there are exactly four reasons why a trans won't shift properly;
1) input gear is spinning; there could be several reasons for this
2) synchros not braking; there could be several reasons for this
3) one of the mainshaft gears is dragging
4) oil is too slippery or too thick, or will not vacate the brake
As to;
#3) this seldom happens,
#4) I have had zero success with full-synthetics. Thick dyno oils are hard to squeeze out, and until the oil is squeezed out from between the brass and the brake cone..... NOTHING can happen. The brass has to make intimate contact with the brake.Intimate. This is why the brass has ridges and or channels, providing places for the oil to go. This is why I have modded my trans with channels ground right into the brake cones, to get rid of synthetic oil, which didn't work anyway, but worked awesome with dyno oil.
I have had the best success with Dextron II ATF. But I worry about the cluster pin, so I run 50% ATF, 50% E/P oil.
Some oils seem to bond to the metals in the trans . When this happens draining it will not make a difference. Even flushing it out with some type od solvent, will not . In this case the trans has to be taken apart and the brakes physically dressed to get it to shift again.
#2) Lots of reasons for this;
Wrong type oil;
Brass worn out and sitting too low on the brake.
Strut springs are weak, allowing the slider to pop over too early
Struts worn too short. Forks worn out. Bearings with too much play. Parts moving in directions they have no business moving.
#1) insufficient Clutch-departure;
> could just be wrongly adjusted freeplay, but could be
> 6-cylinder pedal ratio used on a 10.5 or bigger clutch.
> bent or warped clutch disk
> bad set-up of the Pressure plate
> misalignment of the crank centerline to the trans centerline
> broken Bellhouse
> pilot bearing not releasing
> flexing of the release-system
broken or cracked TO pilot on the Retainer.
Keep in mind that with the engine running and clutch clamped; EVERYTHING inside the box is spinning, unless the vehicle is stopped. At that time, the only things NOT spinning are the mainshaft and the two synchronizer assemblies. When you attempt to shift, whatever gear you are trying to get, has to be speed-matched to the mainshaft speed. Whether shifting up or down, the speed has to be matched very closely. That little tiny brass ring is the guy responsible for making this happen. But it depends on all the other synchronizer parts for support, and specifically the input gear has to be divorced from the spinning crank/flywheel/clutch assembly, so it can it's change speed! because the gear you are attempting to get into is spinning WITH the cluster, which is spinning WITH the input gear. So if you think about it, that brass ring has to speed match a fairly large mass, and the higher the speed difference, the more the mass grows.
Ok now, just as an FWIW, my A833 shifts like lightning, all the way to my self-imposed redline of 7200...... because I tried everything I could think of to make it happen and after many many RnRs I finally got it figured out; see, I'm not particularly bright, but I am persistent, lol. In the end I was dropping that stinking box, and the GVod behind it and the mufflers/ front pipes ...... in 17 minutes. On a 4-post hoist mind you,lol.
Gear clash is another topic;
