OMC 40 hp Powerhead on 35 hp Midsection

[enrico-italy]

quote Mumbles:

So Pappy, I have a ’58 Big Twin here with a frozen powerhead and a later 40 could be adapted to it?

I’m supposed not.

All the Bigtwin 40 newer than 1958 had the "matrioska type " leg, so the PW base flange is different.

Worktwin or Skitwin 40 PW can fit the flange but not the driveshaft spline…

[frankr]

You could put a 40hp Gale powerhead on your ’58 Big twin, but then you have a whole new problem. The ’58 Big Twin clamshell hood won’t work on the 40hp.

[chris-p]

Wayne,

I have been going over parts here, and I think that you will pretty much need to sacrifice 2 motors, to make 1, or use a whole lotta parts, which we have as well.

We need the 35hp swivel brackets and exhaust housings, with the cowls for cosmetics. Then basically a whole 40hp powerhead long block, with everything attached. Need to use the 40hp gearcase as well. Unless there is a way to swap out the drive shaft? If there is, I have a stash of 40hp drive shafts to use.

Another thing. I have done this before, and am wondering if I did it on the Lark you now have? I wonder if that is the motor I put the 40hp powerhead on? I remember having to gut the gearcase as I said, I wonder if I did to install 40hp components in it to accept a 40hp drive shaft.

I dunno, my memory sucks, but that Lark may have a 40hp powerhead on it already.

[chris-p]

I think the easiest thing is to get 2 40 hp super quiets, and install the ’58 SSH cowls on them. Then you get the cosmetics of the ’58, with the power of the 40hp.

Just a thought.

[wbeaton]

Thanks for the heads up, Chris. I am still in the parts gathering stage. Putting the SSH cowls on the 40 hp was my first thought, but since I don’t have any yet I thought it easier to build what I have and drop the 40 hp on later. I didn’t realize the driveshafts were different. I’ll have to open the parts books today and see what I need to do.

Wayne
Upper Canada Chapter

uccaomci.com

[Casey Lynn]

The style of hood does make a difference when changing year models to this drastic of an extent. Cylinder head changes are the best way to go to make sure the old cowling will fit. Watch the compression when or if you do this. Second thing to watch for is the external coils depending on what year you go to. As I said in my first post the splines and driveshaft were different which made using the later model gearcase necessary. Seems there were some other minor issues as well but those were the major ones.

[Casey Lynn]

quote Mumbles:

So Pappy, I have a ’58 Big Twin here with a frozen powerhead and a later 40 could be adapted to it?

Mumbles…I have no doubt you can do it.

[wbeaton]

I’ve poured over the parts catalogues this morning. I’m surprised how many differences there are. I had no idea there were three different drive shafts between 58-70. Since I’m building twins and will want a spare lower unit I will need to find three parts motors of the same build. It appears that the easiest solution would be to use 1960-61 40 hp’s since they are the most compatible with the 1958-59. However, I’m not sure if its worth the risk running the small crank powerheads. Decisions, decisions. This was supposed to be easy. 🙂

Wayne
Upper Canada Chapter

uccaomci.com

[wbeaton]

I thought I’d give an update on the project. I have found and purchased several 1958-63 OMC 35-40 hp motors locally. Too many really. Good thing they are cheap. I am building two longshaft 1961 40 hp Johnsons for use as twins on my cabin cruiser. I am dressing them to look like 1958 Johnson Super Seahorse’s. Fortunately, the Super Seahorse cowl fits the newer 40 hp motors without modification. I am still looking for another 1958 cowl if anyone has one.

Wayne
Upper Canada Chapter

uccaomci.com

[wbeaton]

Also, I’ll be replacing the 40 hp hot water choke carbs with the 35 hp adjustable jet carbs since they appear to be the same size. I would rather have fixed jets, but I have no interest in the automatic chokes and the motors will be mounted on a bracket that stands off the transom so I won’t be able to easily reach them from the rear deck If the auto-chokes fail.

Wayne
Upper Canada Chapter

uccaomci.com

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