1957 Johnson 7.5 lower unit!!!

M

mfw

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So, I have posted before about this motor. I was told by the mechanic that since one cyl has low compression (60) and the other being (74) that re-sealing the lower unit would cost $130 and that I should scrap the motor. I am not one to throw away a motor that actually runs. It sat for many years before starting it a few weeks ago. My question is are there any tricks to sealing the lower unit? I have the kit and am just going to attempt this myself. After that I am going to go through the carburetor and decarb using sea foam. I am hoping to regain some compression and have a great running motor. Any other suggestions on how to get her running well? Thanks

Mfw
 
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Monte1961

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Re: 1957 Johnson 7.5 lower unit!!!

I would take emery cloth to any and all shafts and lightly clean them up. if the old sealing grooves are to bad you can simply off set the seals. You also need to change the one where the shift rod come through lower unit by the pump housing. If you look where the rod comes through, you will see a brass bushing. Oring seal is underneath that. During Assembly, apply grease to seals and shafts Pryor to install. Pressure test lower unit and check for leaks. Good luck and post yer results!
 
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mfw

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Re: 1957 Johnson 7.5 lower unit!!!

Where would I get the tool to test the pressure?
 
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Monte1961

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Re: 1957 Johnson 7.5 lower unit!!!

you can use the tube and fitting from pump bottle you use to fill lower unit. Just be careful not to exceed 15lbs of regulated compressed air. Make sure to turn shafts while in tank and watch for bubbles. Good luck!
 
1946Zephyr

1946Zephyr

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Re: 1957 Johnson 7.5 lower unit!!!

Talk to F_R about a shift rod seal removal tool. It's the best investment you'll make, when it comes to rebuilding one of these old OMC split case lower units.
15psi is a little bit much, isn't it? I thought around 7 - 10 was ideal. You can use a bicycle pump to do your pressure test.
You can sea foam your motor and see if the compression improves. If it doesn't, then you'll just need to check your pistons and rings over. Chances are, you have some slight scoring on the pistons and the rings are stuck. On that model, you can actually remove the bypass cover and get a visual. Scoring is usually caused by lack of oil and/or over heating. That motor, you want to run a 16:1 to prevent this kind of damage from occuring.
 
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mfw

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Re: 1957 Johnson 7.5 lower unit!!!

What an ideal range for compression on the 7.5 hp?
 
nwcove

nwcove

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Re: 1957 Johnson 7.5 lower unit!!!

id spend the money on a good service manual for your motor.
 
M

mfw

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Re: 1957 Johnson 7.5 lower unit!!!

Ok, so I am not that far off with 60lbs and 74lbs. Where would I get the manual? I find the parts catalog on ebay all the time. Any suggestions?
 
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mfw

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Re: 1957 Johnson 7.5 lower unit!!!

Thanks guys just ordered one. Also, I am going to look at a 1956 5.5 hp johnson. The owner says that it runs and comes with the pressure tank. What should I be looking for? Do you think $200 is too much. Cosmetically it looks like its in good shape. I know looks do not mean a thing. Again I am new to the hobby and would like to avoid screwing myself. Thanks
 
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Monte1961

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Re: 1957 Johnson 7.5 lower unit!!!

1st thing is a compression test! Then a spark test. I should jump a 1/4" air gap. Just a thought, you might get any and all service records if available. I would also order a carb and tune-up kit along with a new impeller! Remove lower drain/fill plug and check for water or metal! Good luck!
 
M

mfw

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Re: 1957 Johnson 7.5 lower unit!!!

Is $200 a decent price for motor and tank? If things check out?
 
kfa4303

kfa4303

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Re: 1957 Johnson 7.5 lower unit!!!

Not bad, but many folks eventually convert their old, pressure tank system, to a modern single line system. It's not terribly difficult or expensive, but you may want to consider that in the price too. There are lots of threads here you may want to check out. Don't give up on the 7.5 just yet. Sea foaming can work wonders and is a relatively cheap and easy treatment. If you can get the cylinders within 10% of each other you should be ok. A carb rebuild, water pump and some new ignition components and she'll be good as new. Keep us posted.
 
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