1956 Johnson 7.5 Pistons & Rings

[Scrawny]

I got this old Johnson a couple months ago. I have replaced the impeller and rebuilt all of the ignition. Parts like that seem easy to find.

But I have low compression about the 45 range. Squeezed a little oil in and it jumps up to 75.

I love this engine and would like to rebuild it. Still runs great even with low compression.

I was wondering if I can still get piston rings and pistons for it.

I would hate to see this motor fade out. I want to keep it going for many generations ;D

Thanks!

 

[Daviet]

Re: 1956 Johnson 7.5 Pistons & Rings

Piston = 276790
Rings = 202524, I think this changed to 378411

 

[Scrawny]

Re: 1956 Johnson 7.5 Pistons & Rings

Piston = 276790
Rings = 202524, I think this changed to 378411

Awesome thanks!

So that's why I couldn't find the rings.... thanks for the updated number

What should I do about the connecting rods? I know they are a babbitt bearing. Should I do anything with them or not?

The reason I ask is I restored a 1953 Chevy 216 which had them. I had the rods machined for regular bearings.

 

[Daviet]

Re: 1956 Johnson 7.5 Pistons & Rings

You are going take a good look at them, you can use plastiguage to measure clearence when you have it apart.
277401 = rod, if you need it.

 

[samo_ott]

Re: 1956 Johnson 7.5 Pistons & Rings

45 PSI is low but if it runs great why rebuild it?

You could try a decarb to bring it up some or maybe a new head gasket. I usually try the cheap fixes first.

 

[1946Zephyr]

Re: 1956 Johnson 7.5 Pistons & Rings

I agree with samo_ott. I think the engine could use a decarb and run it for a while to get fresh fuel mix around the rings. It sounds like the rings are sticky, from lack of use. If the motor is in good cosmetic condition, then it's very likely that the inside is great. Run that old motor 16:1 mix and it'll last you for years.

 

[jbjennings]

Re: 1956 Johnson 7.5 Pistons & Rings

I find it hard to believe that it would run great with only 45 psi. I suspect a low reading comp gauge. If it runs nice, you'd be crazy to invest money in rings and such. By the time a 7.5hp from the 50's gets to 45psi, it's bound to be worn completely out. You could get a good used powerhead very easily for what it'd take for brand new rings and pistons. How's the shock absorber? I guess I'm a little biased on a rebuild because I find the 7.5's a little low on power for my fat rear, even though they run super smoothly.
JMO,
JBJ

 

[Scrawny]

Re: 1956 Johnson 7.5 Pistons & Rings

I did do a decarb and cleaned the carburetor and all that fun stuff. I can tell that at WOT the rpm's are lower than they should be. Even when I take it out by myself on my little 12 aluminum boat it seems to be at a lower rpm than it should be.

Its defiantly blow by. When I did the compression test I started it with the one cylinder and revved it up a little. The pressure drops even more. But other than that it does run great. Idle's like a champ and is a great troll motor.

A few weeks ago I used it on my buddy's boat and we trolled for 9 hours and on half a tank. 5 of those hours were consecutive. And there was no problem there.

Its when you try to take it to WOT that it lag's behind. Which is fine for the most part, but there is a little part of me that wants to see it run like it did.

 

[Daviet]

Re: 1956 Johnson 7.5 Pistons & Rings

Is the high speed jet set correctly?

 

[Scrawny]

Re: 1956 Johnson 7.5 Pistons & Rings

Is the high speed jet set correctly?

I took it out on the lake and opened her up then messed with the setting to get the highest rpm. Still sounds lower than what I think it should be. Prop is fine with some few nicks.

I would say that in gear its about half of neutral (not that I open her up in neutral)

 

[levi_tsk]

Re: 1956 Johnson 7.5 Pistons & Rings

i spotted some ring on ebay for a 7.5 theyre 23 bux per set and the same guys got pistons for 27 and some change each store is bob's outboards etc hes a good guy and he'll work with you on shipping and such ive gotten gaskets and my fastwin 15 from him but id also be on the lookout for a power head as they usually only run about 60-80 bux shipped