Rock and roll
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My Dad came to visit one weekend and brought with him a chrome rocker cover for an A-series that I had left in his garage from a long time ago. I took off the old rocker cover and fitted the new chrome cover along with a new cork gasket. These gaskets are a bugger to fit but I find that applying a light layer of engine grease to the gasket will hold it in place as you flip over the rocker, line it up, and then drop it into place. Then insert the retaining bolts and finger-tighten.
Check that the gasket is not stuck out too far and appears to be sitting neatly in the lip of the rocker cover. Then tighten but not too much! I've seen people over-tighten these bolts and they have snapped or ruined the gasket. There isn't really any pressure in the top of the engine so its not as if you are tightening to a particular torque setting. I find that using a socket wrench and turning until I can no longer turn it using just my little finger is enough!