Restoration of my 1965 Fiat 2300S Coupe - Part 24
The 5 speed gearbox conversion
Making the Gearbox Adapter
I put the Alfa 105 and Fiat 2300S coupe gearboxes side by side on the bench and started the ‘measure up’ to ascertain the viability of doing the 5 speed conversion.
Things looked promising – the tip of the input shaft to the output flange is the same length and the output flange has the same bolt pattern (just uses thicker bolts - 10mm vs 8mm). So, there will be no modification required of the tailshaft assembly apart from the manufacture of 3 small spacers to make up the different bolt diameter. First ‘tick’.
The Alfa bellhousing is 12mm longer than the Fiat unit, therefore, a 12mm thick adapter plate could be used to mate the Fiat unit to the Alfa gearbox. Second ‘tick’.
Using the two bellhousing I drew up a cardboard template to determine how the two bolt patterns compare, and to determine any other clearance issues. After looking at all of the aspects, there is sufficient difference in the bolt locations and no obvious ‘show stoppers’ to enable a reasonably simple adapter plate to be made. Third ‘tick’.
The Alfa input shaft is thicker than the Fiat shaft, and is also thicker on the end that goes into the spigot bearing. A quick online check revealed that a suitable spigot bearing is a commonly available part (the Alfa just uses a bronze bush here instead of a bearing which is very simply to make anyway). Fourth ‘tick’. As for the clutch release bearing guide tube which goes over the input shaft – three different ideas have come to mind and I will ‘fine tune’ the thought process down to the best solution at a later time. Obviously, an Alfa clutch plate will need to be sourced and a suitable rear mount will be fabricated as required once I get to the installation stage. So, no show stoppers at all and let the ‘fun’ begin :)
The first thing that I did was to design and manufacture two accurate template plugs utilising my 3D printer. These enabled me to drill the first 6mm reference hole for the centre of the input shaft and accurately mark all of the required bolt holes to ensure proper alignment.
After all holes were marked and checked the drilling began. 10mm holes for the Alfa bolt pattern and 6.8mm holes for the Fiat bolts. The 10mm holes were then countersunk for the flat head (countersunk) screws necessary to flush mount the adapter plate to the Alfa gearbox. The 6.8mm holes were thread tapped to M8x1.25 for the Fiat mounting bolts.
Next came the more ‘fun’ aspect of creating a 38mm x 9mm recess for the input shaft seal. As I haven’t purchased a milling machine yet - this was done by using a series of hole cutters one by one until the desires recess was created, the final 28mm cut was from the ‘other’ side for shaft clearance. This turned out pretty good all things considered :)
The next hole cutter used was the ‘small’ one for the selector shaft clearance holes, and then onto the secondary shaft clearance recess. This was a similar process to the seal recess except that I did not want a guide hole all the way through the adapter plate (as this needs to be oil tight) so a shorter guide was used to get the recess roughly into shape, finishing off with the cutting bur in the drill press.
So, now for the trial fit. A couple of the countersunk screws were ‘radiused’ to allow room for the 8mm bolts and the Fiat bellhousing was marked where the selector shaft rod clearance is required. As for the input shaft sleeve – I have decided that the easiest way forward is to use the Fiat part with the internal diameter increased to 27mm. The mounting flange will be bolted to the adapter plate to stop it from rotating, and this will use the standard clutch release bearing, housing and lever. If required at a later date could be reattached to the Fiat gearbox and used there as is.
Now, I just needed to make the plate look ‘pretty’ :)
Shaping it to match the gearbox and bellhousing with the angle grinder, and finally the flapper wheel to smooth it up and make it look like it ‘belongs’ there :)
I used the original Alfa reverse light switch mounted inside the bellhousing (as per original Alfa), just had to straighten the bracket out to around 45° and drill through the bellhousing for the attachment screws and wires. I’m not entirely happy with the switch inside the bellhousing for maintenance reasons, but, I did note that this unit is a Bosch replacement switch, so I figure that it is likely to be very reliable :)
Before being fitted to the car I will make a little metal cover for the selector shafts between the bellhousing and adapter plate to keep them out of the elements. Now, I just need to machine the thrust sleeve, source a speedo cable and buy a new clutch plate :)
N.B.This Alfa 105 gearbox that I have is a pre 1972 ‘long shaft’ unit where the selector shafts protrude through the top front of the gearbox. The protruding reverse selector shaft activates the reverse light switch that resides inside the Alfa bellhousing, and there is a mechanical lock out mechanism that works off the shift lever to stop any inadvertent gear shifting from 5th into reverse. These are of a ‘constant mesh’ design – more like a motorbike gearbox than your normal car gearbox, and the shift feels very positive and purposefully clunky which perfectly suits the vehicle era :)
A quick comparison between the gear ratios shows that I will have a little better acceleration in 1st and 2nd, a slightly taller 3rd gear, 4th is obviously the same and 5th gives me a 21% decrease in revs for touring :
Gear Fiat 2300 Alfa 105 % difference
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