Orange peel on top of piston

[robbyt8165]

My 1987 Force 125hp engine just blew. 3 cylinders fine but fourth has problems. 70 psi compression (150 for other three). Took off head and found metal shavings in the bad cylinder, cylinder wall scored, carbon deposits on head, and the top of the piston is incredibly rough and pocked - kind of an orange peel like surface. I am assuming that the shavings are a disintegrated ring. Spark plug for this cylinder was kind of black and oily.

Question: What caused all of this? If I fix the the obvious (replace the piston and rings, hone the cylinder), will it just happen again? Is there an underlying carburation or ignition problem? What destroyed the top of the piston?

Thanks

[Petros]

I am not famillar with that engine but it sounds like it might have been pre-ignition in that cylinder. Pre-ignition will eventually melt the piston top, damage rings, head gaskets, burn and melt the exhaust valve, etc. Either that cyl. is running lean or it is running hotter than the others, or something causes pre-ignition in that cylinder (bad spark-plug, carbon build-up). Make sure the manifold does not have a leak or crack in the part that leads to that cyl., check that coolant is circulating properly in that part of the head (sometimes corrosion build-up will block water flow), check for a crack in that part of the head (have the manifold and head crack-checked in a machine shop), make sure the spark plug and wire is good, etc.

Are you sure there are no signs of this in the other cyl.? If so, it just may have shown up sooner in the hotter/lean cyl. Check your spark timing, check your mixture, and your temp.

If you replace the parts you should balance the new piston to match the others within a gram of weight or it might vibrate, hone the cyl. walls (consider replacing all of the rings while you are at it), and I would replace the exhaust valve too (and check the guide for damage from an overheated valve).

Good luck.

[robbyt8165]

Engine recently had a blown exhaust cover gasket adjacent to this piston (it is the bottom piston of a four-in-line engine). Water was squirting out, into the engine compartment. I replaced the exhaust gaskets but don't know how long the engine was running that way. Had about 3-4 hours on the engine after the gasket replacement and before the crisis. Do you think the leak led to less/no cooling? Peculiar, though, how well the engine ran just prior to blowing up.

Engine also made a rythmic clanking sound at the end. Suppose I need to look at the piston rod and camshaft to see what is going at that end to.

Thanks.

[PAR]

Yep, that's pre-ignition damage, likely from an over heated hole (cylinder) and the noise you had was probably piston slap or worse. You'll find out when the main and rod bearings come off.

[BHOFM]

Something as simple as a vacuum leak can cause one or
more cylinders to run lean and cause this kind of problem.
especially with engines that run under constant load. Marine,
aircraft, industrial; automobile engines run under a ever
changing load and not as likely to have this problem.

When you get the damage repaired, change all the hoses.
Cheap, as well a close inspection of the intake parts.

[Petros]

a leaky cooling system could very well cause this. If pressure can not be maintained in the water jacket the coolant will boil away, creating a hot spot. It can warp and crack engine parts because very large temperature gradients can develop across the head or block. have the head and block checked for cracks and warpage.

If they are only warped it can be resurfaced to bring it back true. There are no reliable way to fix cracks in the head or block. Sometimes the crack can be stop drilled and ground out, and then welded. After the part has to be heat treated to normalize the weld. This can work but unless the machine shop is very skilled at this, it will only crack again at the weld. It is usually best just to replace the cracked part.

hopefully the block and head are not that fragile and you only have to replace the piston and rings.

Good luck.

[robbyt8165]

Of course, there is a little more to the story. The engine has been rebuilt once before and was over-bored .030. So this scored cylinder will need a sleeve installed. Local machine shop quoted $250 labor and $100 parts. Should I do it or give up on this engine? If I do, does the sleeve need to be bored .030 over to match the other cylinders?

Thanks.

[BHOFM]

does the sleeve need to be bored .030 over to match the other cylinders?

Yes. The parts must be the same size, weight, or they
can destroy the boat. Shake it to pieces in short order.

The price seems a little high, check a few more shops.

Check the price of a new engine before going on with
any repairs, it might be the way to go if you plan on
keeping the boat for a long time. Used engines are
just "used" engine, last resort only.

[Petros]

You likely can just hone the cylinder, a small amount of scoring, as long as it is honed smooth, will not affect the life or performance much. Just make sure it is not too badly warped, that is what requires over boring. Use a bore mounted dial indicator.

Also most engines can be over bored to .040" but that means all new over sized pistons. Yes, they all must match weight and displacement.

Get it apart and check it out, this is all guess work until you know for sure. let us know what you find.

[jonesg]

I've talked to many marine engine rebuilders and its common to only bore out 1 cylinder, a very common fix.
The over-bore piston usually weighs the same as the original size for this very reason.

I wouldn't begin to guess at what happened but I would find out why it happened or it will again if rebuilt.

[Petros]

it is not just the weight of the piston that is the issue, though that is part of it. Over-boring one cylinder means that one will displace more volume than the others, and it usually means slightly higher compression ratio in that cylinder (more volume displaced into the same combustion chamber volume). So that one cylinder will put out slightly higher power than the others with each power stroke, making the power pulses from each cylinder to the crankshaft uneven, causing the vibration.

If the over-boring is small, like 0.005-.010 inch, and it is a relatively low RPM engine (2000-3000 rpm max speed), and the piston weight is matched, there would likely be no noticeable difference. Marine diesels are much slower than this and it would not surprise me if over boring one cylinder is routine with diesels.

On racing or performance engines the engine builder not only balances the weight of the all of the moving parts, they also make sure the combustion chamber volume is the same in each cylinder just so there is a smooth power flow with little vibrations, especially at full throttle.

Though again if the piston bore can just be honed, and not over-bored, you can just buy a standard size piston and replace it without going to the oversize piston. You still have to make sure the piston is matched weight to the others. I have seen supposed matched sets of pistons be out of balance as much as 8 grams within the set, that would cause a noticeable vibration above about 3k rpms. I like mine to be within a half a gram when I rebuild an engine.

[robbyt8165]

Finally got time to break down the engine. Rings were fine but roller bearing cage (at connecting rod/crankshaft) was totally melted/burned up. Crankshaft is scored where roller bearings ride. So, at a minimum, I need a new piston, rings, and roller bearing cage for this cylinder.

Questions:
1a. Since rings were okay but compression low, does this mean cylinder wall scoring is bad enough to lose compression? Bad roller cage bearing would not have anything to do with compression , would it?
1b. Will honing be good enough to restore compression? Is this one of those try-it-and-see repairs? Kind of a pain to reassemble entire engine to find out that honing wasn't enough.
2a. Does scoring on crankshaft need to be ground out by a machine shop? Would NOT doing this lead to a new roller cage bearing quickly disintegrating?
2b. If crankshaft is reground, is there such a thing as a "over-sized" roller cage bearing, is over-sizing needed, and how is the fit measured/adjusted/etc.?

Thanks for any input.

[pistnbroke]

If crankshaft /rod bearing has failed then piston will not move up bore as much as usual hence the low compression...Usually the crankshaft would be re ground under size and a new roller bearing fitted to match. OR crank can be built up and re ground original size..not so common unless its vintage and not replaceable

[Frosty]

You are looking straight into the face of a new engine, a short motor anyway.

[CDK]

Since piston failures in large outboards concern almost always the lowest cylinder, I assume the buildup of sediment in the cooling channels is a contributing factor. Part of the lower cylinder wall reaches a much higher temperature causing a change in shape, resulting in compression loss, hot spots, cracks etc. It was the main cause of death for the old-fashioned 4 and 6 inline Mercury outboards.

[Petros]

It is possible to save the bore and the crank without going to the machine shop, if they are not too badly worn. You can lightly hone the cyl and hand polish the crank journals.

If the crank and bearings are scoured it sounds like it was run dry of oil. Make sure you clean out all of the oil galleys (use a fine bottle type brush, available at auto parts stores), and your oil pump and the by-pass valve is working properly and within spec. I have seen many blown engines from poor oil pressure caused by bad oil pumps or gummed up by-pass valves (the over-press valve, if stuck partly open it does not allow full oil pressure).

low oil can cause low compression since the rings are sealed with a light coat of oil. Though generally wasted bearing will not have much of an affect on compression (unless they are really bad). The test of low compression being rings or not is to squirt motor oil into the spark plug hole and re-do the compression test, if the compression goes up from the "dry" test, than the rings are worn, if the compression does not go up much, than it is either leaky head gasket or valves. Not possible to do after you dismantle the engine.

As far as the overhaul is concerned. IF you can hone the cyl. AND it is still within the max allowable spec for piston to cyl. wall clearance, you are okay to just hone it. It has been my considerable auto engine experience that a small mount of vertical scoring will not have a noticeable affect on compression or engine performance (I have pulled apart engine with good compression that had rather bad vertical scoring). The key is worn cyl. walls, if they are out of spec after a light honing, than you have to overbore the engine and replace the pistons with oversized ones. Also make sure the piston skirt is also not worn out of spec, often if run low on oil or with real dirty oil (which will not only ruin bearings, but will also scour the piston skirts and cyl.) it will wear out the piston skirts. Too much piston to cyl. wall clearance can lead to piston slap and eventual skirt failure.

A way to get a top notch cyl. hone is to heat the block up as warm as possible. In a water cooled engine this is easy by plugging the coolant inlets/outlest and fill it with boiling water before you hone it. On light engines many machine shop also use a "torque plate", a large alum or steel plate that gets torqued to the block like a head, but allows you to hone the cyl. it can look like a head with large holes through it over each cyl. The idea is that both torquing the head in place, and the heating of the block during running will distort the bore, and both using a torque plate and heating it will make a more precision hone since it will be closer to the condition when running. But I have seen tests that show heating the block is more import than a torque plate, and it is easy to do at home. I have had excellent results by filling the block with boiling water when I hone it (several water changes are required to keep it warm during honing).

I have done low budget overhauls, and just honed the cyl and replaced rings, willing to live with a sloppy piston fit. Sometimes this works okay, sometimes it does not, after the piston skirt fails you have to rebuild it again anyway, and if you are lucky the damage is limited to the piston. So there is no economy in doing a low budget rebuild if it does not last very long (the cheapest way to overhaul an engine is to do it only once!). If you have piston to wall clearance within acceptable limits after you hone it, you should be okay just to replace the rings. If not, either replace the engine or get oversized pistons and overbore the block.

If the crank bearings are not too bad they can be hand polished to a smooth shinny finish. It cost almost nothing to do yourself. Use a strip of 800 grit wet and dry sand paper, with a light honing oil or solvent (I use spray carb cleaner). loosely tape the strip around the journal so it turns freely (with solvent or lite honing oil), and wrap it several times with a length of cotton or nylon cord, and pull back and forth on each end of the cord. This will turn the paper around the journal and put even pressure on it. the action is like polishing a shoe, you pull back and forth on the cord ends as the paper works back and forth on the journal. It only takes about 30 to 60 seconds of this to clean up most journals. Keep checking until the journal is smooth. After polishing carefully measure the journal dia. to make sure it is within spec for min. dia. I have found it will seldom take much off to clean up the journals, like .0003-5" (well within wear limits). If you have only one or two deep scour lines, but the rest of the journals are good, it would be okay to leave these and run it. All they do is slightly reduce the surface area of the bearing, and if it is otherwise good there is no reason to regrind the crank just because of that.

OTOH, if they are badly scoured and gouged and you can not polish them out and be with in spec, they need to be reground. If you have to regrind the crank consider having it rebalanced too.

If the bearings in the cages are scoured or have blue discoloring, they need to be replaced. If the crank is within spec after polishing, you can use standard size bearings, if the crank needs to be reground than you need over-sized bearings.

The key on both honing the cyl. and hand polishing the crank is the clearance spec. You need to have accurate measuring devices, if you do not own them many auto parts stores will rent them to you. Sometimes you can take the block and crank into the parts stores or machine shop after you work on them and they will measure them for you to tell if they need to be machined. Same is true with the pistons, if you do not have access to accurate measuring devices, take them to the parts store or machine shop and they can measure them for you.

Do not feel intimidated by this, most engine overhauls I do I hand polish the cranks and go with standard size bearings. About half of the time I can just replace rings after honing the cyl. I will never spend more than I have to, OTOH every time I cut corners I ended up redoing the job within a few months. these are hard lessons learned for a cheap skate home mechanic (former full time mechanic). But there are a lot of parts that can be reused as long as they are serviceable. Many parts stores will tell you to replace the oil pump, valves, and other parts, but if they are in working order, use them. I have even reused gaskets! If you can get them off without damage, they can be reused. The only exception is the head gasket (unless you have an all metal head gasket, they can be re-used if not damaged). I love metal gaskets, I reuse them all the time without issue.

Good luck.

[robbyt8165]

Could I replace my crankshaft from a different year? My engine is 1987 and have found a crankshaft from a 1986 at a decent price (less than $200 including shipping).

Also am going to take my crankshaft to a shop to see if they can clean up the journal without getting it out of spec. Upon closer look, half of journal is smooth and the other half is more pitted than scored. Since the bearing melted, some of this may even be build-up rather than erosion. I tried the sandpaper-rope. It polished the smooth side nicely but isn't cutting through the pitted side significantly.

Thanks for replies.

[Frosty]

Did I say you need a new short motor? Oh yes I did, I just wanted to make sure I did because in my young days I tried so hard to repair this stuff!!!!