5.5 -> 6hp conversion

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johnyrude200
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5.5 -> 6hp conversion

Postby johnyrude200 » Tue May 05, 2015 3:52 pm

Is this simply a matter of switching carbs?

Forget about the gearcase and shifter. If we pull a 5.5hp powerhead and mount on a 6hp leg/pan, is the carb and manifold swap really all we need to do (to accept the updated style manual starter the 6hp motors had).

I had a '59 5.5hp, and everything in the powerhead had the same numbers, including the crank/rods/pistons/rings. Of course, I forgot there is no provision to convert it to a fuel pump style motor.

The reason I ask is I happened upon a really great condition 5.5 powerhead early 60's, (full motor 90+PSI DRY both cylinders), and have a 6hp complete motor (early 70's) with frozen powerhead.
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Chinewalker
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Re: 5.5 -> 6hp conversion

Postby Chinewalker » Tue May 05, 2015 4:08 pm

There are several significant differences between the 5.5 and 6hp powerheads. First and foremost are the 6hp has needles on the crank ends of the rods. You would also need to make sure the cylinder head of the 6hp fits the 5.5 block (I'm pretty sure it will) as the 6hp head has the angled plugs to clear the cowling. You'll also need to swap the flywheel to work with the spiral recoil. Overall, I think it'll work...

johnyrude200
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Re: 5.5 -> 6hp conversion

Postby johnyrude200 » Tue May 05, 2015 4:29 pm

I think the compartment of a 6hp motor would still accommodate the 5.5 cylinder head. This motor (the 5.5) actually started up, albeit I just threw some gas in to see if it would pop. Runs well, but it's not pumping water, so the powerhead would need to be yanked anyway to change the impeller. Might as well slap it on a 6hp pan and leg if I'm already yanking the powerhead. I'll save the next guy the grief of having to pull the powerhead just to change the impeller.

I read the HP rating for the 5.5 and 6.

The 5.5 says 3500-4500RPM rating operating range, 5.5hp rating at 4000RPM.
The 6 says 4000-5000RPM operating range, 6hp rating at 4500.

I guess my only sticking point is, would it be necessary to change rods. I don't think that it would, but would love to hear the 'panels' weight in on this one. I am thinking the carbs are the same for both motors, less a style difference for the choke and needles. But the change to needle bearings at the cranks wound up squeezing an extra 1/2 hp. So doing the powerhead swap out without changing rods would still yield a 5.5hp motor, since the bearings would remain the same.

Not worth spending the extra 2 hours swapping bearings just for another 1/2 hp...I'm not sure how you could tell a difference anyway.
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FrankR
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Re: 5.5 -> 6hp conversion

Postby FrankR » Tue May 05, 2015 4:48 pm

I really don't know the answer for sure, but I doubt the half horse came from the needle bearings. 5hp Gale motors also got the needle bearings, but it didn't make them a 5.5hp. I suspect the half horse came from the cylinder head (compression ratio) and/or reed plate. Just speculating.

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Chinewalker
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Re: 5.5 -> 6hp conversion

Postby Chinewalker » Tue May 05, 2015 4:55 pm

To use the rods, you'd have to swap the crank, too, as they have different rod journal diameters... And, I believe Frank is correct that part of the power boost came from the head design, so swapping the head would be recommended. If you leave the original rods in place, make sure the future owner knows to run it RICH as they will NOT take the 50:1 mix of the 6hp.

johnyrude200
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Re: 5.5 -> 6hp conversion

Postby johnyrude200 » Tue May 05, 2015 5:48 pm

All good info. So I let the motor sit while I was typing up my last post, I was letting it cool down (it was obviously beginning an overheat since with no water circulation...shut it down at 170°). The purpose was to run it out of gas since it would be a project to do later on.

I went to start it up about 10 minutes later while it was sitting in the test tank and woosh - out came a big glob of debris (probably from bugs or varmint) and it started pumping plenty of water. Operated at normal temperature.

So this has all been educational. I guess we'll just leave the motor as-is - and I will say these little 5.5/6's sure do run smooth and quiet when they have PSI >90. I've actually had a couple of 6's as of recent that were all the way up to 110. Maybe my gauge is lying, but I didn't think these old motors in this small range could even reach that high.

And without sparking the compression debate, they were all sitting for awhile and fairly dry in the cylinders (as in, probably sitting for 2-5 years outdoors exposed to the elements - these were short money pickups from various sources).

On a side note - this motor won't stall out. I have always wondered why the choke knob on some of these early 60's motors had the red triangle "caution" at the end of the knob rather than the more traditional round knob with silver outline. Up to this point, I haven't had any of these small motors with such high compression. Was the choke the old way of shutting down a good condition motor? I had always thought it was just a matter of throttling all the way down to stall it out, not having to choke it off. But that was with motors that generally ran between 65-75PSI.
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benchrascal
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Re: 5.5 -> 6hp conversion

Postby benchrascal » Tue May 05, 2015 10:34 pm

Definitely the choke was the way to stop some older motors .I've got a bunch of them that actually read "Choke to Stop".When you bump into a "crisp' powerhead everything you ever thought you knew goes right out the window .At least that's been my experience .They run so smooth ,idle so well and accelerate like they're supposed to .A fellow once told me that no matter how good the mechanic ,once a car is dismantled ,frame off ,whatever ,total rebuild it never drives like it was never touched ,all original.Same is true of outboards (IMO) ,don't know why but original good condition seems always to perform better.


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