Omc 6hp 1969-1976

[Good Trout To You Sir]

See a number of 6HPs in this age bracket in my local market and wish to know what to expect if I pursue one of them.
Are there any substantive factory differences year-to-year within this 8-year span.
Is any particular year better or worse component-wise?
Was the kW power rating system the same as present day......such that 6hp then is comparable to 6hp now?
What's the bogey for threshold compression for these motors to run well throughout the RPM range?
What fails most often on 6hp OMCs of in this vintage?
Pushing a 15' flatbottom, would I gain noticeable speed from replacing the metal 2-blade prop with plastic 3-blade prop?
Is there a carb kit that includes a new float? (The kits that I see do not --- but perhaps I overlooked.)
Other than preventive maintenance of new coils, points, water pump ---- if you acquired a 6HP of this era with an unknown maintenance profile, what else would you preemptively replace?
Thanks.

 

[racerone]

All the same motor.------Different paint and decals.-------Wonderful motors if you know how to work on them.

 

[F_R]

For the most part, the motors are the same within that date range. all very good motors with no gotsha's that I can think of. At some point, they went from a three-ring piston to a two-ring piston, but both designs work well.

Some like to talk about prop horsepower vs crankshaft horsepower, but I consider that a bunch of nonsense, performance-wise.

What fails most? The owners/people that work on them are what fails. There is a multi part carbon seal at the top end of the drive shaft. If that seal fails, water can enter the crankcase, which over time can severely damage the internals. It won't be noticed for a long time, but that is the eventual killer of many motors. I mention it because in earlier years, you had to remove the powerhead to disconnect the shift rod in order to get the lower unit off for water pump service. That forced you to at least inspect the mentioned seal, and repair/replace any worn or broken parts if you had any common sense. Trouble is, they "fixed" that so you no longer had to remove the powerhead in the years mentioned. Thus, the seal no longer got even looked at. That, in my opinion, is what I would preemptively do. The seal is a fine design, but only if in good condition.

Carb kit with new float is part number 0439071.

I wouldn't hesitate to buy/own one of the motors, if I had use for one.

 

[racerone]

I pick them up for parts / repair / rebuilding.-----About 10 of them on the go at the moment.

 

[oldboat1]

You will be happy with any of those motors. Personally, I hang onto my '57 5 1/2 with pressure tank. I've either owned or worked on a number of motors in the range you mention, but still locked on to the little '57.

 

[Good Trout To You Sir]

There is a multi part carbon seal at the top end of the drive shaft. If that seal fails, water can enter the crankcase, which over time can severely damage the internals. It won't be noticed for a long time, but that is the eventual killer of many motors.

Thanks for the detail! The multi-part seal you reference......it's the complex sitting above the spring of the lower bearing seal?
My questions on threshold compression (assuming balance) and noticeable thrust differences on this motor with metal 2-blade vs plastic 3-blade prop did not prompt response. Should I assume these factors are not meaningful?
Thanks to all for the feedback!

 

[Good Trout To You Sir]

In my hunt for a good 6HP, I ran across a listing for a "WORKTWIN 6HP" with model number E6RSM (1981-1982 commercial model). The motor sold very quickly for what appeared to be a very buyer-favorable price. My 5-hour-long hesitation cost me the deal. I was trying to determine what differentiates these commercial motors from their recreational counterparts. Postings here and on other boards are not in agreement if -- and how much --- they differ in parts compatibility, weight and performance.
In case I see another one at some point, anyone have some input based on their recent experience with these commercial models (parts availability, service manual availability, etc.)?

 

[racerone]

Fact----There really is no difference in commercial models in many cases.---Same engine internal parts.------On larger motors the difference can be the lower units.----We all know that the 2 cylinder OMC gearcases on 50 HP models are not robust.----So on the commercial models they use a 3 cylinder gearcase with a " hybrid " driveshaft.----Some of the bigger motors also feature manual start and no oil injection on some models.-----That 6 HP " worktwin " is no different as far as parts are concerned compared to the 7.5 HP recreational models !!

 

[TN-25]

They did the Lexan props for a few years. They don't seem to be any trouble. We did run a replacement Lexan prop on our old 9½ since that is what OMC sold for a time. I noticed later that the replacement Lexan props were dropped from the 9½ & they reverted to aluminum. Perhaps the larger engines place more strain on the plastic props. I have not heard of any problems with the plastic props though.

I believe 1970 was the year OMC switched to the pressure-backed piston rings.

 

[F_R]

I maintained a fleet of 9.5hp motors back in "The Day". They were used by tourists that got "free" boat and motor use with their rental. Anyhoo, access was via a channel through large oyster beds. People would stray out of the channel and wind up in six inches of water over the oyster bed. Those oyster shells were murder on props. What I'm getting at is that after much doubt, we found out that the plastic props held up just as well, or probably better than, the aluminum ones.

 

[Good Trout To You Sir]

That 6 HP " worktwin " is no different as far as parts are concerned compared to the 7.5 HP recreational models !!

Thank you. Good to know. Conversion of WorkTwin 6HP ==> Recreational 7.5HP is accomplished by nothing more than decal application.......the power output already equates to the 7.5HP recreational model?

 

[racerone]

Main carburetor jet might be smaller.---Easy for you to check out on the parts list.