84 Johnson 115 HP Piston Replacement

[Jerry737]

I have an 84 Johnson 115 HP (4-cylinder) OB that recently had idle problems. I took it to the local OB repair shop and they advised that I need a new powerhead. The guy who worked on it said one of the cylinders was getting poor compression, and he removed the cover and found scoring on the piston. I took a look myself and there is a small chunk of metal that appears to be peeled back from the edge of the piston head. The cylinder doesnt look scored, but I am by no means an expert on this. The guy at the repair shop said that I could probably find someone who could replace just the piston, but that he wouldn't do it any other way than replacing the powerhead (at $3k estimated cost). Since I only have $2k in the boat/motor/trailer, I am thinking this expense is quite a bit more than I want to sink into this. I did a little research and a new piston is about $106. I have a Clymer's shop manual that doesnt provide very good step by step other than pulling the whole powerhead and getting the engine about completely apart. My questions are:

1. What is involved in replacing the cylinder? I imagine I'll have to buy a torque wrench, and possibly other tools as well as gaskets to replace any that I unseal in the dissassembly process.

2. Is there a better book (or website or antyhing that is idiot simple) to use for the step-by-step process of replacing a cylinder? I am mildly mechanically inclined, but this will be the biggest project on an engine that I have tackled.

Thanks for the help!

 

[NHJAKE]

Re: 84 Johnson 115 HP Piston Replacement

I have a 1982 115 with idle problems also. Do you know what the compression #'s were by chance? Mine were 125, 125, 115, 100. Thinkinf I might want to watch your post and take a peek at that 100lb cyl. From what I hear the OEM manual is the best. Good Luck!

 

[wilde1j]

Re: 84 Johnson 115 HP Piston Replacement

Depending on the extent of damage, the bad cylinder can be rebored oversized and a new piston and ring set fitted. If damage is too bad, a cylinder may be re-sleeved (not a dealer or do-it-yourself job) with again a new piston and ring set.

I would suggest contacting some like Mar Fab, who rebuilds powerheads regularly for advice and cost estimates. www.mar-fab.com/ You're in for way more than $100, even if you remove and replace the PH.

The OEM book is the only one worth owning, BTW.

 

[emdsapmgr]

Re: 84 Johnson 115 HP Piston Replacement

Do you know what caused the root cause of the failure? Nice to know before you put it back together and have the same problem. Technically, you can replace just one piston. A machine shop needs to evaluate the bad cylinder and determine if a .030 overbore will clean up the cylinder liner with the scuffing marks. If so, you will need hone the other 3 cylinders, purchase one piston, 4 ringsets and a gasket kit. For insurance on any new rebuild, count on new thermostats and impeller. If it has an OMS fuel pump on it, I would replace it also. If the head firing face is pitted, that is another problem..

 

[wilde1j]

Re: 84 Johnson 115 HP Piston Replacement

Excellent comments. On my own motor rebuilds, I have always bored all holes and all new pistons and rings just because of the labor involved, but it was apparent Jerry was looking for the least cost fix.

Totally agree with finding the cause of the failure, BC if you don't, it will occur again.

 

[SKEETR]

Re: 84 Johnson 115 HP Piston Replacement

I had a similar issue with my 115. Threw a ring and macked up a cylinder pretty bad. I bought it that way and was supprised it still ran after finding what was going on.
It depends on how much time you want to commit to a rebuild. An oem manual is mandatory in my experience since there are very few grey areas and it eliminates the need to label every hose and wire connection. Since they are generally engine specific the procedures are easy to follow.
Have the clyinder work done by the most reputable machinist you can find and make sure they campher the ports so there is no sharp edges.
You may want to replace all the rings, I would, since you are in there.

 

[Dennisanoka]

Re: 84 Johnson 115 HP Piston Replacement

Jerry,

How much you want to put into really depends upon one thing- what do you to have when you are done? You could probably pull it apart yourself -replace the minimum # of parts for a couple of hundred dollars. BTW you really won't know the condition of your pistons and cylinders until they are out.

Or you can pull it apart and replace all the other critical bearings and spend closer to $800 most likely, but you will end up with an engine that will be ready to perform like new and give you many future years or service -taken care of properly.

I was in your shoes 2 years ago, had never undertaken such a project ever before. I first read the manual back and forth talked to people here found the best local shop to bore out the cylinders and select pistons. The only other thing I went to them for was removing a re-installing the main bearings.

Two years later the engine still purrs like a kitten and I an so happy I did it. It won't be a quick deal but you'll know it's done right when your finished.

that's my 2 cents-