I spent all day yesterday looking at seat designs and trying to chase down some measurements for the aft seat rails. There really isn't much out there, but after digging a bit I was able to find the bits of information I needed. Thanks to Wayne Massey's kitlog for filling in the final gaps.
After spending way too much time on seat design, I decided to give my bender a trial run with some 1/2in EMT conduit (3/4in OD) instead of 4130. The practice EMT conduit was cheap at just over $2 a 10ft stick. I could just miter the corners, but I really like the round corners and I think they're a bit safer as well. I marked the conduit at one inch increments and methodically calculated the makeup length and the radius of the bend. I’m using Kevin DeShazer’s seat plans and modifying them a bit. After a few bends, I was able to end up with exactly 16 3/4in outside to outside on the back seat frame, and that’s what the plans call for. I’ll try it with some 4130 tomorrow.
