Diffs: 4 stud vs 6 stud: 4.11 vs 3.89

[slapathy]

Hopefully this may be of use to someone. I have just finished replacing the diff on my 13/60 Estate, having decided to swap the standard 4.11 13/60 diff for a 3.89 one from a Mk IV Spitfire which I bought from Ebay, as I would like eventually to fit a Mk1 2l engine + od gearbox. I imagined this would be quite straightforward but I encountered a couple of niggling problems.

The first related to the number of studs. The diff was listed on Ebay as being from a Mk IV swing spring Spitfire, which meant only the four outer stud holes were tapped the required 3/8" UNF it would appear that the central stud holes are tapped but were (factory) blanked off with aluminium plugs. It is necessary to drill these out 5/16"/8mm diameter and tap them 3/8" UNF. Make sure you split the back casing from the front before doing this so you can clean the swarf out. A new gasket is £1.99 from Ebay.

Having done this, I attempted to refit the diff to the chassis. I have read somewhere (TSSC mag I think) that it is sometimes necessary to bend the front plate to fit the chassis studs, but these were out of position by about 6mm on the LH side, and the RH side by about 3mm. No amount of persuasion with a hammer would have got the bracket holes  in line with the studs.
Easy I thought, I'll just swap the Spttfire mounting plate for the 13/60 one, which would have gone well apart from the 13/60 diff mounting spigot being about 1/16" smaller in diameter than the Spitfire one. So I clamped the two together and filed out the Herald one to match the Spit one.
Once I had done this, it all went together quite well.

[Pete Lewis]

having had one come loose and lock the prop make sure the front plate is fixed with 4 taper shank bolts as the holes in the plate are larger than the bolt thread dia.  you need internal shakeproof washers on these 4 special taper bolts.
having found this out with much noise and clatter the recon diff only had std 3/8 bolts in the plate, unable to get under in clean togs for a day out is the only time in  50 years of driving  returned on a flat bed recovery ..

I have also  sucessfully drilled a 4 hole to a six hole using the spring plate as a jig,
drilled down 25mm , this didnt drill right through so no debris inside.

Pete