On the outside these engines are virtually identical to the original Matchless engine. The only visible differences are that they have a 10mm central spark plug instead of the original 14mm plug, and the inlet flange is a little bit larger and has the carb mounting holes 65mm apart in order to make it possible to fit an Amal GP2 or 1000 series concentric carb. No one has spotted the differences when inspecting the engine for differences.
The internals are greatly improved. The crankshaft is of the well proven ABSAF pressed up type with needle big end bearing and a longer than standard Carillo conrod. The forged piston is of modern design and is much lighter than the original. The barrel is one piece aluminium, nikasil plated, so there is no liner. Piston to barrel clearance is 0.05mm.
The crankshaft bearings are metric roller bearings with brass cages and can take very high loads. The valves have 5/16" stems and are slightly bigger than original. Valve guides are phosphor bronze and the coil valve springs are very high quality.
The G85CS cams are Vernier adjustable and run in needle bearings. Head bolts and studs have been redesigned. All dimensions and threads are as original except for the crank bearing housings. Rockers were also redesigned at the valve end for better geometry and lubrication.
500cc engine bore 86 x stroke 85.5
600cc engine bore 90 x stroke 92.5