Triumph Repair: rear end noise, Triumph, Spitfire

Question
QUESTION: I have a clinking noise in the rear end of my 75 Spitfire which is speed related - reduces or disappears under load. I've replaced Ujoints on the left and right axles and changed the driveshaft by 180 degrees thinking it may have been re-installed improperly but the noise is still as bad.
Is there a differential rebuild kit available? the car only has 28101 miles on it so don't want to replace the diff if at all possible.

ANSWER: Ron,

Check with the parts suppliers.  I haven't seen a differential rebuild kit as such, but the bearings and shims are available.

Keep in mind that the job requires the use of a differential case spreader.  Otherwise you run the risk of damaging the side gears on the ring carrier.



Cheers,

Jim

---------- FOLLOW-UP ----------

QUESTION: Thanks for the info, Jom.
How can I tell the ratio of the rear end?

Answer
Ron,

The absolute best way is to open the diff case and count teeth on the pinion and ring gear.  

Otherwise, there are several things to look for.

Common ratios fitted to the '70s Spitfires and GT6s were:

3.27:1  (GT6)
3.63:1  (GT6 and non US market Spitfires)
3.89:1  (typical for 1500)
4.11:1  (late MkIV 1300)

Most of the Spitfire 1500s were originally fitted with 3.89 differentials. Some of the cars had a 3.63 as an option for relaxed highway driving.  Sometimes racers (or wanna be racers ;) would fit 4.11 diffs for better acceleration.

If it's an original differential that hasn't been rebuilt, the serial number prefix will give the ratio:

3.89:1 Spitfire diffs have an FH prefix,
3.63:1's have FR
4.11:1 Spitfire Mk IV diffs are FK prefix

Otherwise, if you know the speedometer is accurate (or close) and the tachometer seems good, here are road speeds at 1000 rpm, 4th gear non overdrive:

Ratio          Road speed at 1000 rpm (assuming stock tires)
3.27:1          20.15 mph
3.63:1          18.0 mph
3.89:1          16.7 mph
4.11:1          15.75 mph


Cheers,

Jim