Sunbeam Stiletto
Decoke and regrind plans lead to rebuild and restoration of sporting edge
Last time, we left the Stiletto looking lovely with its fettled bodywork and new paint, and able to stop on wet roads thanks to reversion to original drum brakes instead of lockup-prone, too-powerful discs (PC, June 2016). But there was another tripwire on the path to perfection. The engine was lacking sparkle and losing its water, so I took the head off with a view to a decoke, a regrinding of the valves and a new head gasket. If it only it had been that simple.
As I scraped the top faces of the wet cylinder liners clean, they moved. Only slightly, but slightly is too much: they are supposed to be bonded into the block, using super-strong Loctite and clamped for a day under a ton of pressure. Clearly they now were not, and that means oil and water were soon going to be swapping places. The plan to have the Stiletto on stage for the PC Restoration Show was receding rapidly.
So it was out with the engine, take it completely to bits and deliver said bits to Imp engine guru Dennis Allt. He built my last Imp’s engine, a potent little bundle of energy. He also built the Imp engine that PC gave away in a competition back in 2002, and he builds some very successful racing engines. Better yet, I’ve known Dennis for decades and he’s based only three miles from me.
He said that the Loctite bond had probably broken through decades of heating and cooling. The liners would have to come out, and then we’d take it from there. Normally this task is quite a struggle, but he pressed them out with ease.
Measuring up
Dennis measured the liners for bore wear and pronounced them reusable after a light honing, to the joy of my bank balance. There was a brief worry over a little bit of corrosion on one of the liner seatings in the aluminium block, but it didn’t stretch right across the seating so the surface was built up again with JB Weld epoxy. The rest of the block was in excellent shape. The pistons were not so good, with the ring grooves too worn to make it worth fitting new rings. So into the newly refitted, bonded and honed liners went a set of new pistons from tuned Imp parts specialist Mark Maynard, to broadly a Hepolite Powermax design but with modern, thin, low-friction piston rings. The pistons’ crowns needed a light skim to get the right height for the lightly skimmed block
‘All good, apart from one noisy tappet – so back to the workshop’
and liners. The crankshaft and main bearings were fine but Dennis fitted new big-end shells, and he skimmed the oil pump body to take up excess end float in the pump rotor.
Skimming off the top
Next, the cylinder head, also lightly skimmed. This needed new valve guides, in bronze, and new exhaust valves. Dennis also opened out the ports and valve throats to free the engine’s breathing at high revs, then built the engine back up with the latest in bombproof composite head gaskets and an 82-degree thermostat to replace the too-cool 74-degree one. A new old-stock Quinton Hazell clutch completed Dennis’s contribution. Or so I thought...
With the engine back home I fitted all the ancillaries, having already repainted them as required. I had also replaced the gearbox’s input shaft oil seal and the clutch release bearing. That QH clutch is known to have a heavy action, but I elected to go with it because the original Laycock is practically unobtainable in anything other than a dodgily reconditioned state.
The heaviness was to prove too much for the ageing slave cylinder, so it was back to Dennis for a new one and accompanying hose. I also made a mod to the stainless steel exhaust made for me by PD Gough. It had been setting up an annoying
boom around 4000rpm, which I thought might be because it was missing the original design’s bracing piece between its pair of entry pipes. So I devised a clamp between the pipes, and crossed my fingers for success. Back in the Stiletto’s tail, the engine started instantly. A test drive proved that the resonance had gone, and despite its new-build stiffness the engine felt sharper with much more bite up hills.
All good, then, apart from one noisy tappet. Dennis had set the clearances with his customary precision, confirmed when I measured them, but I could see that one tappet bucket was a loose and slightly rocking fit in the camshaft carrier. So back at Dennis’s workshop I extracted the camshaft, the carrier and the tappets. Dennis measured the valve-stem heights, selectively tried new steel tappets in the carrier for the best fit, and calculated the tappet shims required. I refitted the parts, re-measured the clearances and they were all to spec. The man’s a genius.
Several months later the tappets are still quiet and the Stiletto is running beautifully. It was on the Imp Club’s stand at the NEC Classic Car Show in November, but plenty of other things have happened to it in the interim. More next time.