Triumph GT6
How not to fit a small Triumph final drive unit…
Regular readers will recall I recently visited Paul Hughes at 2spec Transmissions to get a new differential built for the GT6 (PC, April 2021). I chose a 3.27:1 final drive ratio, bolted to a Blackline limited-slip differential. This should kill two birds with one stone, giving higher gearing more suited to the little Triumph’s tuned 2.5-litre engine, at the same time being strong enough to cope with said engine’s increased torque.
As soon as I got home with the freshly-built final drive, the weather turned cold, and it was only recently that the permafrost here in Leicestershire thawed to the point that lying on the cold garage floor for hours finally seemed feasible! With a hint of spring in the air, I jacked up the back of the GT6 and got the tools out.
Once the back of the car was settled securely on axle stands, there was just enough space underneath to get the exhaust off and uncouple the propshaft and halfshafts.
On top of the differential casing the suspension’s transverse leaf spring needs to be disconnected. After releasing the handbrake cables and shock absorbers, the rear suspension drops, which reduces the spring’s tension slightly. Then the clamp plate on top of the diff must be carefully removed to finally free the spring, which you do by while working through a hatch in the floor of the GT6’S load bay.
Smoothly does it
Everything was going nicely. The old final drive descended smoothly to the floor on my big trolley jack, and the new one was lifted into place, then bolted into position. Half shafts and propshaft were coupled up. Next, I clamped the suspension spring back on top of the diff casing and reconnected the shocks and handbrake. Exhaust on, wheels refitted, the car was soon back on the ground. That left just the final jobs inside the car to be done.
Armed with a torque wrench, I squeezed into the back of the car to finally tighten the leaf spring clamp nuts. And… disaster struck.
As I wound the last nut up to its specified torque there was a muted ‘thunk’ sound, accompanied by that sickening feeling of the nut going slack, as happens when a thread strips. If only the problem had been as simple as a stripped thread: under the car, I spotted a hairline crack running from the offending stud down the side of the differential casing. The casing of my new final drive had instantly become scrap.
Why this happened remains a mystery, as I’d done this job by the book. The studs were inserted finger-tight, the nuts were tightened evenly, and my newish professional torque wrench was set to 28 lb ft, when the workshop manual specifies 28-30 lb ft. It was either old age (the diff casing not me) fatigue or sheer bad luck, but the unit would have to be dropped out again.
One hour later, the new final drive was back on the garage floor. Looking at the cracked rear casing, it dawned on me that the situation could have been worse. All the clever work of shimming the crownwheel and pinion and setting the bearing preload happens in the front section of the case. The broken rear casing could be changed without disturbing the diff setup.
Giving in to the bling!
Online adverts didn’t reveal any cheap diffs to cannibalise for a replacement casing, so I rapidly gave in to temptation and ordered a glamorous alloy version from ebay seller Moto-build Racing. I’d been thinking about fitting one of these racy lightweight extravagances when the diff was first built, but recognised it was money that I didn’t need to spend. This time, I succumbed to the power of bling, even though it’s only visible from underneath the car!
The new casing soon arrived, and I took it over to Paul Hughes’ workshop for him to swap over, also asking him to drill and tap two extra stud holes for the spring mounting. The alloy casing comes tapped for four studs, correct for later Spitfires and GT6S, but I wasn’t going to take chances fitting fewer than the six studs my car started out with. Back home, the final drive was refitted for what I fervently hope is the last time. When torquing down the spring clamp plate, I showed an abundance of caution. Torquing initially to 12 lb ft, I worked up to a figure of 24 lb ft in small steps. That should be tight enough for the plate, without overloading the casing’s alloy threads
The final drive certainly looks like it means business; out for a test drive, the taller gearing clearly suits the GT6’S bigger engine. Job done.