100hp johnson what to do???

[cdoliver]

Ok, here's the deal. I have a 1979 johnson 100hp (javelin? what does this mean anyway and are the parts the same as a non-javelin?) It had been lightly used over the years, and it was used a lot last summer with no problems.

This is my situation: Last weekend it broke a ring which in turn busted a hole in the piston at the exhaust port(where it broke). The part of the piston that came off, dented the head up pretty good (7 or 8 spots where there are nice little pecs in it over the top of where the piston was) The Cylinder walls are maraculusly ok. I can't believe it. Here is my question(s)
1. Can I clean-up and reuse the damaged cylinder head with a new gasket?
2. Do I need an entire powerhead gasket set for this piston replacement?
3. Being that this engine is 29 yrs old with no previous rebuild, should I go ahead and re-ring the other pistons while I'm in there? The other pistons have good compression.
4. What can I contribute to this failure and what do I need to check on the motor that could have caused this? oh, ahem... it had never been decarbed. (or was it just time for this to happen)?
5. I'm guessing the part of the piston went out the exhaust. Do I need to look for it?
6. Any tips for this project?
Thank you so much. This forum is very helpful.

 

[Dhadley]

Re: 100hp johnson what to do???

I think you'll find the corner of the piston burned away first and then the ring broke / burned. At any rate you'll need to figure out what caused that or it'll do it again on the fresh / new powerhead. It could be something as simple as the wrong spark plugs.

I think that you'll find, once you measure the cylinders, that you'll end up boring it all. But measure it first - or have your machine shop measure it - before you order parts. Regardless you'll need a gasket set, no big deal. It's easier that way anyway.

Your 100 is the same as any other OMC 1979 V4 100 hp crossflow. The crossflows use a lot of common parts for a lot of years. Especially crank, rods, pistons etc. Look on the good head - not the damaged one - and write down the casting number. The bad head should be only one digit off. You'll want to replace the damaged head with a mate to the remaining good head.

 

[cdoliver]

Re: 100hp johnson what to do???

Thank you for the post. Tell me if I'm barking up the wrong tree here. I was planning on replacing just that piston and honing that cylinder out. Are you saying that it may have natural/unnatural wear to the point that a standard size piston will be too small? Also, funny you should mention plugs, because my "genious" brother replaced the champions with NGK. The auto parts person said they were exactly alike... This is likely the issue here then??? Thanks again

 

[cdoliver]

Re: 100hp johnson what to do???

Hello again,

I have a chance to get a set of 1979 johnson 140 hp heads for cheap. I was wondering if these are the same/work as my 1979 johnson 100hp. Thank you again