Fitting the 4.2 all-synchromesh gearbox to a 3.8

The gearbox introduced with the 4.2 litre car in late 1964 was Jaguar's own design, with synchromesh on all four forward gears. It replaced the Moss unit that every 3.8 litre E-Type left the factory with. Owners of 3.8s have been fitting the later box ever since, because it removes the one part of driving an early car that most often puts people off.

Why owners do it

• Synchromesh on first, so the car can be put into gear while rolling and in traffic.
• A quicker, less obstructive change between second and third, which is where the Moss box is slowest.
• Less need to double-declutch, which matters most to anyone who did not learn the habit.
• Later boxes and their parts are more numerous than Moss internals, which affects how easily a repair can be sorted.

It is not a straight swap

The two boxes differ in casing, length and mounting, so the job reaches further than the gearbox itself. Expect to address the bellhousing and clutch arrangement, the gearbox mounting and crossmember, the propshaft, the speedometer drive and cable, the clutch release mechanism and its hydraulics, and the gear lever position where it meets the tunnel and the console. Some cars need the transmission tunnel altering, and some do not, depending on which combination of parts is used.

That is why the sensible route is a kit or a set of parts assembled by a specialist who has done the conversion before, rather than a secondhand box bought on its own. Sourcing the box is the easy part. Sourcing everything that has to go with it, in versions that match each other, is the work.

The alternative, a five-speed

Owners considering this change usually also consider a five-speed conversion, which adds a higher top ratio for motorway use as well as a modern change. It is a larger departure from standard and generally a more expensive one. If the reason for changing is town traffic and a stiff change, the 4.2 box answers it. If the reason is long-distance cruising, look at the five-speed options before committing.

Originality and reversibility

A 3.8 with a later box no longer matches its build record, and the gearbox number will not agree with the Heritage Certificate. That matters to some owners and not to others. Keep the original Moss box, keep it dry and labelled with the car, and keep the parts removed during the conversion. A car that can be returned to its as-built specification is a different proposition from one whose original transmission was sold on. Describe the conversion openly when the car changes hands.

Before starting

• Confirm whether the existing box is genuinely worn or simply being driven too quickly. A healthy Moss box driven properly is better than its reputation.
• Decide the target specification first, then buy parts to suit it, rather than buying a box and working backwards.
• Agree with whoever is doing the work what happens to the tunnel, the console and the lever position, since that is where the visible compromises appear.
• Ask the specialist to confirm the clutch, release bearing and hydraulic parts as a set, because mismatched release components are the usual cause of trouble afterwards.