Differential bearing failure is the most common. Often shows as oil leaking from the drive shaft oils seal. Fitting a replacement seal will prolong a gearbox but if the bearing has wear, it won't be long before the leak starts again. Noisy driving shows this up. The faster you go, the louder it becomes.
Occasionally the input shaft bearings fail. This tends to be more on gearboxes that have run low on oil or never had an oil change. These can be found by sat with the engine running and the gearbox is noisy. When the clutch is pressed all the shafts stop inside the gearbox so the noise also stops. Not to be confused with clutch release bearing failure where the noise is only when the clutch pedal is pressed and quiet at idle.
In track cars I have found the brass selector fork bushes come adrift. This then means gears become difficult or impossible to select. These can be removed and squashed back into place in a vice or replaced with the earlier cast iron forks as these don't suffer this problem.
Best is more related to a requirement of a car.
On track cars, find the absolute top speed you will ever do and base a gear ratio to max out just over that speed. Most UK track cars using around 200bhp cars will max out at 125mph. So building a PG1 to this spec means using aftermarket gears.
In a road use car, choosing the cruising rpm at 70mph is important. Peronally I think aiming between 3000 and 3400rpm is perfect for a NASP petrol car but the more sporty cars do rev higher than this. In a petrol turbo this can be lowered slightly but other than being a bit quieter, gives little advantage in fuel economy. The diesel cars do like the longer gearing if doing long motorway journeys. This can be done but parts are difficult to source. There are 4.2. 4.06, 3.923 and 3.65 final drives available which is an output shaft plus crown wheel. These can be standard or U spec and when building a gearbox you must use the case type for the shaft.
Take a look at our gear calculator page which has the list of factory gear sets listed.
Rover fitted 2 Torsen Limited Slip Differentials into their cars. The type A and a type B. Type A is more aggressive. It was only fitted to a Rover 220 turbo coupe. As many people have driven these cars hard, the type A has broken giving these a poor reputation. I have found these work perfectly well in a NASP car, especially Mini or Metro conversions.
The Torsen type B was fitted into a Rover 620 Ti. Rover 820 Vittesse turbo and Rover 218 BRM. These are a more reliable unit but not as aggressive. I have used these in rear engine cars too which helps grip.
Quaife make a very good unit for the PG1, I have one in my own car. It has proved very reliable, even in motorsport. These come with a lifetime warranty.
Gripper make a plate Limited Slip Differential which gets very good feedback from the race and rally drivers. The downside to these is they do need a service every few years which means the gearbox stripped and the unit sent to Gripper for service.