"Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix it?

"Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix it?

Hi All,

My posts started out with questions about a knocking noise - see previous posts for background if interested.

Anyway, I pulled the crankshaft and pistons out (1991 75hp 2-stroke) and the top piston is scored in multiple places see pictures below:



The abbreviated version of the background is the fuel pump check valves failed so I'm assuming the engine was running lean. Knock is worst at idle. No knock immediately after running under load at 2500rpm back to idle in neutral. Probably was a lot of carbon build up - did a concentrated SeaFoam treatment before I removed the power head. Compression when warm 110 - 105 -110 - (T, M, B) questionable HF tester.

I measured the cylinder bore as close to in parallel and perpendicular to the wrist pin at the top and bottom of the cylinders and I don't have my notes with me right now but they measure good, in fact a little under 3.375. OOR is less than 0.001, same for taper. I think the average bore diameter was around 3.3720. Even the top scored cylinder measurement on the worst scored section was 3.3735 as I recall but the gauge would have measured the peaks...

The two bottom pistons and cylinders show no sign of contact. Nice cross hatch pattern on the cylinder walls. The top piston was contacting the cylinder walls in a number of places all around. The last picture is the worst area that extends the entire length of the piston. On the other side there are smaller spots that do not extend the whole length.

When I pulled the pistons out, the top ring on the bad cylinder was not totally free. All I had to do was rotate it a little to free it up.

I haven't taken the rod off yet so I don't have the journal, rod, and pin measurements yet but there isn't noticeable play at either the crankshaft or the wrist pin. The top piston linkage feels exactly the same as the other two to me. The rod is discolored more than the others near the piston (brown) for about 1.5" below the wrist pin. It is not noticeably warped.

So, my question is what should I be looking for? What additional measurements should I make?

I want to make sure that I understand what happened and replace what needs to be replaced, but not what does not need to be replaced. Assuming the rod, journal, wrist pin, and bearings measure good and I replace the rings and either re-hone or bore to the next oversize piston it seems to me that it will just happen again.

I'm thinking that the discoloration of the rod probably means the engine overheated but not because of lack of water circulation and the sensor did not get hot enough to trigger. Now with the cylinder walls scored, I suppose the rings get hung up a little and that makes the piston tilt? Was the top ring really stuck due to carbon buildup that I mostly cleared with the SeaFoam but by then the damage was done?

Anyway, if you have any suggestions, please post. I guess I will be taking it to a machine shop and those guys will double check my measurements.

Thanks in advance,

Greg

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix




After looking at other posts on piston scoring and taking a closer look at the cylinder in better light it looks like aluminum from the piston has been deposited on the cylinder walls. One post talked about the possibility that a carbon chunk or chunks may have gotten lodged between the piston and the cylinder and that is what makes the most sense to me given the multiple contact areas between the piston and the cylinder wall. I can feel the biggest deposit on the cylinder wall and it seems to be raised.

From what I have read given this is looper is that even though the compression chamber compression is "OK" the crankcase compression is probably not.

Do you think the carbon chunk explanation makes any sense?

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Guess engine has been overheated some time, typical bad cooling and top cylinder goes first.
Difficult to se on the pictures,but its possible that cylinder could be ok with a honing.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Iceman66 said:

Guess engine has been overheated some time, typical bad cooling and top cylinder goes first.
Difficult to se on the pictures,but its possible that cylinder could be ok with a honing.

Click to expand...

Yeah, It was running lean due to a failing fuel pump. Didn't trigger the temp sensor thou - maybe I need to check that. The cylinder measures good so I'm hoping a few passes with a ball hone will clean it up. From what I have been able to find at least from Flex Hone, I have to choose from 3 1/4" or 3 1/2". Web site says the hones are over sized so I don't know what size would be better. I'm also considering going after the transferred aluminum with Tidy Bowl but I don't know if that is safe yet.

Thanks for the encouragement!

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Would not use a ball hone,may hook up on the ports,better to use one like this( adjustable): Remember to use right speed and up and down for correct hatches.






And DO check your temp sensor.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

A ball hone is just for deglazing and a spring hone is worthless, you need a rigid hone like a Sunnen to hone cylinders. Finished bore size after light hone should be no more than 3.3775. The piston top does not show a lean burn pattern and the rod is discolored from factory heat treating. The only way to know if is to remove piston and lok at bearing surface, if a lean oil problem the surface and roller will have a bluish tint to them. Check the poppet and t-stat as its look more as if running to cold and that the reason for excessive carbon bulid up a possible cold sieze of piston.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Before sticking a hone in that cyl you need to clean all the aluminum from the walls. Otherwise you risk clogging the hone stones, making them almost worthless as they will not cut much once clogged.

Muratic acid works well, but stand ready with a water hose to thoroughly rinse all traces of the acid from the motor.

Use gloves, mask and eye protection, scrub the cyl wall with a scotch-brite dipped in acid until the wall comes clean. Rinse well, scrub while rinsing to be certain all traces of acid are removed from the cyl AND from all alum surfaces of the block.

Dry and lightly oil all cyls.

Toss that piston and replace with a new one, size depending on your finished bore measurement.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

First,

Thanks for the responses -

Fazt - does look like a cold sieze (now that I have read up on what one is) and I will check the poppet and thermastat. If I have to do any more than just break the glaze then I will take it to a machine shop. I do have access to a mil but I'm not confident that I wouldn't ruin it. I'll take a look at the wrist pin needle bearings post what I find.

Charlie - thanks for the cleaning tips - I also read that applying some baking soda afterwards is a good idea to neutralize the acid. Of course, I hope the cylinder walls won't be damaged by the acid given that I don't know what they are made of.

Iceman - I agree with you that it seems as though a bunch of balls banging into the port edges is a really bad idea but but it seems as though the majority opinion is to go with a ball hone to break the glaze. I will test the temp sensor.

I called the Flex Hone (BRM) technical support line and got some information. I'm embarrassed to say that I don't know and can't find what the cylinder inserts are made of - it's probably so obvious that no one bothers to mention it.

Anyway, if I were to use a flex hone (just to break the glaze) BRM recommends a GB series 3 1/2" hone. The flex hone is not recommeded for a plated i.e. nikasil or other platings that I am not even aware of on ported cylinders. Unless the cylinder wall is aluminum BRM recommends silicon carbide. They say there will be no problem with the ports even with a 3 1/2" hone in a 3 3/8" cylinder.

It seems as though almost all the posts I have seen on ball hones say to use an aluminum oxide 240 grit to break the glaze and I guess I will go with that.

Sure would like to know what those inserts are made of...

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Cylinder sleeves or liners as you call them are cast iron and you need a 180 grit to properly deglaze. The best solution to hone with is deisel fuel, just slowy pour in cylinder to keep balls wet. When done throughly wash wash block with dawn dishwashing and hot water and a round or oval commode brus.h Wash untill you can dry cylinder with a white paper towel and no residue(grey) is visible on towel then coat walls with 2 cycle oil.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Faztbullet said:

Cylinder sleeves or liners as you call them are cast iron and you need a 180 grit to properly deglaze. The best solution to hone with is deisel fuel, just slowy pour in cylinder to keep balls wet. When done throughly wash wash block with dawn dishwashing and hot water and a round or oval commode brus.h Wash untill you can dry cylinder with a white paper towel and no residue(grey) is visible on towel then coat walls with 2 cycle oil.

Click to expand...

Thanks - at least I am learning a lot - never have had to go further than the head before... Given that they are cast iron, do you think I should go with silicon carbide, or aluminum oxide for the hone? My father was a machinist - I have his tools wish I had gotten more know-how.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Acid won't harm the liners, but it is hell on aluminum. If it will desolve the alum smeared on the liner what do you think it will do to the rest of the block? Have a garden hose ready so you can quickly rinse it off completely. Once you think you have it all rinsed off, rinse it again as it will set and eat inside the microscopic 'pores'.

Acids are regularly used to clean pontoon boats. Muratic is a vicious one, I don't recommend it for pontoons but some use it.

A safer choice is many of the toilet bowl cleaners, they aren't bear as strong and can take multiple applications. Far slower due to the lesser acid % in the solution. And not near as harmful to your nose. Still a good idea to wear a face shield and gloves.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

CharlieB said:

Acid won't harm the liners, but it is hell on aluminum. If it will desolve the alum smeared on the liner what do you think it will do to the rest of the block? Have a garden hose ready so you can quickly rinse it off completely. Once you think you have it all rinsed off, rinse it again as it will set and eat inside the microscopic 'pores'.

Acids are regularly used to clean pontoon boats. Muratic is a vicious one, I don't recommend it for pontoons but some use it.

A safer choice is many of the toilet bowl cleaners, they aren't bear as strong and can take multiple applications. Far slower due to the lesser acid % in the solution. And not near as harmful to your nose. Still a good idea to wear a face shield and gloves.

Click to expand...

Charlie, thanks for the tips. The aluminum removal is complete. I didn't take a garden hose and do any major dousing but I did coat all the metal that could have come in contact with the acid with baking soda. Hopefully there are no holes being etched I am not aware of. Now that I can really see the scoring in the cast iron, I am re-thinking my decision to not bore to the next piston size. I can't really measure the scores but I can just feel one of them. There really isn't any scoring from the middle of the exhaust port to the spark plug so I don't think there will be a compression problem but I'm not so sure about what might happen to the piston skirt. I measured the cylinders with a telescoping gauge and I was getting repeatable numbers about .002 less than the bore spec of 3.375 assuming that is right I could hone a little and that is now the plan. I'm not all all confident I will use the flex/ball/dingleberry hone I have on order correctly especially given that this cylinder is blind. I have a cheap dial bore gauge (.0005 graduations) on order to double check my previous measurements. All that being said, the engine performance was fine as far as I could tell with the nocking (slapping) so I'm not looking for more compression - just a setup that stays together. Now that I know what the problem is it makes sense that the knocking went away when the engine was warm and did not come back until it was idling for a while and cooled back down again.

Anybody know how much material I dare take off with a flex hone and still have a serviceable cylinder?

Thanks

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Really all you want to do is remove the glaze and leave a decent cross-hatch pattern to hold oil and get the new rings seated. any more material removed only increases clearance and lets the new piston 'rock' even more, which tends to shorten the life of the rings.

If you can 'catch' a fingernail in a groove them you may want to consider boring to an oversize piston. If the scratch is small and only in the lower half of the cyl wait until you hone and see what is left, you may be able to just leave it.

Pictures if you can get clear ones so we can see the damage.

The main thing is to find what caused the initial problem.

Rebuild the fuel pump and carbs and water pump. You want to be SURE timing is set correctly.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Anybody know how much material I dare take off with a flex hone and still have a serviceable cylinder?

Click to expand...

You can run a ball hone in a cylinder till its worn out and you might take a .001 off . Also a ball hone will not take any tapers out of cylinder as it will only follow cylinder shape. You need to mike it and if its over .003 out of round its needs boring. You need to check your mikes as the cylinder should be measuring 3.375 + as 3.375 is the finished hone size from factory

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Charlie and Fazt,

Thanks for the replies. The scores are not deep enough to catch a fingernail and you can still see the factory cross hatch where the piston did not rub against the cylinder walls. I'll try to get some good pictures and send them. One thing is that even though I thought I coated the walls with enough 2-cycle oil it rusted a little. I was able to wipe it off with a shop rag. It is good to know that the ball hone will not take more than 0.001 off so I don't have to worry about doing too much damage. I must be measuring under. I think I'll measure one of the good pistons and see if that measures 3.371 as a sanity check.

This is a low resolution shot of the worst area of the cylinder I just took. Staring at this picture I can see that I have not removed all the aluminum...





This is the same picture cropped at full resolution.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

To me this look like a bore is required, to deep damages.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

The scores are not deep enough to catch a fingernail and you can still see the factory cross hatch

Click to expand...

This means nothing as 20 year old motors will still have the crosshatch marks in them and fingernail is worthless...use the edge of a penny. I suggest you take to a marine/cycle/auto shop and have a rigid hone run thru cylinder as this wil show if its out of round and if grooves are removed. You do not want grooves in cylinder wall as combustion will blow by and stick rings or etch piston.From the looks of it you need to possibly bore it...

I think I'll measure one of the good pistons and see if that measures 3.371 as a sanity check.

Click to expand...

The pistons will be undersize when measuring ..you need to measure cylinder bore.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Finish removing the alum, lightly hone then measure.

I don't even own a ball hone, only a straight hone, NOT a spring type.

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

As ususal thaks for the replies, you guys are really helpful!

I should get the ball hone in today as well as a dial bore gauge. The machine shops I have found that are willing to work on it are each about a 2 hour drive. They and you guys have more or less said that the ball hone won't hurt anything so I'll give it a shot and measure. I'm not sure why I am measuring under now but at a minimum I'll be able to get relative measurements on the bad cylinder for OOR and taper and I can compare to the other two cylinders for diameter.

On a related topic, neither the piston pin nor the piston pin bearing needles have a blue tint. The ends of the needles are blue/black. That along with Fazt's earlier observation that there was not a lean burn pattern on the piston would indicate that the cylinder was not running lean? If that is the case, then the question of what happened remains. I didn't mention this again in this thread, but when we launched the boat for the first time this season, it was stalling at not quite WOT. I had my son pump the primer bulb when it started to stall and that fixed it. I bought a fuel pump rebuild kit and new fuel filter - the filter fixed it (or so I thought). I guess the point is that it did run lean for a while and then of course I discovered the bad check valves in the fuel pump after the knock started.

The poppet seems to be OK, lots of spring tension and the diaphram and seat look good. There is a small bypass "notch" which I assume is part of the design. There thermastat was closed when I removed it - have not put it in hot water yet.

I also have a few embarassingly basic questions to ask.

1) I've read a number of posts saying that the connecting rod cap bolts cannot be reused because they don't streatch right the second time so I ordered 2 new ones for the cylinder I am working on. I was thinking that I would not even take the rods for the other two pistons off the crankshaft but now I'm thinking through how hard it is going to be to get it back together... Neither the Mercury service manual nor the seloc manual say to throw those bolts away. They do say to throw away the piston pin circlips, though so I am wondering if the cap bolts can be re-used. (I figured the less I did, the less I would harm as far as taking the outher two rods off the crankshaft).

2) It is not clear to me exactly what surface on the crankshaft end cover I am supposed to spread the perfect seal - it is not supposed to touch the o-ring, right?

3) Can I use blue loctite for the bolts in general instead of the 271 red that is called for in the service manual?

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Re: "Rod Knock" Turns out to be top cylinder piston slap - what caused and how to fix

Follow the factory service manual directions to the letter. Lok-tite products cannot be switched, the holding strength is totally different and cannot be substituted.

The cost of the new rod bolts are not as bad is possibly breaking a ring in attempting to reassemble the crank with rods and pistons attached. Bite the bullet and replace the bolts, do not re-use them.

Bolts may be re-used, but you want to eliminate every possible fault to maximize the motor lifetime. This is the reason most reputable builders rebuild carbs, fuel pump, and water pump, to remove possible failures.

Piston circlips, the open ends must be either straight up or straight down. If positioned at either side the clip could 'spring', when stopping/changing direction at TDC and BDC that portion of the clip can 'spring' out of it's land, eventually work hardening the circlip until it fails, the pin then works out eating a nasty gouge in the cyl.