1969 to 77 lower unit swap

[Grayzer86]

Hey guys, sorry my first post is a question but i am in a bind and needed some advice quickly. We have been working on my friends boat which has a 1977 90HP Chrysler. The lower unit is shot, seals, and gears and all. Parts are pretty tough to find but we found a great deal on a different boat that may have parts. The other boat has a supposedly 1969 Chrysler 70 hp engine on it that has a good bottom, starts, but wont stay running. It also comes on a two year old trailer and his is screwed so the price is right. What we are looking to find out is if the lower unit, or whole lower and mid from a 1969 70hp would possibly fit his 1977 90hp. They look very similar but thats just from pictures as the boat is currently 2 hours away. Any and all info you could share would be hugely appreciated. Thanks in advance.

 

[Frank Acampora]

Although the lower units look slightly different the bolt pattern is the same. HOWEVER: the 70 most likely has a different gear ratio than the 90. During those years I think the 90 was the only engine with a 2 to 1 gear ratio. All the rest had 1.76 to 1. If this is the case, you may need to step down 2 pitches on the prop and even then, performance will not be quite as good as with the 2 to 1.

The later lower unit has a mounting bolt hidden inside the exhaust snout. The exhaust snout must be removed to access this bolt. The earlier lower unit has a 6 inch long 12 point 5/16 bolt running through the exhaust snout into the mid leg.

If the price is really good buy the whole package and swap lower units. Save ignition parts, distributor, and starter. In fact, save the whole block . Pistons and rods are the same as is the crank. The difference in horsepower is due to porting and carbs. The 70 has 1 3/16 diameter venturi carbs. If you measure and port the 70 carefully and put on bigger carbs (1 5/16 diameter venturi) you can upgrade the 70 to a 90