G3/cetme bending jig dimension?

[My375hp302]

Im going to start doing my own cetme receiver so I need to build a jig to bend them. My question is what is the correct diameter for the ram rod that contacts the center of the receiver. I have found several people using 3/4 drill rod but since the germans are on the metric system I doubt thats the actual spec for it. I have all the nice machienes to turn it to any size so I want to do it right. The inside of a completed receiver is supposed to be 20mm inthat area but I was thinking you may need something slightly smaller to account for spring back. Any help is appreciated!

[Holescreek]

I use a 13/16" diameter rod and mill the sides flat to 3/4". the internal diameter finishes at 20mm but the 13/16" press bar gives the proper spring back in the sheet metal. Final forming is done with the straightening mandrel.

http://www.youtube.com/watch?v=AI5if5NUklA

Over the years I've found some receivers react differently using the same tools due to the differences in stamping methods and sheet metal thickness used. The latest batch of Gewher Works receivers with the squared rails tend to want to collapse in a tiny bit.

If you are a weaponsguild member here is a link to a compilation of about 13 different cetme/HK flat bending jigs. Several have dimensions that will make your jig build easier:

http://www.weaponsguild.com/forum/index.php?topic=20128.msg271567#msg271567