated with pre-drilled holes in rails and holes too close to a bend had to be addressed. 

This is why I made the holes undersized to have them drilled or reamed afterwards. The trigger pin hole stretches because the hole is too close to the bend. I made the hole a little smaller, but it still slightly bends outward and will need to be tapped on to bend it down.  After bending that down then it’s ready to ream and will turn out better than any flat with the hole that was drilled to full size before bending.

To install the rails you slide them all the way up against the front trunnion. Make them parallel with the top rail and lined up with the front trunnion so the bolt slides in and out without jumping. Some people say to use a certain size drill bit, which is a good idea if you test fit to get the right size. 

A)  Oversized mag stabilizer on ejector and right
side rails will need to be trimmed. It’s easier
to do this before welding the rails in. The best
way would be to get them close to the right size
and final fit them after welding. That way if the 
mag opening is off center it can be accounted for.

B)  The embossed dimple on the right
rail will help center the hammer in
the center of the receiver. It is easier
to install the trigger guard if it is done
before welding in the rails.

After installing the rails and trimming the ejector it will need to be heat treated on the tip to prevent the cases from denting the ejector.

Center support hole will be drilled to 5/32”
after welding in the rails

The Hammer/Trigger pins will need to be 
reamed out to 7mm after bending.

These holes will need to be heat treated after reaming.


Center support hole will be drilled to 5/32”
after welding in the rails

The Hammer/Trigger pins on the Operator side will
need to be reamed to 5mm. The hole for the hammer
pin should be drilled at .177 first then reamed to 5mm

Heat Treating Tips

Since 99% of most people will not have a kiln you will only be able to spot harden the hammer/trigger pin holes and the ejector. 

Using a MAPP gas torch heat up each area individually and quench in oil.  Be aware that standard oil will probably flame up and create lots of smoke.  After doing this to each of the four hammer/trigger holes and the ejector you will want to clean the oils off of the blank.  Once that is done put it in a regular oven at 400 degrees for an hour to get rid of the brittleness.