Replacing Crankshaft 70hp Mercury

" Helow, just wondering if the

" Helow, just wondering if theres anyway I can put a 72' 65hp Mercury Crankshaft and good set of pistons onto 76' 70hp mercury powerhead that has one scored piston and rusty pitted shaft. I was told that their is little differance between the two motors but not sure if model years are compatible. Thanks, Mark B. "

" Mark,

The crank is the sa

" Mark,

The crank is the same, but the pistons and rods may be different. The 70HP uses two different piston and crank numbers depending on the serial number. What is your serial number on the 70HP engine?


Tony "

" Just my two cents...

The

" Just my two cents...

The '70 may use power-ported pistons. These pistons have a 1 inch hole in them on the intake side. It matches a cutout in the cylinder wall. You can also measure the deflector to determine if the pistons are high or low dome. There is a 0.10 inch difference. There are two types of rods; those that use a bolt and those that use a nut and bolt. Don't mix the two since they weigh differently. I'd give it a 95% chance of working ok.

While you are at it, you may want to move the tell-tale to the top of the exhaust water jacket. There was a service bulletin about doing this to prevent steam pockets from forming and overheating the motor.

Good luck!

Kirk "

" Thanks for the input Tony an

" Thanks for the input Tony and Kirk,
The serial number on the Mecury is 4982289. The rod's on the 72' are a differant number of 622-3665 from the 76' with 622-3665. But the piston #'s are the same of 745-4973. Both crankshafts came with ported pistons, and both set of rods have a nut and bolt connnection. The 72' has a reed block # of 542-7507, while the 76' has no #'s. The original problem was a roller bearing that came lose from the rods retainer and scored up the piston and block. Has been sitting for 5-6yrs, plan to replace water pump and impeller? Hope will fit on 17ft. classic montauk? So far so good. Thanks again, Mark B. "

" Mark,

According to the pa

" Mark,

According to the parts manuals I looked through, the two engines you speak of should not have interchangeable cranks and pistons.

The newer engine is a 1977 according to my manuals.

Tony "