A 606 with a 603 injection pump is the way to go.

Still mechanical. Just starter, alternator, glow plug, and temp sensor wiring. Same as a 617. I've noticed more and more 617 turbos developing knocks and or failing over the past 5 years, starting to leave a sour taste in my mouth. In fact my 617 currently sounds like it has a knock of some sort at 209k miles, but I need to rule out injectors and accessories first.

If you can financially swing it, go for the 606. Hell it has 24 valves, and a crossflow head.

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1981 300TD 4 speed manual
Euro bumpers, zender valance and skirts, H&R springs, billy HD's, leveled sls, real AMG Pentas 16x8 et11, vdo boost/egt gauges intergrated into ash tray, eurolights, led 3rd brake light