Re: New here and posting first question
My guess is that you could make the swap work.
There are quite a few differences between the two powerheads in terms of part number comparison. Some of those differences may be nothing more than situations where one part number supercedes the other. There are some, however, that appear to reflect real differences.
The crankcase assembly (two halves) for the '68 motor are physically different than the those for the '75. The difference that I saw (there may be more), is that the '68 motor has an oil drain valve on the front, starboard side of the front casing half. The '75 powerhead does not have this feature.
The driveshaft assembly and all of the lower crankshaft seal components are the same between the two models. That's a good sign and means that there shouldn't be any incompatibilities between the '68 crankshaft and the '75 driveshaft.
The exhaust housing for the '75 is also a different part number than the one for the '68. Whether that really means anything, is hard to say just by looking at a parts diagram.
What is interesting, and may say quite a bit about whether the older powerhead will mate up with the '75 motor, is that the exhaust housing gasket, which fits between the exhaust housing and the powerhead base, is the same for both motors. That tells me that the bolt pattern is the same, as is the water passage configuration.
On the carburetor, the float bowls for the two motors have the same part number, so they are the same. The body on each carb is different, however. This is true not only of the part numbers - in the diagram, you can see that the casting is physically different. The main feature change appears to be in the design of the cam follower. The orifice plug and slow speed needle valve for the two motors also have different part numbers. Once again, its hard to say if the numbers are just updated, or if the parts are physically different.
In the case of the carb, however, the gasket tells a good story too. They part numbers are the same for the two motors, so the '75 carb (which you will probably have to use if the choke lever is positioned differently) should bolt up to the '68 powerhead.
Next on the list is the intake manifold. There are different part numbers for the two engines. I looked at this because the intake has the mounting point for the recoil starter on it, and there are differences in the starter between the two motors. Once again, the gasket that sits between the manifold and the leaf plate tells an encouraging story - it is the same for both motors, so you should be able to swap manifold covers if you have to. What appears to be different between the two motors behind the manifold gasket cover, is that the leaf plate and the leaf plate gasket on the '75 motor, don't have a cutout in them for the oil drain. In both cases, the part numbers for the leaf valve and the leaf stop are the same.
All of that said, and while there are some subtle differences in the recoil starter assemblies, the general positioning of them appears to be the same. From a practical perspective, that may mean that the setup on the '68 motor will work just fine.
I looked at a couple of other things too. The armature plate control lever, which is the vertically oriented "throttle lever" on the port side of the motor, is the same in both engines. Since the steering bracket is the same for both engines too, it is positioned on the respective powerheads the same way, so a swap shouldn't be a problem. The cam shown in the magneto group sections for each motor is the same, so if you have to go to the '75 carb, you shouldn't have problems.
While I realize that a long, written narration of differences between the motors is hard to process, if you load up shop.evinrude.com in two windows, with one for the '68 motor and one for the '75 motor, you will be able to follow the points made.
Hopefully, all of this info helps you get an idea if your plan will work, but it isn't foolproof. In the end, sometimes you just have to gamble. Whether or not it is a good gamble in your case, depends on the asking price for '68 powerhead and its general condition. If it was known to have decent compression before removed from the rest of the motor, and is priced relatively low, I would probably "go for it."
