I think the stock bottem end parts are fine. I like the heat disapation of plasma coating and the endurance of cryo treating.
Absolutly you'd want it balanced and blueprinted, but the thing is it already IS from the factory(another reason they seem to last forever). I think a very, very careful rebuild to tight specs with hand gauged parts, cryo treat the riciprocating assembly, plasma coat the pistons(and what ever that low friction coating on the skirts is). Maybe beam the rods like we do with V8's. I wonder if ARP makes any rod or main bolts, or head studs.

Porting and poloshing has got to help some. Any reduction in the friction of the air comeing or going is good.

A newer turbo that spools faster/higher, intercooler, someone who actually knows how the injection pump works internally so that they can tune it to flow more fuel, ditto the injectors.

I doubt the cam can be changed much, there was great discusion about this with the VW diesels some time ago. But I'm not as familier with the 617, so maybe..

Head gasket, and a way to lower compresion noticably. I wonder if a fresh motor will start with 16:1 in the cold with just glowplugs?

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K