As bulletproof as the 61X motors are, with the routine servicing needs, my vote is for the 603.
Sure, the #14 head is a bummer when overheated, and sure, the front head bolts in the chain area back out, allowing oil burn… but… overall, the power, sewing machine smooth running, the far easier maintenance… did I mention the power?

Of course, they need the vacuum pump upgrade. They need delivery valve seals every 100K… a #22 head upgrade is a dream improvement (I have one) the serpentine belt tensioning system is kooky… and the alternator really needs more output, but then you have to change to a clutched pulley on it… the stock aluminum fan is too weak, and the 606fan needs CAREFUL attention to prevent flexing into the radiator….
Why do I like it so much again?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!