Triumph GT6
Nigel concentrates on the bottom end
The 2500 engine build for my GT6 has mushroomed, as a light refresh became a full-on rebuild, with a few performance tweaks thrown in. As a result, costs and timescales have soared, and I can often be heard muttering: ‘Well I’m only going to do this once, so let’s do it right’.
This no-half-measures approach to the engine has resulted in various essential components being scattered across central England, while assorted specialists worked their magic. This month, I’m greatly relieved to report that the big Triumph motor is coming back together. Following a rebore at Ward Engineering in Rugby, I’ve collected the crank, rods, pistons and flywheel after regrinding and balancing. Jason Howe of JHPG works in a unit on a farm in south Leicestershire and specialises in crank grinding and tool making.
Meanwhile on the other side of the same building Bill Eales runs Midland Balancing. Most of his work is balancing rotating and reciprocating components for competition car or motorcycle engines. Bill balanced my Triumph crankshaft as an assembly with front pulley, flywheel and clutch cover attached. Separately, he matched the weight of all six connecting rods and pistons individually. Perhaps balancing isn’t strictly necessary for my road-going engine, but as
I needed to mate a flywheel and crank from different sources, it seemed a sensible option.
With most of the engine parts now back in my workshop, I ordered the appropriate undersize bearing shells from Triumph big saloon specialist Chris Witor and set to work on the bottom end. Taking no chances with contamination, I spent a good few hours cleaning the block and crank, paying particular attention to the oilways. These were syringed through with paraffin, probed with long, thin brushes and finally blown through with compressed air. Eventually the time came to start bolting bits back together.
Lubrication’s what you need
The crankshaft journals were liberally smeared with assembly lube, as were the main bearing shells. I don’t get to build engines very often, and I find these early stages of engine assembly exciting and anxiety-inducing in equal measure. Exciting because there’s real progress but anxious that something may go wrong, too. Nervously I lowered the heavy crankshaft onto the new main bearing shells and fitted the thrust washers and bearing caps. After torqueing the bearing caps, a trial turn of the crank showed very little resistance – excellent!
The next task was to check endfloat. Before stripping down, the crank endfloat was well out of tolerance. Fortunately, with new, standard
thickness thrust washers, my dial gauge revealed the endfloat is just on 8 thou, which is the top end of specification. Oversize thrusts are available, but not necessary here. Having fitted the new pistons to the balanced connecting rods, I set about the tricky task of fitting the piston rings. Fortunately this all went without a hitch, none of the fragile rings getting broken in the process.
Finally, I could slide the pistons into the block and assemble the big ends on the crank. Working slowly, all went smoothly again. With all the pistons in and the big ends torqued up, the engine still turned, but with more resistance from new rings in machined bores.
Now crank and pistons were installed, I needed to fit the front sealing block. Triumph’s cast iron engines use an alloy block to bridge the front main bearing cap and provide mountings for the front engine plate and the sump. It’s well known to the Triumph community as a source of persistent oil leak. Notorious for threads stripping in the soft alloy, I’ve opted to upgrade to a stainless steel replacement supplied by Mark Field of Jigsaw Triumph. The tiny corner gaskets were smeared with Wellseal then fitted, and the block was pulled squarely into place with the Allen screws that Jigsaw supply as part of the kit. This little job was finished by hammering a pair of wooden wedges into either side of the new sealing block to complete the seal.
On a plate
The final job for this stint in the workshop was the front engine plate. I needed to fit a Gt6-style front plate to the 2500 saloon engine and chose to go for a lightweight alloy version, again from Jigsaw Triumph. With its gasket stuck in place using Wellseal once more, the front plate slid into place over its two locating dowels before being bolted up. Next month’s jobs will be to fit the camshaft and build up the top end.
practicalclassics@bauermedia.co.uk