I dont know about the Td42 but i know Nissan do like to change the rocker cover colours on a few diffrent motors and "usually" means nothing other than diffrent year motor. It was done with the RB20det goin from red to silver, also the SR20det went from red top to black before going black top VVT.
And there is always people who will claim that "xxx" colour rocker cover engine is so much better than "xxxx" colour engine and so on and usually means stuff all as they are the exact same motor.
Mike my Gq ( safari ) is built 1989, its a manual low roof silver top base modelbest for turboing:
GU turbo black top, has the turbo oil drain cast into the block. 28mm gudgeons, different bolt pattern on the back of the crank. Vacuum pump driving off timing gear below fuel pump.
Good for 900+ Nm if you build them properly with ceramic bits etc. EG: One Wild Diesel in Perth build by United Fuel injection and Zac. Shawn Swaffield's various vehicles over east.
precombustion chamber ports are larger. Compression ratio 22.5:1
Engine number starts with TD42Tnnnnn
Silver top (which dont all have silver rocker covers) Build date to somewhere around August 1995. Vacuum pump on back of alternator. Good for 650-700 Nm at 30 psi in short bursts if you watch your gauges with standard internals. 30mm gudgeons. "small/standard" back end on crankshaft. small precombustion chamber ports. Compression ratio 22.7:1. I've just blown one of these up being an idiot and the block and liners have disintegrated between #3 and #4. I'd keep it to under 650Nm with these. Engine number starts with TD42nnnnn
Good for mild boost only:
non turbo GQ and GU "black tops" built post august 1995 approx. Vac pump driven off timing gear below fuel pump. No turbo drain cast into block, "small/standard" back end on crankshaft. 28mm gudgeons.
12 psi non intercooled, 15psi intercooled, Thou Shalt Not Pass this boost level. reasonably lively engine due to the lower mass pistons, especially if you use 4.375 diffs. small precomb chamber ports. compression ratio 22.7:1
the pick of the bunch for moderate applications because everyone wants the "strong" ones and the lower reciprocating mass helps performance.
Factory turbo Y60 Safari (GQ safari import)
ALL TD42s SHOULD LAST FOR AT LEAST 600,000KM WITH A TURBO AT MODERATE POWER LEVELS WITH REGULAR SERVICING.