Johnson AD needs help

[adam1961]

I pulled apart what looks to be a low hour AD 7-1/2 hp.
This motor hung on the rack in a long closed dealer for at least 25 years. Cracked coils etc.
It must have been run low on oil melted the big end of the top rod. Compression before tear down was 65 top and 90 on the bottom. Based on the condition of the notes and pistons, I don’t see the reason for such a large variance. I have not measured the notes yet, but will do so soon.
The pistons look good, rings are free, no carbon build up in the grooves. Bores look OK to me… if anything, the bottom looked worse.
I would like to get away with a crank and set of rods, but the compression variance has me stumped. The gauge used has always been reliable.

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[frankr]

Funny (or not), I just got done explaining what causes that. viewtopic.php?f=2&t=11048. It is caused by water intrusion into the crankcase. You can plainly see the corrosion pitting near the wrist pin on the piston on the right. It usually from a bad carbon seal at the bottom of the crankshaft/top of drive shaft. But can also be a pin-holed exhaust cover plate, or leaking head gasket. Once the water gets in there, it is circulated throughout the powerhead by the oiling system. I’ve seen lots of them. Any normal, sane person would jump to conclusions and blame it on the oil. Wrong conclusion.

[adam1961]

Frank,
That does make perfect sense. I was thinking that oil starvation would have caused further scoring on the pistons and also heated the main journals. The lower cylinder walls did look like they might have had some rust pitting at some point.
Is there any reason that it would take out the top rod first, or is it just the luck of the draw?

[fleetwin]

quote Adam1961:

Frank,
That does make perfect sense. I was thinking that oil starvation would have caused further scoring on the pistons and also heated the main journals. The lower cylinder walls did look like they might have had some rust pitting at some point.
Is there any reason that it would take out the top rod first, or is it just the luck of the draw?

Well, the water that is drawn in by a bad/missing lower seal is actually pumped up to the top crankshaft bushing through the recirculation system, then it makes its way into the number one crankcase.
Like Frank says, you better pull that exhaust cover as well, especially if you are going to invest in a new/used crank and rod. Those bores look pretty glazed/rusted over, which might explain the compression variance, but again, heed Frank’s advice and check that head gasket/surfaces carefully. This situation may be a combination of water intrustion and someone thinking these engines can run on a lighter oil mix due to "today’s improved oils".
Be sure to measure the bores, then examine them closely after honing to ensure there isn’t excessive rust pitting.

[frankr]

That motor actually has an oil pump. Don’t laugh, it does. See the key slot on the lower main journal of the crankshaft? Now look at the lower main bearing. See those two little holes that align with the key slot as it passes by? At that moment, there is pressure in the lower crankcase section. The pressure forces oil that has accumulated at the bottom through those passages & keyway, through the hose to the upper main bearing, where it spills out to lubricate the upper bearing (where there is a partial vacuum at the moment). And of course it then enters the upper crankcase chamber to do its dirty work, if it contains water mixed with the oil. The 12hp in the other thread is oiled the same way.

[adam1961]

Thanks to both of you for these excellent explanations. I will be sure to find the source of the water intrusion. I will measure the cylinders and de-glaze them. The rings are nice, but if breaking the glaze, is it necessary to fit new ones?

[fleetwin]

Well, I don’t think I would go through all the trouble of finding a crank and rod, then use the old rings. You should be able to find some NOS rings without too much trouble/expense. But, if you choose to reuse the old rings, I would certainly mark them and reinstall exactly the same way they come off the pistons. You should check the ring end gap as well. Probably a good idea to measure the piston skirts, especially after replacing the one bad rod.
Unfortunately, some of your measurements may be "marginal": not out of spec, but close, forcing you to make judgement calls. These powerheads are pretty forgiving, and I’m sure the engine will run fine even with marginal fits/gaps/clearances. Please be careful trying to remove the exhaust cover screws, you sure don’t want to break any of them off. I would work on getting the exhaust cover screws out first, before hunting down new/used parts, just in case you run into trouble.

[Mumbles]

Adam, check your PM box.

[bkrsdoz]

If it were me, I would look for another power head. They are plentiful, and can be had very reasonably.
JMHO

[adam1961]

Fleetwin,
Good advice on removing the exhaust cover screws first….I always like to get past the tough points before ordering up a buch of parts.

The block looks good. I would rather build it up with good parts and know what I have. I have pulled apart this engine and 3 parts motors. All have damage to the upper rod journal. I would not have guessed that they pump water up there, but I guess that is the new think that I leafed for yesterday. 😀

I am on the hunt for a good crank. Have rings on hand and will hone it and make sure the bores are withing the acceptable range.

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