I see no one has any answers yet....difficult to find facts on this. But I have a few books...the 221 rods have a smaller crank end and will not fit the Merc 4" crank. You would have to get 8BA, 0BA or 29A rods. Journal diameters on all were 2.139", but 8BA and 0BA rods could only be used with insert bearings and on journals with two oil-feed holes. The 221 engine rods were 1.999" at the journal end and designated "21A". The rod lengths are the same at 7". Hope this helps some.