Complex sentence: Main design differences between the OM-601 and OM-616 include use of an aluminum head as opposed to cast iron, this helps make the OM-601 weigh 100 lb less than the OM-616 it replaced.
Split into simpler sentences: Main design differences between the OM-601 and OM-616 include use of an aluminum head as opposed to cast iron. This being a key element in making the OM-601 100 lb less than the old OM-616 it replaced.