Normally the M21 will have one groove on the input shaft, the M20 has two grooves. However, many of the early transmissions do not have the ID grooves. Something to check for. There is also the small and large sycnro hubs and blocking rings issues. You'll need to know what you have. The early small design are junk, small blocking rings that always break or crack. If you have the small hubs and sliders, you'll need to upgrade to the large style. A must!
Don't buy any parts kits. They are usually thrown togehter with cheap parts from Mexico and what ever, I buy all of our Muncie rebuild parts out of Detroit and NY. it's best to buy the parts seperately. Make sure you ask questions and who the manufacturer is. We rebuild and restore 5-6 Muncies a month at CHP, probably more than most shops do today.
Jerry
