Which diff?

[rossybrown]

Hi All,

After 19 years and seven years in my ownership my £60 1968 spit is finally MoT'd and on the road.  I've got a few niggles to sort (exhaust bangs on underside, diff leaks, roof fit is poor,  i need some sun visors etc.) but she seems perky to drive and very chuckable.

However, I find the gearing too low with 60mph being nearly 4000rpm. Therefore given the leaking diff I'm considering upgrading to a higher geared one (3.89 or 3.63 for £85 from spitfire graveyard), my question is whether it's a straight swap and which to go for?

Also how can I tell which diff i actually have?

Cheers,

Ross

[bob dunn]

my std 1975 spit 1300 is 4000 rpm at 60 in third  there is a good site for gears on minty lamb .

[GreenV8Machine]

you could get a recon one from fitchetts and you'll have to ask for small output flanges to be installed.

[Velocita Rosso]

Be wary of second hand diffs , ..unless you are going to strip it and rebuild.
You may find replacing with second hand , getting more whine than the other half does for wasting £85.00
Then on the other hand you may pick up a decent one

[James]

A Mk3 should have a 4.11 with small flanges - it sounds like you have 4.11 based on those figures (60mph is around 3750rpm)

Don't go for a 3.63 because you will kill performance - that is in no way an upgrade - you'll get to hills and have to change down a gear.

Mk3 Spitfires work best with a 4.11 - the engine should be smooth and last all day at high speed. If you want a lower cruising RPM then get an overdrive gearbox rather than fudge it by changing the diff. 60mph would then be 3000rpm.

As Mike says above fitting secondhand diffs can be hazardous - there's no way of knowing until you fit it if it's any better.