“I presume you are not saying that a Detroit Diesel has its exhaust ports in the cylinder liner - even though the above tends to read that way.”
The Detroit Diesels have the intake ports on the cylinder liner and the exhaust poppet vales on the cylinder head.
In case the Detroit Diesel is to operate with the arrangement proposed by the guy in Eng-Tips (i.e. in case of a modified Detroit Diesel having exhaust ports and intake poppet valves), the piston skirt has to thrust not onto the cold side of the cylinder, but onto the hot exhaust ports, etc.
“Another question - why are ship 2-stroke diesels of crosshead layout? Why not just a giant version of a Detroit Diesel? Ship 4-stroke diesels are "trunk" and appear to be just like giant automotive and truck diesel engines.”
A few reasons:
The stroke to bore ratio can be as high as you like in a cross head engine, giving a way smaller surface to volume ratio for the combustion chamber during combustion. In a 4-stroke, if the stroke to bore ratio is over a limit they are necessary cuts on the lower end of the cylinder liner to avoid connecting rod to cylinder liner collision (the Hanshin GEL44 is a big 4-stroke having stroke to bore ratio 2).
From Taylor’s “The internal combustion engine in theory and practice” for the Hanshin GEL44: The Bore is small to allow very high bmep (294 psi) with consequent high mechanical efficiency resulting in remarkable fuel efficiency…”
The best efficiency is at a mean piston speed about 7.5m/sec. For direct drive of the propeller (the big ones have best point somewhere between 80 to 160 rpm) of a big ship , the resulting stroke is from 2.8m to 1.4m, otherwise you need revs reduction (gearbox), the overall efficiency (brake thermal efficiency of the engine multiplied by the efficiency of the gearbox) drops, the cost increases.
The two stroke running on the same bmep as a 4-stroke of same displacement and same revs, makes double power.
The cross head takes the thrust loads away from the cylinder liner. With the heavy supercharging used, the thrust loads is difficult to be taken at the piston skirt to cylinder "contact" without deformation (the sealing of the combustion chamber degrades if the piston rings are abutting onto a deformed cylinder). And if the necessary quantity of lubricating oil exceeds the quantity the scapper rings can scrap, a part of the lube gets into the combustion chamber.
The cross head, on the other hand, uses as much lubricant as necessary, has the perfect form for the forces it takes and finally allows short connecting rod to be used, reducing the overall engine height.
The medium speed Wartsila 64 is the most powerful 4-stroke marine (it may also be the biggest: 640mm bore, 900mm stroke). The Wartsila RT96 low-speed 2-stroke has 960mm bore (i.e. 2.25 times the piston area of the biggest(?) 4-stroke) and 2500mm stroke.
The best with the small bore is that the loads are way smaller. This PatMar arrangement :
matches to heavy truck use (90mm bore, 400mm stroke, straight-four, even firing, balanced as the best V-8, 10lit 2-stroke). Its small bore makes the loads on the piston, on the connecting rod, on the cross head and on the crankshaft similar to those of a small 4-cyliner, 2-liter supercharged direct injection engine.