Mitsubishi Repair: mitsubishi l200 pickup 2003 model diesel turbo, mitsubishi l200 pickup, piston ring

Question
i recently had the head gasket blow on my motor i got it repaired all belts changed and serviced i now  have serious trouble starting the motor it is  like the compression is on one cylinder only i went back to garage they said it could be piston ring prob but once started it runs well no missing  or bad tick over it is just starting it however i do get a lot of black smoke once it is fired up but it clears unless i rev fast then the smoke  is there again  . could you help me please?

Answer
Lee,
I'm not familiar with your model, but your problem sounds like perhaps your glow plug is not igniting your fuel.  If your compression is too low at times, your piston will not move, causing excess fuel to build up in your chamber.  But once compression returns, your glow plug will be able to ignite as much fuel as it can, and you'll see a plume of black smoke that is unburnt fuel.  

Again, my expertise is neither on this model nor diesel engines.  But your mechanic may be correct to assume a worn piston ring.  It may have little compression when the engine is cold, but as the engine warms up, the piston ring may expand enough to show adequate compression.  If that is the correct assumption, unfortunately, that would require another head removal to replace the piston rings.
Good luck!