Hello Clive,
Rambolambo's answer is certainly the best way to go. Also the mentionned link gives nice examples of where the problems are.
For further understanding, you can do a little mathematics : basically we can expect the UN1 primeshaft steel to be around 95000 psi of ultimate tensile strength (common value for shafts). Thus, a 17.8 mm should break around a force of 55 mkg and suffers permanent deformations around 32 mkg.
Does this means that if your engine output is above these amounts, your gearbox will break? (I will try to write a very simple and basic answer to give you some paths to investigate)
Obviously no, as these numbers reprensent the opposite force the shaft has to meet to break. For a car, it represents the resistance to motion of your car. I posted somewhere on this website some quick calculations but basically considering all the forces the highest opposition comes from air penetration at high speed wich follows an exponential curve. So the moment where to maintain/reach a speed of X km/h, your engine would have to produce such power at Y rpms, the shaft will break. This is also why UN1 shaft, in most cases, in our lightweight cars, doesn't break on 1,2,3,4th gears but on fifth (high speed so max opposite force). Also as the setting of the 5th gear is very bad : shaft with changes in diameters (so a clear "tear here") and a bushing setting with poor lubrification system creates at the weak point an hot zone surrounded by a cold one : the worst situation and the type of engineering error a 1st year student in mechanics would not do...deliberate mistake? hard to answer.....
Solutions : Do not use 5th....
, do not drive at high speed
etc ..more seriouly the solution from rambo is great, you can also upgrade your primeshaft, consider a 25 mm with no "tear here" would be able to sustain 150 mkg before breaking and 89 mkg before deformations....I recently saw an interesting solution for a low budget : The guy has a sleeve machined wich was heat shrunk to all the 5th gear shaft location. the sleeve (inside) adopts all the changes in diameter but externally is strictly straight. It gives the shaft a nice upgrade in terms of strength and this for a small budget : rear bearing would have to be changed and 5th gear hub to be enlarged to the new splines diameter.
The UN1 is a very good option : The housing is really bullet proof with this construction with walls wich is a pain in the ass to cast but gives strength with no weight. Secondary shaft is very strong too with some nice finals available. Adding some lubrification systems and some ehancements to bolt sizes etc....would make a very reliable starting point. An LSD is a must have too and a primeshaft upgrade is certainly a must have too.
%In any case, a gearbox is an expensive part, but accordiing to the prices I know, a UN1 basic should cost around 300 euros, a quaife lsd around 1200 euros, The pain is the quaife primeshaft upgrade... around 3000 euros. Doing your rebuild/assembly yourself will save some bucks but you will end with a budget of 4500 euros to have a very relaibale setting..This is more the price/cost of a good gearbox than trying to believe that you can have something reliable for 300 euros.
Cheers
Stephane
