Spitfire Mk4 Differential

[Paul H]

I’ve got the opportunity of buying a Spitfire mk4 diff 3.63 for my Vitesse mk2 OD with 3.89 diff . The seller has stated “minimum backlash” 

The 3.63 would have advantages with longer drives . I travel from Suffolk to the Wirral 230miles regularly and would like to use the Vitesse more on these runs .

I know the diff would require 2 extra studs and I would also add a drain plug . At a minimum  the bearings on the inner axle shafts and the front flange oil seal and bearings would be changed and although I have not done this before reasonably comfortable about doing this . My question is how do I check if the diff is worth refurbing ? Does the diff need checking over by an expert or could I do the necessary tolerance checking . If work was required on the internals I know this would be beyond my capabilities and then part ex with Fitchetts  or hand over to Mike Papworth . Fitchetts current price for a recon is £540 which seems reasonable but what do you get with a refurb , accepting you won’t get anew crown wheel and pinion. 
Any advice welcomed 

Paul 

[johny]

I thought the 3.63 ratio was only used on 1500 Spits? Also will it come with its original 4 hole case?

[trigolf]

Paul, I've just bought a recon 3.89 diff from Fitchetts. Couldn't afford new crw/p. My understanding is all new bearings and seals fitted. Planet gears and crw/pinion reshimmed. All drive flanges refaced as necessary. It runs quietly.

[JumpingFrog]

44 minutes ago, Paul H said:

My question is how do I check if the diff is worth refurbing ? Does the diff need checking over by an expert or could I do the necessary tolerance checking . If work was required on the internals I know this would be beyond my capabilities and then part ex with Fitchetts  or hand over to Mike Papworth . Fitchetts current price for a recon is £540 which seems reasonable but what do you get with a refurb , accepting you won’t get anew crown wheel and pinion. 
Any advice welcomed 

Paul 

Condition of CWP is the main thing, minimum backlash is a good starting indication, you can also pop the back off and visually inspect the CWP. If it's very shiny, it's a bad sign that the hardening has worn through. Some guide to this here: https://www.differentials.com/technical-help-2/failure-analysis/

From a good refurb you should get all new bearings (preferably decent brand, like Timken), new seals, the pinion bearing preload should be reset (with new crushable spacer for a 3.63). The carrier should be checked, sun and planet gear washers replaced if required, carrier preload should be reset along with the backlash setting.

[trigolf]

Forgot to say, Tony at Fitchetts builds them in batches of 50 at a time.I asked about a drain plug. That would be a special order.

[johny]

41 minutes ago, JumpingFrog said:

From a good refurb you should get all new bearings (preferably decent brand, like Timken),

Unlikely to get known brand bearings these days as imperial so hard to come and expensive. Likely to be Korean but ok as most of our cars arent going to do many more miles now....

[JumpingFrog]

3 minutes ago, johny said:

Unlikely to get known brand bearings these days as imperial so hard to come and expensive. Likely to be Korean but ok as most of our cars arent going to do many more miles now....

You can buy Timken bearings for all of the diff (at least for Spitfire Mk. IV), unlike the gearbox bearings it's not a big issue and the cost is not that bad. Last year it was about £150 for the whole diff, I can dig out the part numbers if needed, but there's several different setups for carrier bearings depending on the exact year the car was built.

[Paul H]

4 minutes ago, JumpingFrog said:

You can buy Timken bearings for all of the diff (at least for Spitfire Mk. IV), unlike the gearbox bearings it's not a big issue and the cost is not that bad. Last year it was about £150 for the whole diff, I can dig out the part numbers if needed, but there's several different setups for carrier bearings depending on the exact year the car was built.

the diff has the later oil breather 

Paul 

[johny]

And a 3.89 serial number....

[Paul H]

18 minutes ago, johny said:

And a 3.89 serial number....

Thanks Jonny , back to the drawing board ! 
Paul 

[johny]

Oh dear although you do know youll lose some of that great acceleration with a 3.63? Think the Vitesse was advertised back in the day as out accelerating an E type up to 30mph😁

[Paul H]

Deal might be back on course . Seller says it’s a 3.63 as tested by turning the drive shafts and measuring and the prop is a nycro but not a castellated nut and split pin. So a suggested rebuilt diff using a 3.89 casing 

Paul 

[johny]

Hmmm if it had been done by any professional itd probably have been overstamped so I would be very wary. There are some right bodges out there now after so many years and the increasing value of some of our diffs....

[johny]

I opened one up recently to find the crush spacer had been reused by hammering it out! Of course torque to squash one like that is a lot lower than a new one so by the time you tighten it to the correct value the bearings are being mashed. Or another trick is to slacken off the torque or remove shims on old notchy bearings and then by hand they feel great again....

[daverclasper]

On 03/06/2023 at 14:05, johny said:

Oh dear although you do know youll lose some of that great acceleration with a 3.63? Think the Vitesse was advertised back in the day as out accelerating an E type up to 30mph😁

My 2litre is the only one I've ever driven and had a 3,27 and overdrive when I bought it. Great for cruising and get 38mpg on Motorway runs at 65mph. I do wonder how much more nippy it would be with an original 3,89. 

[johny]

Not just that Dave but the 3.89 does allow for a lot more time in 4th around town. You really can burble along in it for most conditions and then still accelerate away cleanly👍

[Wagger]

On 05/06/2023 at 20:22, johny said:

Not just that Dave but the 3.89 does allow for a lot more time in 4th around town. You really can burble along in it for most conditions and then still accelerate away cleanly👍

Just showing off now, but my 2500 engine will do 37 mph in first and pull from 18 mph in 4th using a 3.63:1 diff.