Modern oil filter

When one of mine did that it was caused by overheating on one side due to one of the needles being accidentally set low. There are piston makers' web sites showing piston marks and what causes them. That looks like too close clearance I believe.

Full flow filters can be fitted (to slimline). Norville sold me my neat stainless steel bracket which fits on the cross tube behind the gearbox.

I am not a piston expert,but I would say thats typical of a four corner seizure when for whatever reason the clearance is not enough. Maybe overheating, distorsion, etc. The piston usually can be re used if cleaned up with a swiss file. I would check you have at least 41/2 thou clearance at the thrust faces  ,as that should hopefully ensure you have enough clearance everywhere else , trusting a bit blindly that the piston manufacturer has correctly cam machined the piston and its made of the correct  low expansion alloy. Is your barrel powder coated?, mixture and timing correct etc?.At 41/2 thou it may not run as quietly as others when cold ,but thats what I have had for many happy years.A bit of honeing  may be needed .In the past I have just hand filed pistons to releave the non thrust faces ,cleaned the bore with emery paper and all has worked. Shade tree mechanics!. 

A filter used to collect any debris in the system like blast media is pointless. An oil filter keeps the oil clean, it is not designed to catch large particles that should not be in the engine.

The damage occurs as it travels to the filter. This can be seen on tappets where glass bead and chill hard blast media has come from the head. The tappets will be scored at the cup end and the score mark gets smaller as it breaks up and passes past the tappet to take out the cam and tappet. 

If you really need to blast a cylinder head, degrease it, clean it, then clean it again. Degrease it, because there may still be particles trapped in corners and in threads. Collect the water each time until nothing settles in the bottom of your wash container. This can take a few hours, do not rush it, and due to the time taken don't assume whoever blasts it has cleaned it properly, like wise don't assume your engine builder will clean it either.

There are now media free blast systems, but even these can be contaminated with particles if not maintained correctly. 

4 corner seizure +1, not too little clearance but excess heat from too weak a mixture or advanced timing. Look for tiny pieces of alloy on the plugs, combustion chamber and a small depression on the top of piston where a hamster has been nibbling. The 4 corners relate to the thickest part of the piston, these expand faster than the thin section parts and are for and aft of the gudgeon pin.

Some years ago I ran a Triumph Stag as my daily transport, doing close to 20,000 miles a year. Of course it needed constant upkeep. The valve covers were looking a bit tatty so I had them bead blasted and powder coated. A few weeks later I noticed the oil pressure fluctuating wildly and unfortunate noises coming from the engine.

It turned out that I hadn't noticed that one cover had a wire mesh "filter" under the oil filler orifice and this had collected loads of beads. It then released them into the lubrication system. The oil pump was scored to an unbelievable extent; mains and big ends were scrap and in fact my only option was to fit a good used engine.

So I've never gone near beads again.