Johnson 1952-1954 Jw-10 3hp Piston & Ring Combos

I would like to increase the power/compression of my JW-10. Currently, the compression is reading about 90 lbs (if my gauge is accurate) in both cylinders. The compression increases to about 145 lbs (if my gauge is accurate) when adding 2 cycle oil to the combustion chambers, testing the engine with it laying horizontal and combustion chambers facing up.....oil uniformly sealing the combustion chamber.

Option #1 Inner diameter & outer diameter of piston ring is standard size, standard size piston

Replace the old, standard sized piston rings with new, standard sized piston rings (part #378412).

In this particular case, compression could be increased by replacing the worn out rings. I believe that the cylinders should be honed & "cross hatched" before installing the old, cleaned standard size pistons & standard size piston ring assemblies.

Option #2 Inner diameter of piston ring is standard size, outer diameter of piston ring is over sized, standard size piston

Is there a point where too much honing will force me to go with over sized piston rings ? Or, if the cylinder walls are scored to the point where boring them out is appropriate, I guess I'll have to go with over sized piston rings (part #378414).

In this particular case, compression could be increased by replacing the worn out standard size piston rings with new over sized rings and creating a clean, uniform cylinder walls with very little space between the over sized piston rings and cylinder walls.

Option #3

If the cylinders are in very bad shape, have them bored way out and go with over sized pistons and way over sized piston rings

In this particular case, compression could be increased by replacing the worn out standard sized piston rings with way over sized piston rings, using over sized pistons to accommodate the way over sized piston rings and creating clean, uniform cylinder walls with very little space between the way over sized piston rings/over sized pistons and cylinder walls.

I think all 3 options are viable as long as parts are available. Were over sized pistons ever made for this engine ? If they were made, does any one know the part number ? What size piston rings would fit the over sized pistons ? And, does anyone have part #s and/or specs ?

Does anyone know what the compression of these engines were right out of the factory ?

Wow.....that was lot to say. Any knowledgeable and wise advice is appreciated !

Re: Johnson 1952-1954 Jw-10 3hp Piston & Ring Combos

My advice would be to lay down until the feeling goes away. Your compression figures are very respectable for that vintage motor. If you need more power, find a larger HP motor. You're not going to see the kind of results you're looking for by refitting larger pistons. Shaving the head would be the easiest way to increase the compression figures , but not advised.

Re: Johnson 1952-1954 Jw-10 3hp Piston & Ring Combos

Boy, I agree with Xcusme 100%. That's fine compression for your JW. It is only 3hp, after all. Is it running perfectly? Getting it to run perfectly is the very best performance booster you can do.

If it's already running perfectly, and you have a nice clean, undinged prop, you're getting out of it all it has to give with that compression.

Those are sweet little outboards. If it's going well, it's best to leave it alone. You're more likely to mess it up than improve it, and that's the truth.

Re: Johnson 1952-1954 Jw-10 3hp Piston & Ring Combos

I had a JW-10 but understood it to be a 1955 model. If you decide to make some mods, be sure to verify year/model number. Good luck!

Re: Johnson 1952-1954 Jw-10 3hp Piston & Ring Combos

Simply put, the motor won't take any serious modifications. It is a plain bearing (bronze babbit) motor with aluminum rods. Try to stress it too much and you'll have a couple bucks worth of scrap metal. 90 psi in each hole is fantastic for a JW-series motor. Leave it be...

As a side note, OMC did eventually "hop up" the little 3hp and turned it into a 4hp in 1969. Same bore & stroke, but they opened up the breathing with more reeds and a tuned exhaust pipe. Seat of the pants - I've driven both and you really don't notice much of a difference... and that's with an advertised 33% increase in power!
- Scott

Re: Johnson 1952-1954 Jw-10 3hp Piston & Ring Combos

Willy, 1955 was a the JW-11. The JW-10 was built from 1952-54.

Nothing else to add that hasn't already been said.

Re: Johnson 1952-1954 Jw-10 3hp Piston & Ring Combos

A sweet running motor capable of about 8 mph on a light free moving boat.