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The aim is to achieve a ‘nip’ on the outer sealing rings whilst also achieving a ‘nip’ on the plug ferrule washers. The original copper/asbestos washers are a pain in the proverbial, as a lot of torque has to be applied to the locking rings to get them to seal, and this risks cracking the water dome casting. I have given up on the granulated cork outer rings as antifreeze leaches through it and spoils the paint finish on the barrel. Put your chosen sealing washers, perhaps neoprene flat washers, on the plug ferrules, and tighten down the locking rings to a moderate tightness. (No silicone yet !). Now measure the gap between the water dome and the barrel. This is often different between sides, due to the old rubber Limit Gauge…
Now select outer sealing rings that are a couple of thou” MORE than the gap, preferably the rubberised canvas type, rather than the granulated cork. Next put a goodly amount of silicone mastic on both sides of the plug ferrule sealing washers, and a thin smear on both sides of the outer sealing rings. Now reassemble, and tighten down the plug ferrule locking rings firmly, but don’t go at them like a gorilla ! Wipe away any excess silicone that has squeezed out, before it sets, and leave the job for at least 24 hours before refilling the coolant.