1986 force50 hp w/45hp power head?

[dkonrai]

so i blew a piston in my 86 50hp. i have a spare parts motor, chrysler 45hp vintage 70's motor, and i am wondering if its worth the trouble to swap out the power head?

this is what happened to the old motor

think the head is ok?

this is the spare, it is a 45hp vs a 50hp. it looks like the walls still have the original lapped grooves.

what do you think? i figure i just wont run the motor at wot, and i should be ok, also im gonna run premium gas.
dino

 

[tater76]

Re: 1986 force50 hp w/45hp power head?

How bad is the scoring on the "blown piston" cylinder? Did you compression check? Here is a pic of my 73 Chrysler with a melted piston. It still retained 125psi after this. I made sure the engine was clean of shavings or chips, then readjusted the carb to a richer mix. I ran it for the entire last season, approx 17 trips with no issues. This year I rebuilt it, but put a 1985 50hp force ignition system on it. The cases of the 73 and 85 were identical, as were the cranks. I actually pulled and used one of the good 50 pistons in the 55, as it was the same size, just put new rings on it. I don't know about the 45 interchanging with the other, might check piston diameters/bore sizes, crankshafts? not really sure on that one sorry?

 

[dkonrai]

Re: 1986 force50 hp w/45hp power head?

scoring is minimal on the walls. the piston looks like it has a hairline fracture. look close at the first photo. the piston is melted away, and near the top of the dome is a crack. the motor wouldnt start, i guess bc of all the aluminum on the plug. mechanic told me he retarded the timing 18 degrees to help prevent this. i guess its common? with the crappie diluted fuel in norcal. said i should have been using 91 octane at least, due to the high compression (145 psi) and it being a 2 cylinder.

 

[tater76]

Re: 1986 force50 hp w/45hp power head?

A melted piston is due to a lean mix. It was likely your carb misadjusted that did the damage. That is exactly what happened to mine, look at the pics. It would run, then bog down. I would pull the plug to find melted balls of aluminum stuck in the plug. I rebuild the carb, adjusted it per advice on here, and never had any more issues.

 

[dkonrai]

Re: 1986 force50 hp w/45hp power head?

A melted piston is due to a lean mix. It was likely your carb misadjusted that did the damage. That is exactly what happened to mine, look at the pics. It would run, then bog down. I would pull the plug to find melted balls of aluminum stuck in the plug. I rebuild the carb, adjusted it per advice on here, and never had any more issues.

yeah, i know. hard to say really. maybe a combination of many factors. im just hoping the 45 power head is gonna swap out ok on the 50?

 

[tater76]

Re: 1986 force50 hp w/45hp power head?

Set them side by side, I know its old school way of doing things, but you may be able to see if the cases are at least the same. Most of my repair manuals show from 35hp-55hp, so... and like I said my 73 55hp had the exact same block as the 85 50hp force. If it were me I would drop the lower unit from the toasted motor, and the good one. Check driveshaft diameters/lengths, if they are the same then?...... I would try the swap.

 

[dkonrai]

Re: 1986 force50 hp w/45hp power head?

tater,
the lu's look the same, and both interchange so im gonna give it a try.
thanks

 

[tater76]

Re: 1986 force50 hp w/45hp power head?

Thats awesome! love it when something works out

 

[Frank Acampora]

Re: 1986 force50 hp w/45hp power head?

You can swap the block but I think you will be disappointed. The 45 was rated at the crank and the 50 was rated at the prop. The 45 has a cylinder diameter of less than 3.187 and less than 44 cubic inches--Don't remember but probably around 3.125 and 42 cubes. The 50 has a 3.375 bore for about 51 cubic inces displacement. You will see a big difference in hole shot--acceleration-- and probably a couple MPH less top speed. If you are only using it to fish or putt arouind it will be ok but if you are looking for performance--as I said, you will be disappointed.

 

[dkonrai]

Re: 1986 force50 hp w/45hp power head?

You can swap the block but I think you will be disappointed. The 45 was rated at the crank and the 50 was rated at the prop. The 45 has a cylinder diameter of less than 3.187 and less than 44 cubic inches--Don't remember but probably around 3.125 and 42 cubes. The 50 has a 3.375 bore for about 51 cubic inces displacement. You will see a big difference in hole shot--acceleration-- and probably a couple MPH less top speed. If you are only using it to fish or putt arouind it will be ok but if you are looking for performance--as I said, you will be disappointed.

frank,
can i use the head off the 50hp for the 45? the head off the 50 is in better condition than the 45 (i am too lazy to clean the 45hp off).
thanks for the reply. i was hoping you or jerry would have chimed in. i basically fish, so the plan is to get the motor running, then sell it. hoping to get around 500 bucks or so. if the motor isnt running its a boat anchor, but running i can possibly sell this. the boat is great (ok as good as a bayliner can be) but the motor has me gun shy.

 

[Frank Acampora]

Re: 1986 force50 hp w/45hp power head?

You can use the head off the 50 on the 45 but not the other way because of the different sized piston baffles. It will give even less compression and power though.

And, please don't try to sell the engine as an 86. Be upfront and explain what has been done. If you mean selling the engine by itself, 500 is very optimistic for that engine, even running. Boat, engine, and trailer, OK

 

[dkonrai]

Re: 1986 force50 hp w/45hp power head?

You can use the head off the 50 on the 45 but not the other way because of the different sized piston baffles. It will give even less compression and power though.

And, please don't try to sell the engine as an 86. Be upfront and explain what has been done. If you mean selling the engine by itself, 500 is very optimistic for that engine, even running. Boat, engine, and trailer, OK

frank,
to clarify, if i use the 50hp head on the 45hp powerhead, i will have less compression? so i should use the original 45hp head?

as for selling the motor, yes, i would let the buyer know. i guess it would be listed as a 45hp motor, with a 50hp dog leg and controls??

dino

 

[Frank Acampora]

Re: 1986 force50 hp w/45hp power head?

Look at the piston baffles on the 45 and then ion the 50. The baffles on the 50 are larger so the head must be relieved to clear them. More space in the head---less compression. I don't know how much less and how much loss of power but if it were me I would use the 45 HP head on the 45 HP block.

I would call it a blown 1986 50 HP with an older 45 HP replacement powerhead and let the buyer decide what he has. That way, you are being honest, but not shooting yourself in the foot either.