18 hp Johnson - 1961 - Mark on top end of cylinder

Hey guys, I had a previous post where I had concerns regarding the performance of my 61 Johnson 18. I replaced the water pump, lower unit oil, and rebuilt the carb before putting it back on the water. It started up beautifully as normal and idled exceptionally well (which gave me some hope that maybe the engine would run correctly), however, once I gave it some throttle it began to emit a vast amount of white smoke and began to shudder. I immediately shut it down and took the boat off the water. When I got home, I did another compression check, which read 72 on top and 67 on the bottom. At this point I realized that compression was likely the issue. I pulled off the cylinder head and immediately noticed that the head gasket was in pretty rough shape. It didn't have any blow outs, but the corrosion was so bad that I couldn't imagine it created a good seal. When applying pressure to the pistons, I was able to move them around in the cylinder pretty easily, not so much as to warrant a total power head rebuild, but the rings will definitely need replaced. My concern lies with two small chips or impressions at the top end of the cylinder (check pictures below). Can I continue to use these cylinders without having them machined professionally, or can I just hone them out, clean off the faces, and put it all back together. Any help would be greatly appreciated. I am trying to avoid going to a shop. I have rebuilt car engines before, but this is my first outboard. Just looking to make the right judgement call before I get too far into it. Thanks guys.

:WELCOME: to Tin Boats.
Please complete your profile when you have time.
Knowing what part of the country you hail from helps
us give you more accurate information.


what mark on the cylinder are you talking about ?
I have poor eyesight. Maybe a yellow arrow on the spot will help.
I just get a flat piece of 3/4" plywood and glue some wet r dry
sandpaper to it and hone down the cylinder and head mating surfaces.
Aluminum is easier to dress than a cast iron car motor block.

Let me know if this helps.

I know they are pretty small, but the one on the left looks as though it might be questionable since a small part of it overlaps with where the head gasket would sit. I still have to get the whole power head off of the engine. Not really sure how to go about doing that yet... the flywheel is giving me a hard time too, which I'm sure is a common problem on these older engines.

in my world, I would dress it down first.
then, form your own opinion if it is a concern or not.
for me, if it sets a good seal, I don't bother with it.
Dress down both mating surfaces to flat shiny smooth.

Those two marks are above where the piston rings travel, which is good. Check to make sure you can't feel a pronounced bump into the cylinder bore. I doubt they are high enough to cause any contact with the piston. Additonally, it doesn't look like there was a gasket leak around them.

Then check to make sure that they don't stick up beyond the face of the cylinder (toward the head).

Dress the whole area with a piece of 400 grit sandpaper adhered to a small piece of glass (more consistent than wood) and clean the whole top surface of the cylinder block.

Put a new head gasket in place and you should be good to go.

Let us know where the compression lands afterward.

Good luck!

Well I was considering changing the piston rings too... obviously I'd rather not go that far unless I have too, but I am a little concerned that the pistons aren't sealing properly either. I guess it just comes down to me wanting to risk having to break it all down again and buying a new head gasket, since it can only be torqued one time.

Based on what I can see, I doubt that those marks or the head gasket had any significant impact on the compression. It looks like you've got a well used motor that may need some new rings. The head gasket isn't too expensive where giving it a try is a big deal, but I wouldn't get my hopes up too high.

Kofkorn is on the right track with you.
Couple of things to add here though. I would check the water pump again. The only cause I know of for white smoke is that the smoke is steam. You know where we are going with that......
Secondly. If you are contemplating a tear down why try and do it on the cheap with just rings?
Measure the bore or have it measured for you. You can then determine whether or not rings alone will do the trick or whether you need to source oversize pistons and rings. Pistons and rings for that engine will not be any more plentiful later on down the road and that engine is a good one to keep in service.
In a bore that is at or near the end of it's service life a hone and ring job will put it past that mark in all probability. One of the most critical clearances that need to be maintained in a two stroke is the piston to bore clearance. Specially around the skirt area (chances are you already have a taper in the cylinders). This needed clearance is to assure you will have enough pressure and vacuum in the crankcase to produce a good idle as well as power and ease of starting.
Your compression numbers did not come from a blown head gasket.
From the photos I can see scuffing in the visible bore. Pull your exhaust covers and take a look at the rings and the skirt area below.
Am thinking you many have an "AHA" moment on the source of the compression loss. Shoot a couple of photos of the exhaust areas of the pistons and the bores.

Thanks for the info Pappy.. the rings and pistons were my original concern. I will get some pictures of the exhaust and skirts as soon as possible. I've been slowly working at getting the flywheel off for the last week (one or two turns at a time, not to force it too quickly). Once that is off I'll probably just remove the entire power head. I don't have the appropriate tools to diagnose significant wear to the cylinders or decide if I need to have them bored. I may just have to suck it up and go to a machine shop. Obviously I've been trying to avoid this because I'm on a pretty tight budget, but I want to make sure I do right by this engine. I've heard nothing but good things about these motors and would love to see this one continue to perform.

I guess another problem I have Pappy is that I've never torn apart a 2 stroke outboard before. I've worked on car engines and have done a few rebuilds, but this stuff is all new to me. I'm not really sure what to look for in all aspects of the engine. I will post as many pictures as I can and if you guys want/need more to help me diagnose it, I'll provide them. Thanks again for everyone's help!

k2sno070 said:

Thanks for the info Pappy.. the rings and pistons were my original concern. I will get some pictures of the exhaust and skirts as soon as possible. I've been slowly working at getting the flywheel off for the last week (one or two turns at a time, not to force it too quickly). Once that is off I'll probably just remove the entire power head. I don't have the appropriate tools to diagnose significant wear to the cylinders or decide if I need to have them bored. I may just have to suck it up and go to a machine shop. Obviously I've been trying to avoid this because I'm on a pretty tight budget, but I want to make sure I do right by this engine. I've heard nothing but good things about these motors and would love to see this one continue to perform.

Click to expand...

A flywheel puller is needed to remove the flywheel.
Shouldn't cost much to have a machine shop measure the bore.
I can understand a tight budget but....there is a saying based on what I see on at least a weekly basis.
"Ya' never have time (insert money here) to do it right but ya' always have time to do it twice"
This pertains mainly to workmanship but a lot of workmanship issues are based on money for the repair.
If you are able to disassemble a car engine and re-assemble and time one correctly you can certainly do this.

I am using a flywheel puller for the job. It just doesnt seem to want to come off. I've just been tightening it up every day a little more and hitting it with penetrating oil. Hopefully it'll come off soon. I'll have a chance to get at again this afternoon.

The procedure using the puller on a keyed, tapered shaft is to tighten (like you are doing) and then smack the center bolt with a hammer. Not too hard mind you and it is even better if you are lifting the flywheel before you smack it.
The nut is off the crank I take it?

yes the nut is off the crank, but I threaded it back down a few times so the flywheel doesn't go flying. I haven't been trying to force it. I'm going to do my best to get it off later tonight. I'll send over some pictures of the exhaust ports and skirts if I can get that far.

I just acquired a 1960 18hp today.

Pappy said:

I just acquired a 1960 18hp today.

Click to expand...

I hope that it's in better running condition than mine...

Not really. Good thing is that it hasn't been run probably in a couple of decades....or more.
Bad thing is that it looks like Bubba, Gomer, and Billy Bob have been trying to make it run.
Spark plug threads damaged and stripped in one hole, rewind not working (rope too large) , carb needs work, a bit of water in the gearcase although not bad.
Tiller handle cracked, needs a water pump but zero signs of an overheat, Coils are original and cracked which I like! Means the engine has not been run on 50:1 and damaged.
Front latch missing from cowling, throttle sector gear broken. All in all no big deal, just time consuming. Paint is pretty good overall.
Like yours, it will be a runner.

Dead giveaway is to take a look at the paint on the cylinder head. If there is any color change of the paint (typically browning) near the head, then it heated up enough to burn the paint. Most motors max out around 140 to 160 degrees. If the paint on the head is original and the color looks even all around, it's a good chance that it's not been overheated. However, that doesn't guarantee good compression.

There occasionally will be some browning close to the spark plugs, but typically not.

This can vary from motor to motor, as the cooling channels are different in all of them, but generally, if I see a picture of the motor with the cover off and discolored paint, I stay away.

Here's an example I found on the web:


Bad top cylinder with a little concern on the lower too.

You can also check the wiring around the cylinder head and exhaust cover. If there is any indication of melting, you've also got a potential indicator of an overheat.

If you're looking for signs of overheating while running your current motor, then an easy test is to either put an IR thermometer on the head while running. Temp should be between 130 and 160 when warmed up. If you don't have an IR gun, then you can do a quick hand check with a wet hand. You should be able to hold your hand on the head for about 3 or 4 seconds when it's warmed up. If you get a quick sizzle or find it to be very hot, shut it down immediately and figure out where your issue is.

Good luck!