- Triumph Spitfire Diff Ratio Change
of you know, to varying degrees of amusement or astonishment,
that I have recently changed my Spitfire diff ratio from 4.11:1
to 3.89:1. The reason was that the crown wheel and pinion were
worn, and I thought slightly longer legs would be a good idea.
final result is wonderful. Through the gears, the car is not
noticeably sluggish. The legs are only 5.4% longer, but what a
difference! The diff is so quiet that I now realise how noisy the
what’s the bottom line? Here it is: You can’t do it unless you
have a diff housing suitable for the 3.89:1 ratio. Full Stop.
on if you want the Ts and Cs but first some technical terms:
Diff numbers starting FC = Mk 1, 2 and early 3 up to GA housing.
FH = Mk 4 and other 1500cc cars, and more. 3.89:1 ratio
FR = 3.63:1 ratio and, I was told by reliable sources, that parts
are interchangeable with 3.89:1 diffs.
But 4.11:1 and 3.27:1 diffs are not interchangeable with others
FM = American Mk4. 3.89:1 ratio
why you can’t put 3.89:1 (FH) parts into a 4.11:1 (FC) diff
The front housings are machined differently so that the pinion
meets the crown wheel at a slightly different angle. This is the
show stopper, there are no shims or spacers to fix this.
The FH front housing diff carrier bearing mounts are closer
together and smaller OD to suit smaller carrier bearings. Both
carrier bearings have the same ID.
The front housing and diff carrier must be used together and can
not be interchanged. The FH flange to which the crown wheel
is bolted, is thinner - to allow for a thicker crown wheel.
that you probably could use an FC diff carrier if you are
prepared to machine the crown wheel flange by almost 1mm. Then
the modified FC carrier might be used in the FH housing. In that
case the FC rear housing and side output shafts might be used.
Personally I think it is risky to machine original parts into
something non-standard. But if needs must....
FH crown wheel bolt heads are flatter, to give clearance inside
The FH diff carrier (sun gear) output shaft splines are bigger in
The FH side output shafts and splines are thicker, bearings are
bigger, drive flanges are bigger.
side output shafts and half shaft flanges are centred by a large
diameter boss and recess which is the same for both FC and FH
diffs. Thus the flange bolt holes can be slotted to match. This
is what I did.
Front and rear housings are interchangeable w.r.t. casing joint
and alignment of the output shafts. But FH output shaft bearings
are bigger, thus front and rear housings should go together.
The FH pinion front bearing and seal is bigger
The FH pinion input flange is bigger and the splined length is
longer. I had had my FC input flange splines slotted to fit the
FH pinion while I was thinking that the FC housing would work.
Thus my FC input flange fitted the new FH pinion.
the input flange was done by wire EDM cutting. Grindex
Engineering in PE do this. It is very accurate and costs a good
deal less than conventional slotting. Worth knowing.
Note: The pinion supplied by Rimmer was, in my opinion,
incorrectly manufactured. The pinion is assembled with a
collapsible spacer which fits between a shoulder on the pinion
shaft and the toe bearing. As manufactured, the spacer would have
needed to collapse by about 7.5mm - far more than it’s design
allowed. I resolved this by making and fitting a 6mm spacer
behind the toe bearing cone. The added advantage of this, I found
out later, was that my FC input flange (now slotted to fit the FH
pinion) was about 5.5mm shorter than the FH input flange. The
spacer thus corrected the input flange position on the pinion
splines - a gap is necessary between the pinion splines and the
washer / nut which holds the assembly together.
thanks to: Beyers Vermaak
tried very hard to find a FH diff for me and put me onto Jaco and
advertised for me in Sabrina. Jaco Van Vuuren
in Centurion makes his living by restoring and fixing Triumphs.
He confirmed exactly what I had to look for, and he finally found
a diff for me. Frank Dreher
in Cape Town also fixes Triumphs, confirmed what I needed and
could have sold me a 4.11:1 diff. The guys at ITAC
issued an import permit to me efficiently and free of charge,
just in case I got really desperate. The technical guys at Rimmer
UK who also confirmed what I needed. Jeff
Nagel at RDG PE was a mine of
technical info and physical help and finally set up my diff
professionally. And many more folk who I spoke to along the way.
of luck it you should ever have such “clever” ideas!