1986 Johnson GT150 Powerhead Rebuild

This is my first time rebuilding a 1986 Johnson GT150 Powerhead (Mod J150SLTCDC) after I had a piston start to melt down. I’m needing everyone’s help to make this work and anything would be helpful. I was able to hone out the cylinder that went bad and it came perfectly clean. I honed the rest of the cylinders and removed all the rings because I was just planning to re-ring while I had it tore down. I replaced the bad piston with another one I had from another powerhead, exact same part number as the one I’m working on. Now I have everything cleaned up and ready to go back together. The problem I have ran into is with the piston rings. I preordered the rings (GLM 24210 replaces 394930) which I assumed would replace the standard rings. Well I was wrong. I gap tested one set of rings on the starboard side and found that the lower ring was .035 and the upper ring was .030, even further out spec on the port side. The spec I have is .007 to .017 in per the Clymer Shop Manual. The cylinder measurements on the starboard side are all 3.499 and the port side are all 3.527. So I was thinking about returning the rings and getting a 30 over set and file them down to the specs. Please let me know what you think and any help would be greatly appreciated.

Need to do more machining to the cylinders. If I read your post correctly.

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Originally Posted by ChampioNman

Need to do more machining to the cylinders. If I read your post correctly.

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:thumb up2: Also must check for cylinder taper and out of round. Measurements must be taken at different depths and different areas around the bore,,especially around the ports. Hard to believe a 30 year old motor still has a standard bore at 3.499(unless you are inserting the ring just in the top of the cylinder where the rings never touch) 3.527 is way too big and you just cant hang .030 over rings, it will not last at all. Start with round/in spec bores. Do it once and do it right. Throw the Clymer manual away and get an OMC repair and service manual. I can E-mail you a copy if you want. You need to double check your bore measurements per the OMC manual.:thumb up2:

If you can send me the OMC manual that would be great and I will check the lower bore measurements. Thanks

After re-reading your post, it looks like someone took the port side out to .030 OS and left the stbd side stock.

So should I put .030 over rings in that side and the standard in the other? Plus this is on a 1986 Champion Fish and Ski and it's done me well. Trying to come up with a cost effective repair.

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Originally Posted by jwcert10

If you can send me the OMC manual that would be great and I will check the lower bore measurements. Thanks

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Shoot me your e-mail address,,,:thumb up2:

It's what I run, when I bought it new I had a GT150 on mine. Mic your pistons and see what you have. With the port side at close to .030 OS you're close to the end of the line next you would have to re-sleeve it. As Art says use an inside gauge and check the cylinder in two areas. 90* across from each other.

I checked the port pistons and found they are 30 OS so I'm going to get the rings ordered for it. As for the cylinders, I checked the bore and tapper and everything is within the specs. As for the starboard side, they are standard pistons but when I put in the new ring( .5 in in) the gap is to close to .017, way over. Will it still be ok or what is the recommendation. Thanks

If you can afford to do it take the starboard side to .030 over. The OS piston kit will come with rings, piston, wrist pin and circlips. You will need to order the wrist pin bearings separately. Don't mix and match the manufactures though you make get a difference in piston weight and throw off the balance of the internals. Clean the piston crowns on the port side and look for a part number.