Will '73 20hp power head fit '79 20hp Merc?

[randys]

I have a 1973 20hp Merc. It runs. It looks like hell, having been "rebuilt" at some point, with a poor spray paint effort. I have just been given a 1979 20hp Merc with a frozen head. Previous owner told me that "it just got hot, then stopped running". He gave up trying to fix it, not having time or interest to get very far into it.

Other than the head, the 1979 is in better condition than the 1973. Especially regarding the tiller, which is very worn on the '73. Oil in both lower units looks "ok", the extent of my inspection of LU. I don't know which has the most life left in it.

I'd like to put the '73 head on the '79 body. That assumes my check of '73 compression shows as 'good'. I checked the online parts catalogs, and many parts (by part number) are shared, except the cylinder block part numbers are different. Same part numbers on drive shaft housing, drive shaft options, exhaust gaskets, water tubes, lower units. I assume removal/install involves dropping lower unit, removing some tubes and connections, and removing a few bolts. I've done it before for a small Evinrude.

Will this work easily? Or does someone know that am I missing something about compatibility between the two years? Are the lower units likely to be geared the same? (Same lower unit housings, but I there are options to drive shafts and gears.) Throttle, shift or control connector issues? Comments?

Thank you for your replies.

 

[Laddies]

Re: Will '73 20hp power head fit '79 20hp Merc?

The engine will bolt to the mid section just fine but the have 2 different ign. systems and the upper main caps are different and I believe the caps will interchange so if you talking about a complete swap with accessories, I believe it will interchange but not just changing powerheads.

 

[8075mercury]

Re: Will '73 20hp power head fit '79 20hp Merc?

You are lucky to get a '79 in good shape ! and a spare lower unit.

I agree and the easiest and best thing to do is to put the '73 powerhead complete with all the trimmings, ignition, carb, flywheel, ect on the '79. It will work great ! (The upper end cap changed as Laddies says and the stator coil assembly changed in 76 to 77, so changing the ignition would be dicey in my opinion. If you are forced to go that route, check the little bleed hose drain holes and hose arrangement for a match too).

Good luck !

 

[randys]

Re: Will '73 20hp power head fit '79 20hp Merc?

The engine will bolt to the mid section just fine but the have 2 different ign. systems and the upper main caps are different and I believe the caps will interchange so if you talking about a complete swap with accessories, I believe it will interchange but not just changing powerheads.

Maybe another way to say it is "I want to put cowls, drive shaft housing, and lower unit from the '79 onto the '73." Yes, anything attached would go as one unit with the power head, unless I need to remove it to get to head bolts. The shop manual not having arrived yet, I'm not sure of the procedure to remove throttle connections and the like. This is supposed to be relatively simple, saving time over rebuilding the head on the '79 or replacing parts on the '73.

Ignition. One item I did not think of, is different.

How much of an advantage is the upgraded ignition on the '79 over the '73? From a forum search "Merc 20 Hp", comments about those engines in the '73 range, Merc 200, seem to be well thought of. '79 was the conversion date to the newer system?

Edit: 8075 answered the conversion date question while I was typing. Thanks.