2.25 & 2.5 N/A and TD engines have a 'V' in the crown and 200/300TDi's have a round recess in it. In the TDi engines, the combustion happens in the piston crown, whereas the others have the combustion occur above it. I think that if you put a TD piston (for instance), in a TDi engine - compression will be rubbish. If you did the reverse, then the engine won't turn - that is until all 4 conrods have bent! :)
300TDi std pistons - ERR2410
200TDi std pistons - ERR1390 or ERR1391
2.5TD std pistons - ETC8670S or ETC8676
2.5N/A std pistons - RTC6442S
Les.