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2001 Johnson 3 cyl - bottom piston rod bearing melted

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AZRic

New member
The connecting rod bearing on a 1991 Johnson J70PLSIS has melted and I'm attempting to determine why it failed. Seems it could be multitude of possibilities. I'm hoping someone has experience to help me narrow the reason down from my findings.
After dismantling the never taken apart power head here is what I found:
  1. Only the bottom piston overheated.
  2. The lowest cylinder connecting rod bearing was melted.
  3. One of the rod bolts holding the melted bearing was loose on inspection.
  4. The bad cylinder has very little damage - a barely seen small vertical scratch which can probably be honed out.
My thoughts:
a) Item 3 could be the cause. If the rod bolt somehow loosened, this would immediately cause the entire problem. Is this even possible? Or would the overheating cause the bolt to back out?
b) Since only one cylinder overheated, I am thinking it is unlikely be the VRO fuel pump. Seems all cylinders would have overheated.
c) I'm more inclined to look at the lower carburetor & tubing to see if somehow it failed and no fuel/oil was injected into the lower cylinder causing the overheating/failure.
Any help is much appreciated.
 
The bottom carburetor has clogged, gummed up, fouled in some manner, resulting in a extremely lean mixture, which is turn results in friction and therefore... Heat!
 
OK. Do you suggest me buying a carburetor rebuild kit, or just a good cleaning: checking the bowl and blowing out the hoses/jets/etc?
I'm still a little concerned about one of the rod bolts being 2-3 turns loose on initial inspection. Can this actually happen due to the overheating?
 
AZRic said:
OK. Do you suggest me buying a carburetor rebuild kit, or just a good cleaning: checking the bowl and blowing out the hoses/jets/etc?
I'm still a little concerned about one of the rod bolts being 2-3 turns loose on initial inspection. Can this actually happen due to the overheating?
Click to expand...

Yes... heat loosens and this is why we apply a propane torch flame on the metal that surrounds tight bolts.

Question: Did you inspect the connecting rod bearing surface and also the crankshaft throw? If not, I suggest you do.
 
I'm just a little bit surprised that the bearing failed before the piston, if it was a lack of oil (lean/fuel). I'd rather think it broke a bearing cage. Or, water got in and rusted / pitted the rollers.
 
Thanks for your help Joe!
Yes, I looked at both. I am planning to replace the connecting rod since the bearing had seized to the rod, and replace both bearings for the new rod. The crankshaft will be inspected and the throw polished by the same place that is honing the overheated cylinder. They will also inspect the entire head for cracks or other possible damage.

I also talked to a guy at TSM Performance and he suggested replacing all the pistons/rings (getting a reduced cost kit) and honing all cylinders. His thought being it would cost slightly more but I would have a like new power head. After running the numbers, it would cost me about $150 more and really no more work.

What are your thoughts on this and the carburetor rebuild? Thanks again for your continued help.
 
Hi fdrgator,
No rust was found on any component of the power block or entire motor. We live in Arizona so have little rust on anything! ;) Not saying it wouldn't if there was water in the fuel. I will check the gas tank when I drain it and the oil container for water.
However, the hot piston did have the top ring "frozen" and a little burn/melt mark on the top of the piston. The lower ring was fine. None of the other pistons show any damage at all. Not even discoloration from overheating.
 
AZRic said:
I looked at both. I am planning to replace the connecting rod since the bearing had seized to the rod, and replace both bearings for the new rod. The crankshaft will be inspected and the throw polished by the same place that is honing the overheated cylinder. They will also inspect the entire head for cracks or other possible damage.

I also talked to a guy at TSM Performance and he suggested replacing all the pistons/rings (getting a reduced cost kit) and honing all cylinders. His thought being it would cost slightly more but I would have a like new power head. After running the numbers, it would cost me about $150 more and really no more work. What are your thoughts on this and the carburetor rebuild? Thanks again for your continued help.
Click to expand...

Sounds good to me. New rings on all pistons would be a must in my book... Also, in the 2 or 3 powerheads I rebuilt in my 30+ year stretch... cleaning and rebuilding all of the carburetors fell into that "must" category. God forbid I'd crank up a rebuilt engine and find a cylinder running lean.
 
Hi Racerone. After I go to the machine shop I'll let you know the results of their inspection. I do know that the crankshaft rod journal of the failed cylinder will be cleaned up by the machine shop along with honing all cylinders. From my visual inspection everything looked surprisingly clean for a partially seized engine.
 
I have never put one together with a crankshaft that melted a bearing like that.----This is precision stuff.----Do you or your shop have the dimensions and tolerance ?-----Or will it be a case of -----" it looks good to me "
 
Good question.
I would hope that the machine shop either has the measurements or can take measurements from the non-overheated journals and verify that all is OK. This is a very reputable shop with a top rating that stand behind their work.
Are you saying you would buy a new crank shaft and replace all new bearings on a 2001 motor?
I wonder what others think about this.
 
You said that the bearing was melted. I would be very surprised if the crankpin survived that. But if the shop is any good at all, they can advise you on that.
 
Maybe we are having a bit of a disconnect on the term "melted". I'm going to add a photo to this thread so we can all see the damage to the head and bearing. I call this "melted", but others may say it is "damaged". I'm sure there are much worse cases than mine.
rod bearing.jpg
 
Well,yes it is melted, but I've seen worse where everything was welded together from heat. My gut feeling is still something broke, such as the cage or roller. The cause is debatable, I suppose. Hey, doesn't that piston skirt have corrosion on it (water intrusion????)
 
Yes, I agree with others, that the crank can't be used as is. Racerone, here's a picture of the overheated journal on the crank. The machine shop I'm going to said they can probably repair it, but they haven't seen it yet.:confused: Any comments on them repairing this? Hoping they can since it would add around $800 to the bill to buy a new one.
JournalClose.jpg

Also, to answer frgator, the picture makes it look like piston corrosion but its not. I haven't spent the time to clean it since I'm replacing all 3 pistons and rings - available in a head rebuild kit with seals and bearings at about $50 more so seems like the thing to do.
 
I say and many others wull say that is SCRAP-------Those that say it can be polished up simply do not understand how these bearings actually " work with the oil "
 
I would say------Rod bolt loosened.------Then rod cap misaligned.----Then bearing rollers and cage jammed.-----Not working like a bearing " works in oil "------Then motor came to a " dynamic halt "-------This is going to be EXPENSIVE.
 
Gotta give credit to the shop---they haven't seen the crank. If they do look, it will take less than half a second for them to agree it is junk.
 
Thanks for all the bad news gentlemen! But your experience and honesty is why I'm on this forum. :cool:
I hate to send you the next pictures.
-Looks like the leaf valve on the leaf plate assembly has a larger gap (1/8") on the damaged cylinder that the other 2 leaf plates. I'm assuming this is probably an issue too? (there is a reflection of the opening in the pic)
LeafPlate.jpg
Here is the damage to the cylinder head:
CylinderHead.jpg
Final picture of the cylinder wall (note that only the vertical lines can be slightly felt with fingernail - the swirls cannot be felt at all):
CylinderWall.jpg
So, from these, what is the vote on trashing the whole power head or take this stuff to the machine shop and see their opinion on rebuilding this one?
And, if trashing is the outcome :(, is buying a refurbished power head from a place like TSM Performance a good idea?
Or, since I still don't know the real cause of failure, trash the whole engine it's just not worth a rookies (i.e. my) time.
Thanks for all your expertise on this thread...I'm learning a ton!
 
Bore all 3 .020, find a crank, replace reed block/reeds,mill head.020 as minor dents will eventually fill up with carbon,rebuild carbs, reassemble and go boating.
 
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