The 455SD was designed as a race engine but GM got their hand slapped when caught messing with emission.The block is a 4 bolt main with all joined lifter bores like the RA Vs and also provisions for a dry sump oiling system.Diff cam altogether as it has a smaller dizzy gear on the cam and a larger gear on the dizzy to handle the 80 LB supplied in the engine.Too bad they could not get it ready for 1970 with high compression and a IV cam. They had to go back,run a smaller cam and a iron intake from the the engine that was given for emissions.Pre dated the VW cheating scandal.FWIW,Tom
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