Replacing the Powerhead for my 70 HP Force. Can I go bigger?

[Psycho_Fish'r]

My 1996 70 HP Force has a stripped crankshaft. The shop said that this part is no longer available. I see used crankshafts on eBay, but I don't want to put used parts back in and spend more time, money, and grief later down the road. I'm looking to replace the powerhead with a re-manufactured. Hopefully I can get years of use out of it. Will a bigger power head of the same year fit for more increased performance? Any suggestions?

 

[Frank Acampora]

Re: Replacing the Power Head for my 70 HP Force. Can I go bigger?

Re: Replacing the Power Head for my 70 HP Force. Can I go bigger?

While to engine block is the standard 3 cylinder Force and the mounting bolt pattern is the same, If you have the Mercury controls, and with a 96 engine they most certainly are, the controls and mountings are different. Therefore, you are limited to using another 70 engine unless you want to get into lots of modifications. Do-able and not too difficult, but custom work and time consuming.

I have the same engine (1990 something--I forget the exact year) and have bored the block. I am currently contemplating whether or not I want to buy new pistons for it or swap in a 90 block.

I just re-read your post. Did I misread it or did you edit it? I don't remember seeing the sentence about the crankshaft. At any rate, a crankshaft from an 85 or 90 will fit in your block. If it is just the flywheel threads (for the nut) then yes, you can swap. You will not gain any horsepower because the cranks are all the same stroke.

 

[Psycho_Fish'r]

Re: Replacing the Power Head for my 70 HP Force. Can I go bigger?

Re: Replacing the Power Head for my 70 HP Force. Can I go bigger?

Thanks for the info! I'm getting ready to place an order for a powerhead and I see that the 90 HP is the same price as the 70 HP. If it requires modifications, I guess I'll have to pass on going bigger since my boat is at a repair shop and their labor rate is about 3x of what I earn per hour. It would've been an easy way to gain additional performance if it didn't require additional modifications.

 

[roscoe]

Re: Replacing the Power Head for my 70 HP Force. Can I go bigger?

Re: Replacing the Power Head for my 70 HP Force. Can I go bigger?

FYI, you don't have a 1996 70 hp, as they made the 70 hp from 91-95, then made the 75 starting in 1996.

 

[Psycho_Fish'r]

Re: Replacing the Power Head for my 70 HP Force. Can I go bigger?

Re: Replacing the Power Head for my 70 HP Force. Can I go bigger?

FYI, you don't have a 1996 70 hp, as they made the 70 hp from 91-95, then made the 75 starting in 1996.

You are correct! I ran the serial numbers and the motor is a 1995! I get the year of my vehicles mixed up.:facepalm:

 

[Frank Acampora]

Re: Replacing the Powerhead for my 70 HP Force. Can I go bigger?

SORRY--- I picked up the wrong driveshaft when I went to double check the splines on the 70 HP crankshaft. The 85-90 HP crankshaft will fit into the 70 block, BUT--The driveshaft is a slightly larger diameter thus it will not mate with the lower unit.

On triple checking, I observed that the spline count is close--14 for the 70 and 15 for the 85-90. The shaft diameters are different just enough that the larger crank will fit on the smaller 70 driveshaft, but I would not dare to run an engine this way

 

[fucawi]

Re: Replacing the Powerhead for my 70 HP Force. Can I go bigger?

You hear of many people cutting drive shafts and welding them ..this was done often in Australia ..in your case the bottom half of a 70 hp and the top from the 85-90 . the proper way to do it ..if proper is the word is to make a sleeve from 1 inch bar about 3 - 4 inches long . and bore each end to match the shafts on a lathe BUT you dont weld the ends that weakens the shaft . On each side you cut a 1/4 slot in the joiner (milling machine or row of holes and a file !! ) ..thats 4 in total and stick weld through that ..so you in effect get 4 woodruff keys made of weld ......just a though and not a big job for a shop to make for you ......use stainless weld rods .

 

[rebeldog]

Re: Replacing the Powerhead for my 70 HP Force. Can I go bigger?

i was also wanting to know will a 120hp tracker powerhead of the same year 1998 bolt on 90hp elpt tracker force mercury lower unit?

 

[Lakeside92]

I have a 94 70hp force and was wondering if a 91-94 90hp powerhead will bolt in. Can I use the 70hp carb and manifold on a 90 block?