To fit the J/GPZ trans in the Z1/KZ case the mods are as follows: Input shaft: Using a J input shaft you must bore the threaded end so the 3/8” ball and the Z1 pusher fits inside. Using a GPZ shaft the threaded end must be shortened and re threaded as well as boring.
The 5th gear input needs to be turned down to clear the shift drum and the drum must also be turned because you cannot take all the material from just one of them.
The 4th – 5th fork must be clearanced on both sides to clear 3rd and 4th gear on the input shaft.
A conversion bearing, spacer and seal are also necessary.
There is also some re shimming to be done on both shafts to obtain proper neutral clearances and engagements. If this is not done correctly, it will get itself into 2nd and 5th at the same time and cause considerable damage. I’ve seen it plenty of times. Chewed up teeth and broken dogs—Very ugly!
There are two different 3rd gear sets and output shafts. The J has 21/30 T and the GPZ has 19/ 27T. They are both the same ratio, but the GPZ set uses a larger tooth pitch with less teeth. The J output shaft uses a bolt to secure the sprocket and the GPZ uses a nut and also has about 3mm. more spline. The GPZ parts are the more desirable ones. If you cannot find these parts, the gpz output shaft (13128-1048) and 3rd input (13129-1566) are still available from BB Racing. The 3rd output is no longer available, but I make a replacement that is stronger than the stock one.
There are a few more small details, but this is the majority of what has to be done.
BB Racing carries the full Robinson Industries Bearing conversion kit. It costs $125.00 And is available Here