Mercruiser 470 vs. 170 Crankshaft

Rinker captiva 206 · Mar 30, 2019

I have a 1978 mercruiser 470 and a 1985 mercrusier 170. I spun a rod in the 1978, and the rod journal is almost out of tolerance to be undersize .020. I noticed this crankshaft seems to be a forged crankshaft. I had the 170 sitting on a shelf and to my knowledge the internals are the same aside from the way the water pump is attached to the camshaft. However, the crank in the 170 seems to be cast. Any thoughts? Did mercruiser decide that a cast crank was sufficient later on? Thanks.

kenny nunez · Mar 30, 2019

They probably went to cast crankshafts as a $ saver. I do not see any reason to not use the cast crankshaft. The main thing is to make sure all the chips from the burned crankshaft are removed from the oil passages. The early 470s had a problem with the water pump shaft pushing the impeller through the timing cover if the engine ever “kicked back” on a false start and the shaft would unscrew that held the pump impeller.