Unfortunately the Motobecane/Puch frame brace found on most websites isn't a direct bolt-on for most Puch Maxi frames. You will find that a touch of extra grinding, sanding, painting, and some ingenuity are needed.
First off, place the brace in place on top of the frame. You will notice that the back fork brace will not slide down over the backbone of the seat post due to the frame seam.
The easiest way to tackle this is to remove the fork from the tube (considering your tube is the adjustable type.) If not, you have to use the entire bar as a guide.
Now, most forks will be wider at the ends than near the bar, this is due to the fact that the Motobecane frame is square and wide near the seat post. Considering you're working on a Puch, you can place the fork mount in a bench vise, or simply use large channel-lock pliers to bend the ears of the forks parallel to one another. DO NOT HAMMER THEM! You may accidentally fold them over.
Now that the ears are straight put the mounting fork in place, the easiest way to do this is to INVERT the fork, so that it's up-side down. Most models will have a drain hold punched into the tube, this should aim up. Over the course of this project you will find that it is much easier, and much less of a hassle to mount.
Take the top bolt of the seat brace out and slide the fork back into place. The holes should be off about 1/2" to 1". For this next step you will need a grinder, or dremel tool with appropriate attachments.
Measure where the bottom of the fork hits the frame when the bolt hole is in-line with the existing holes. Make a mark. It should be about 3" down from the top of the seat clamp. You need to grind the seam completely down to the frame to match the tube from this point up.
After this step, re-fit the fork to check the hole alignment. If you're still off a tad you will need to A: Shave down the inside of the forks where they hit the frame. AND B: elongate the bolt holes a tad.
Now you should be able to mount the fork to the frame. If not, you MAY need a longer, and possibly one-size smaller bolt, instead of the 13mm's you may need a 12, or 11mm bolt with appropriate washers.
Next, screw the pieces of the brace back together, fit the front, and adjust the rear so that it fits snugly into place. (It's better to install it snugly, than loosely.) Use a rubber-mallet to hammer it into place if you cannot push it down manually. Again, make sure the fork is UP-SIDE-DOWN compared to the rest of the frame brace.
Now, Install your seat, Install your bolt, and off you go.
You may need to periodically check the snugness of the bar as you ride considering it may loosen/wear in over time.
Originally Written and Edited by DannyMoped