My joiner is an old 4 1/8 inch Craftsman. It runs pretty good but the knives are not easy to set. This jig seemed to be just the thing but there are two short comings I found:1. When the jig is lined up with the knife at top dead center, the screws holding the knife in place are not accessible. So the jig has to be moved forward enough to get the knives to rotate just a bit. By this time, the knives are to high. With this set up, no amount of tweeking got both knives parallel and at the same height.
The way I tested whether the knives were parallel was to join a piece of scrape (3/4 x 3 1/2). This joined edge (3/4) was placed on the cast iron saw table and a square was used to see if the 3 1/2 side was 90 degrees to the table. I never had a test run where this was the case.
Another way I came up with was a ruler. The ruler was placed on the infeed side with the "6" on the very edge, side closest to the fence. Then the cutter was rotated until the steel blade hit the ruler. The contact point on the ruler was noted. In my case, this was about 7 1/16 on the ruler. Then the ruler was moved to the other end of the knive and the process was repeated. Most of the time I could not detect a difference so maybe my machine is the real problem.
2. The magnets were not strong enough to pull up the knives every time. Sure, the jig would stick to the cast iron table but the knives would often stay put. I used a spare rare earth magnet to pull the knives up to the jig magnets. Once in place, the knives would stay put most of the time. I ended up making another little tool with the magnet screwed to a scrap piece of wood. This was used to pull the knives up to the jig magnet.
So, I'll keep the tool as it came by way of UPS and sending it back would not be free. But I would be very careful about recommending the jig.