the piston top has a lot to do with compression and combustion...pistons come in many varieties....
I've always known diesel pistons to come all the way to top of the cylinder walls.....ie, the top edge of the piston being level with the top of cylinder, even the cylinder
head is shaped to accommodate the piston crown.....a 2mm gap here simply does not sound right to me...
if there is a gap....the cylinder head
gasket is where the gap will be.....this thickness can be added to the 2mm....diesel cylinder head gaskets usually have a metal ring around the cylinder...and I would guess that
gasket thickness is probably in the 1mm range....so now you have a 3mm gap....doesn't sound like much, but if you are using compression as the combustion agent...it's a lot...enough to prevent combustion....
diesel engines are built to extremely fine tolerance to make this all
work.
the piston rings MUST be placed on the piston in a certain way...another area where compression could be
lost...doing this wrong could also lead to compression problems..
finally, doing your own valve job is an art....the slightest imperfection will allow combustion pressure to escape at the valve seat.
I've seen cams/valve springs/valve retainer clips being placed back on valves, that were not done correctly....another source of compression leakage...
all these things need to be reviewed and addressed.......a compression test will tell you if you have sufficient compression, but won't tell you where the problem is.
The smaller
Volvo engines can be hand cranked....this is done by a lever that changes the cam position to reduce the compression so you can crank it...another possible area of concern, if so equipped.....this is a good example of how even a small difference in cylinder head pressure allows the engine to be hand cranked. It's not possible to hand crank even a small diesel without the de-compression lever...
the injectors and
fuel delivery system are precision made, the average home
mechanic cannot
service these...they must be taken to an appropriate shop for service...
many things......