TOPIC: Interchangability Merc power head

Hey Yall. I have a 68 Merc 100 hp who needs a lower unit. I can buy one here,outright.But, Ive seen a 68 Merc 650 for sale that they claim runs well and its slightly more than a lower unit alone. The question is, can I drop my 100 hp power head on to the 650s lower leg. I think so but I wanted a second opinion before I commit to buying either one.I could end up with two running motors I thought the lower units were the same on the 650, 70 85 90 hp etc of that era but my memory fails me. Opinions and knowledge please. Thanks Dave

Dave,

Far as I can recall the powerhead bolt pattern on the 65 is the same as the 100hp and you should be able to plop a 100hp powerhead on the 65hp base. Worst case, you'd need to transfer the 100hp exhaust tuner assy to the 65hp mid-section.

But even simpler is to remove the 65hp lower unit and install it on your 100hp. They are identical except possibly for gear ratio. Some 65hp models had a 2.3:1 gear ratio instead of the 2:1 gears that you'd find on a 100hp.

I can't recall when the switch to 2.3:1 gears was made but sometime in the late '60's.

Doesn't matter too much, except it may affect your prop selection. The change in gear ratios would be similar in effect to installing a prop with 2"-less pitch.

This actually works very well in the 100hp model. I had an older 1000 with the 2.3 gearbox (from an 800-4) and it pulled well while having a broader selection of pitch sizes.

At any rate, if the entire engine is that cheap, it's well worth buying for parts.

If you find the 65hp powerhead is decent, it's worth keeping around until you can find another lower unit. Craigslist is a wonderful resource!

Let us know how you make out with that.........ed