Decided to thoroughly measure everything as can't put gearbox back together as waiting on some sealing goo. Initially I thought the shaft was one diameter apart from the obvious diameter change at the chamfer down to the spline. If we look back at post number 5 I took a measurement from the spline end to the face of inner race which was 87.7mm and you will also see in that photo that the inner race was beyond a very clear line which is were the shaft diameter of 20.0mm starts. You will see from my sketches that there should be a gap of 15.8mm (possibly is 16mm by design and my calipers are a little out but if I stick to my caliper measurements everything still remains consistently out together.) As the bearing is 14mm I want it to fully bear onto the 20mm diameter portion of the shaft and not 19.78mm portion, so if I shim 0.6mm from the gear face, the inner race should line
Just for the sake of some completeness for this thread I will add some final points and observations.
1) As per the post above I did fit the bearing with 0.6mm clearance and all has been fine.
2) Not sure if this has been mentioned elsewhere but if attempting to use one of these type of centring tools, don't bother as they will not work on this particular model. The hole in the end of the crankshaft/flywheel is actually larger than the spline diameter of the input shaft. Therefore when you select the correct size dowel to fit that whole and then slide on the dowel that fits the hole, you will quickly realise that if you tighten the pressure plate, you can't get the flywheel centring dowel back out through the spline of the friction plate. Old friction plate used in pictures just to demonstrate.
3) Then there is the plastic type centring tool which supposedly centres as it is drawn into the pressure fingers. It sort of works in principle but when you look it is not centred accurately and I think it's useless. I know some people will think it gets it close enough but I think that is a problem in itself and part of the reason why I had a problem when I did the clutch 4 years ago. I have seen people getting it close, then they just use brute force and ignorance to force the input shaft in. I have seen the engine lowered so far to one side that it is putting a lot of strain on the left side engine mount and so they can get a better angle to try and get the clutch housing/ g box to enter more easily. I believe this method could actually be the start of problems down the line with bearing and seals. The issue with entry is twofold, the friction plate not being centred accurately and the peaks and troughs of the splines on both the friction plate and the input shaft not aligned.
4) I did use the plastic centring tool but only as a clamp. I divided the splines into 3 equally spaced (120 degrees apart) and then measured from the valley of each spline to the front edge of the holes the pressure plate that locate to the dowels on the flywheel. It is then very accurately centred, when the 3 measurements are equal and immediately removes that as a potential issue when trying to enter the input shaft. I would use calipers as the blades locate perfectly and assure you are measuring in exactly the same place each time. A ruler would make it too hit and miss IMO.
So the plastic centring tool is just being used as a clamp and not to actually centre the friction plate. Measuring leaves no doubt at all that it is indeed in the centre.
5) So with the clutch centred and fixed to flywheel that is one half of the potential issues taken care of. Now the process of raising and aligning clutch housing/g-box. I think it is much better to not let the engine lower to one side, find a way to raise g/box in such a way that it stays on the same plane as the engine. So the two mating surfaces of the clutch housing and the engine are parallel then once at the correct height and the input shaft is at the same height as the spline on the friction plate you only need to do a tiny rotation at most of the gearbox to align the peaks and troughs of the spline and the clutch/g-box slides straight in no bother. No wrestling with it at weird angles, no spline damage from attempts at using the g-box like a battering ram.
6) For gearbox reassembley sealing I did not use Loctite 573 as recommended by Fiat , at the time of this post 50ml cost about £25. I did find a product on ebay (Delta D510 multi-gasket) that seemed to fit the bill and a more reasonable price plus it was made in England ( I prefer to support home businesses were possible too ). I rang the company and they were very helpful too and at a cost of £13.99 for 75ml delivered for what appears to be an almost identical product, I decided to give it a shot. I used that for all mating surfaces on the gearbox with the exception of the end cover plate that goes over the 5th gear, for that I used V-Tech Vital Red RTV which I already had. I did leave a full 24hrs before filling the gearbox with oil as the data sheet for D510 did say that was the full cure time. I am not endorsing these products just sharing my own experience and approx 1 month later now have had no leaks or issues.
So there you have it. Unlike the last time I fitted a new clutch to this car, there is no whirring noise and very smooth operation. I did go on and replace cambelt and water pump too but did not document that and fairly straight forward. The clutch and gearbox experiences all seem to be a little different from what I have read here so worthy of posting I felt. I hope some of the points I have made may help some people in the future but please note I am not a professional but as an engineer of another discipline do find that careful measurements help in all fields. I think this brings an end point to the thread but will try to answer any questions in the future should they arise and will be totally honest if the life of the new clutch and gearbox bearing and seal fail prematurely, I will update the thread accordingly.