silsleeve - how it works
The cylinder failure is typified by a ‘D'-shaped piece of the liner barrel becoming detached at the very top where it meets the cylinder head gasket. It also seems most common on cylinders 2 or 5, in other words the middle cylinder of each bank. Until now, such engines would be considered to be scrap. Many theories have been put forward on why this breakage should happen, but Nick Fulljames, our Engine Developer, believes that it is due to flexing of the engine block itself.
‘All engines twist to some extent,' he suggests. ‘And the harder you work them, the more they tend to distort. It's like the wing of an aeroplane. It visibly moves up and down as the aircraft passes through turbulence, which can be pretty alarming if you don't like flying, but if it didn't flex it would simply snap. Here that twisting starts a minute crack, and then it's only a matter of time.'
This crack spreads through the liner wall, eventually providing a path between cooling water and combustion chamber. Often the engine will run in this condition, perhaps a little down on power, but otherwise surviving by expelling any ingested water through the exhaust ports. However if the car is left unused for a day or two, and as the metal cools and contracts, sufficient coolant is drawn in to the combustion chamber that the next time it's cranked on the starter motor the hydraulic pressure simply bursts the affected cylinder wall like a balloon.
'Sometimes you'll see a failure in the affected cylinder's big-end shell bearing, too,' adds Fulljames. The good news is that the block is undamaged, only the liner needs replacing. Our project partner, Perfect Bore of Andover, machine out all the existing six ‘Lokasil' cylinder liners and replace them with re-designed liners that have the low-friction ‘Nikasil' running surface that Porsche have used on all 911 variants for many years, these liners also benefit from a widened top flange.
‘The standard blocks have a very open design to the water jacket,' says Nick Fulljames, ‘but what we do effectively creates what's called a closed deck. It gives a nice big footprint for the cylinder head and the gasket to clamp down on and certainly makes the finished engine much more rigid. But obviously not so rigid that it'll then be too brittle! Actually, what we're doing here has been used in all sorts of other engines for many years,' says Fulljames. ‘But we believe we are the first company to offer this repair process as a fully engineered solution to solve the M96 engine's single biggest problem, the cylinder walls cracking'
We will also supply piston rings which are correct for the new Nikasil liners, plus if we're building the engine, we'll fit all new bearings and seals, plus all the latest upgrades, e.g. Variocam solenoids, chain guide ramps, chain tensioner and of course our upgraded Intermediate Shaft. There have been some problems with the rear main crankshaft oil seals (RMS), though we believe that this is more attributable to the design of the seal rather than any mechanical issues. We replace the seal as a matter of course, utilising the latest design and materials as used on the 997, which we have found to work effectively even on a very grooved shaft contact surface.