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If it's not really about tuning or wiring. Then it belongs in here.
I've tried to research to find out more about these topics. Looking to find out more information on these topics for modifying manual transmissions.
Figured this would be a great place to ask.
Anyone know detailed information on these practices?
Corner Rounding, lead edge trim back, tip relief, modifying gearstack offsets and modifying the shift rail and selector.
Sorry about the delay. I don't know how much folks can add (including me) as usually the box' would be swapped out for something stronger, with better ratios, better shift (sequential), number of gears and/or other reasons. However, that's not always an option and certainly back in the day there wasn't much option other than do it yourself. Some of the modifications you mention may be known by a different name here, but here goes.
Usually modifications were made to enhance shift speed and reliability (reduce missed gears).
I believe most of the first queries relate to reducing the baulking and/or clashing of the syncro's by reshaping the leading edges of the gear and/or synchro' teeth to ease the meshing. More specifically they normally have a fairly blunt end with a relatively small gap to get through - by removing the back-cut that's there to help hold them engaged and making them parallel it both opens up the gaps and ellows them to be pulled out of mesh more easily. Way back in the day this was taken to extremes by also removing every other tooth of both the gears and the synchro' cones to give huge gaps for engagement. By doing this, and slotting the sychro' cones so the fine grooves are less effective, one can achieve very fast, clean, shifts BUT the trade-off is rather horrendous shunting on light throttle.
I think the gear stack may refer to shimming gears on the 3rd motion (output) shaft so they best align with the matching 2nd motion (lay) shaft gears. This would usually be done in conjunction with shimming the bearings on the 1st (input)and 3rd motion shafts, and the thrust washers on the 2nd.
Not too sure about the selector shaft and selector (forks?) except maybe it's ensuring the detents and selector forks are aligned so they place the synchro' hub for full engagement but short of pre-loading them? Maybe it also address the problem with some transmissions of the selector forks being too weak for spirited shifts which can bend them, so to replace them?
Most production gearboxes will run the gears directly on the shaft, and it can be beneficial to improve the oil supply to thee bearing surface? Remember, because the gears are always in mesh, the gears will be rotating at many times the engine rpm in the higher gears and this can lead to excessive wear, even seizure, of 1st and 2nd gears.
A quick GOOGLE search using "books on modifying manual transmissions" pulled up some books that may even have your specific transmission there.
If you can't change gearsets, replacement off aluminium sandwich plates/bearing carriers and thicker/stronger oil pans can help.