Well, starting from the beginning, that fluffy white stuff on top of the pistons is oxidized aluminum
, probably from seawater, but could be from condensation
in an engine that has been sitting for a long time. My guess is that even after you get the engine turning the rings are going to be corroded into the ring grooves, which will necessitate pulling the pistons out and unsticking the rings at a minimum, and probably replacing the pistons.
On a diesel
engine the cylinder wall surface is crucial for the engine to make full power, any pitting is bad, though sometimes an engine will run reasonably well with minor imperfections. Since the cylinders are cast into the block generally you just get the block bored and honed. I'm pretty sure pistons are available in .25 and .5 mm oversize.
The good new is that these engines are used in a wide variety of applications, the important part is identifying which one you have. I was under the impression that the m20 was an ISM/Perkins 103-07, but I could be wrong. The actual engine number, be it 103-06 or 103-07 may be cast into the side of the block, a sure fire way is to measure the piston diameter, the 103-06 is 64 mm, the 103-07 is 67mm.
Many of the parts
between the -06 and -07 will turn out to be interchangeable, a copy of the service
and a set of calipers is a good way to start figuring out which ones are and which ones aren't...
A lot of these engines, in variety of sizes were used in AC units for 18 wheeler reefer units and medium big lawn mowers, parts are everywhere and are available in original and aftermarket. The supply seems a little confusing, I suppose because of the many applications, but a few phone
calls to different parts suppliers should eventually get you in touch with a knowledgeable person. Perkins prices are generally reasonable, try and stay away from strictly marine suppliers. As an example my first search for '103-07 piston' turned up a piston for 76.00 US