Triumph GT6
Nigel gets expert help to create a bulletproof differential
It was obvious when I put a 2.5-litre engine in my GT6 that the transmission would eventually need upgrading, so I’ve been collecting the parts required. A Dolomite 1850 gearbox with J-type overdrive turned up on ebay a few months ago, and Practical Classics reader and TSSC member, Chris Povey, kindly donated a worn high ratio final drive. I decided to tackle the final drive first for two reasons. First, against the odds, the original gearbox and overdrive are continuing to function without signs of distress. The other reason for doing the final drive now is the rather low ratio of the original unit. Triumph fitted overdrive GT6S with a 3.89:1 ratio final drive, meaning that at 60mph in direct top gear the engine is spinning at a busy 3500rpm, while cruising at 70mph in overdrive top equates to about 3400rpm. Even the standard two-litre engine could pull a higher ratio, and the new 2.5-litre lump certainly needs a step up in gearing.
Triumph’s parts list offers two longer final drive ratios: 3.63 from the Spitfire 1500, or 3.27, which was factory fit for non-overdrive GT6S.
My decision wasn’t straightforward; the Spitfire’s ratio only gives a seven per cent increase in gearing, not quite enough for the bigger engine. The 3.27 final drive is 14 per cent higher than standard, meaning 70mph cruising requires a much more relaxed 2800rpm from the engine. The torquey, long-stroke 2.5-litre engine should also pull the taller intermediate gears with ease.
Higher power
The answer may seem obvious, but there’s a snag. The highest 3.27 ratio comes with a differential that’s known for being rather fragile, and the torque of the bigger engine could quickly cause failure. After a bit of head scratching, I called Paul Hughes at 2Spec Transmissions for a chat. I’ve known Paul for several years, following a chance meeting at the NEC Classic Car Show that led to him sorting out the tricky rear hub bearings on the GT6. He runs a transmission business from a unit in Kenilworth and is a UK distributor for Blackline limited slip differentials. I kind of knew the way the conversation would go before ever picking up the phone!
Paul did indeed recommend a limited slip
‘Building a new diff is like assembling a three-dimensional jigsaw puzzle’
differential. As he says: ‘there’s nothing wrong with Triumph’s 3.27 crown wheel and pinion, it’s the diff that’s the weak link’. Fitting the Blackline LSD was the obvious solution but, to accommodate the high ratio crown wheel, the diff case needs machining slightly. I took Paul my worn 3.27 final drive unit, which he stripped while waiting for my new Blackline diff to come back from the machine shop. Unsurprisingly, it turned out the crown wheel and pinion were badly marked, so we sourced new replacements from nearby Triumph parts specialist Canley Classics. Paul also collected up new Timken bearings, oil seals and plenty of shims – we were ready for the big build!
Precision work
Building a differential is like assembling a threedimensional jigsaw puzzle with thousandth of an inch accuracy. The crown wheel and diff need shimming in one plane to mesh correctly with the pinion while, perpendicular to this, the pinion also needs to be shimmed to meet the crown wheel accurately, minimising backlash. Simultaneously, the pairs of opposing taper roller bearings must have the correct pre-load, without end float. Inevitably, the subassemblies of the crown wheel, bearings plus diff, and the pinion with its bearings, must be fitted and removed multiple times while shims are checked and adjusted. It Is precise, painstaking work and requires real expertise.
By the time I left Paul’s workshop with my new diff, I was confident it would be ideal for a little GT6 with a big engine. I also knew that I’d made the right decision to go to Paul, rather than attempting the final drive build myself.