Good post by "GregGT" on CGOA ...
http://www.classicglastron.com/phpbb/viewtopic.php?f=22&t=5692&start=10******************************************************
Here is some useful info from an inline six expert that specializes in rebuilding them:
1968/1969 Merc 1250 (125hp) Is a critter unto itself and only interchangable with a '68 or '69 1250.
In 1970 the new "direct charge" powerhead came out. The 1970,1971 Merc 1350 (135hp) as well as the 1970, 1971, and 1972 Merc 1150 (115hp) all used this new powerhead. Also I can convert those early 115's to 135's for a small additional fee. This cannot be done with the 1972 and up powerheads.
The 1972 1400 (140 hp) marks the beginning of the "new" exhaust ports and base gasket / bolt pattern. This pattern lasted to the end of the inline six. In 1973 the 115 also got the new bolt pattern. This means that (with the exception of the 1972 115 that interchanges with '70 and '71) all the distributor ignition inline six powerheads made from 1972 to late 1979 will bolt on interchange. Also, this means that you can bolt a 140 or 150 onto a 115 leg to upgrade your outboard - the only difference in the powerhead is the internal porting. You cannot convert these 115's to 140/150 hp version, the internal porting is radicly different. But you can bolt a 150/140 powerhead to the 115 leg with no problem at all, the lower units are all the same. Also the 1972 140 is a "true 140". But, the later 70's 140's are identical to the 150's. Once Mercury came out with the V-6 150 in the mid-70's, the marketing guys renamed the 1500 inline to a 1400 so as to not have two 150's competing against each other.
Lastly, In late 1979 mercury did away with the distributor ignition and went with the modern ADI ignition. In 1982 they re-rated the powerheads to "propshaft HP" as opposed to earlier "crankshaft HP". The old 115 was called a "90" and the 140 became the "115". All the ADI ignition powerheads will interchange.
Here at the shop we differentiate the post '72 powerheads as "distributor or ADI ignition?" and "low or high output?"
