Ripping its head off.

From the tool guide;

Borescope: noun. A special tool that allows you to look inside an engine and see in magnified detail that it is knackered, just like you first thought. Also see "Compression Tester"...

I put that in as in my experience, what you see on the screen can look very dramatic, whereas in reality it isn't too bad.

If the mice have been at the pistons, and started nibbling, then only you will know whether you wish to refit them or not. Unlike pre-ignition erosion, the usual place for detonation erosion is the top lands, (between crown and top ring) which means barrels off to check.

Regarding welding, it can be done,(have done this with prototype car engine pistons back in the '80s) but on a used piston, you will need a very good welder who will need it scrupulously clean before starting, and if it is the top lands, then not only will you need the lands machining, you will need the ring grooves remachining too. How much is a new piston again?

As for your head gasket, ask two Norton enthusiasts what is best and you will get at least 3 different answers and that is before you get onto what sealant to use! My view? Use the supplier that you trust the most, or at least had the best experience with. 

Here's hoping that it isn't too bad when you lift the lid off.

Regards,

George.

The barrel rear drain hole is too small for the job it is asked to do. Until the oil gets hot it will not drain down this hole. Plus while it is trying to defy gravity, the engine crankcase pressure is pushing it back upwards.  Consequently, oil pools in the inlet rocker box and then gets sucked down the valve guides to cause smokey engines. The solution would be to enlarge this hole but in the head is there is not enough metal around to enlarge it. safely. 

If you enlarge it in the barrel you end up reducing the amount of gasket fighting to keep the oil inside the engine.  Especially by the rear right head bolt.

I have added the pic of the Atlas Barrel to show the weakpoint of the 650 and 750 engines. The rear oil drain hole is the only point in the whole set-up where oil under pressure has an open gasket face trying to hold black goo at bay. All the other openings have a ring of metal to help seal the works. ie the pushrod tunnels and combustion pots. Though on cheap head gaskets the pushrod tunnels often have a joke for a sealing ring in this area.

In the rubbishy Composite gasket the oil tracks along the inside of the gasket to the head bolts (just 10mm away) and then leaks upwards and over the fins. Some sealant can help but is difficult to apply around the rear drain hole without blocking it.

The solid copper gaskets are a better solution but the wrong grade of Copper and/or  poor annealing often leads to the metal burning and thinning just where you need it most.

As has been pointed out above, getting a piston totally clean of oil for a welding job is not going to be easy.  Many years ago I met a chap who had sorted out a hole in his big single by drilling and inserting a nut & bolt to seal the piston.  On another engine he had fabricated a piston out of wood as a temporary fix.