This video shows the rebuild of the 915 gearbox in my 1978 Porsche 911. I am just an amateur enthusiast - so bear that in mind! Symptoms were:
1. Frequent difficulty engaging 1st gear
2. Crunching into 2nd gear
3. Rattling and dropping out of 3rd gear
4. ~One inch movement of gear lever when on and off throttle in 4th gear
5. A singing differential.
Solutions were:
a. Repair damaged mainshaft
b. New dog teeth/gear teeth, synchro bands, synchro sliders/hubs, and bearings
c. Shimming of the main pinion bearing outer race
d. Big changes to shimming of differential ring gear and pinion
e. Speedi-sleeves on the differential output flanges.
All problems were fixed. However, when in 3rd gear, if the gear lever is pushed up or down (north or south), you can feel a pulsation. This existed before the rebuild, and is also present in 3rd gear in my 1955 Triumph TR2!
My feeling is that this is some lack of concentricity in the 3rd gear synchro band/gear teeth, which transfers to the 3-4 synchro slider, then the 3-4 selector fork, and then to the 3-4 selector shaft. When the gear lever is in its sweet spot, the gear selector ball-end in the gearbox does not touch the 3-4 selector shaft, so all is sweet. But move the gear lever a bit and then contact is made and you feel the pulsation.
Did I get this right? Maybe this is quite common. I think and hope it is a minor issue. Any ideas welcomed in comments below.