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

[Faztbullet]

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

You do not use loctite of any kind on rod bolts, only thing you use is 2 cycle oil. The factory manaual is what you need and throw the Seloc away.

 

[propgun]

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

So the question now is build or bore?



Just to be sure about the blue tint or not on the piston pin and bearings I included a shot of them. The cylinder shots are after 28 passes with the ball hone.

The cylinder is about 0.00075 out of round (bigger vertically). That is also the case with the two good cylinders. It is also tapered about 0.001 being bigger at the top (spark plug end). That is also the case with the other two. The middle of the bad area is now re-surfaced (scratched up). I could hone some more but when I got to .001 OOR I decided that was as far as I wanted to go.

The dial bore gauge is pretty neat. I set the dial to read 0 on the small dimension (horizontal) of cylinder 2 about an inch down from the bottom. My calipers measure 3.3735 which is the same as when I used the telescoping gauge so I guess the problem with the measurement being under was not my ability to use the telescoping gauge but the calipers themselves. The biggest micrometer I have goes to 3 inches - not quite big enough. My measurement of the ID of the connecting rod on the piston side was exactly what Fazt posted some time ago for a 1991 90Hp engine so the calipers might be ok for that dimension.

If I build, I suppose I should clean out the ring grooves for the other two pistons? They are free. I can afford to break a ring since I ordered rings and clips for the new piston no knowing that they were included with the quicksliver piston which is a little different that the others - the original pistons had well defined rings around the outside.

As for the loctite, I wasn't asking about bolts in the crankcase (actually I thought I was supposed to install those dry) but other bolts that the manual specifies locktite for. A mechanic at a local shop also used the term "oiled in" so I guess I should put two cycle oil on them. I called to ask if they had the bolts and he told me to re-use the old ones.

Thanks for the info on the clip orientation, I had not heard that before.

 

[Faztbullet]

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

Bore it.....

 

[propgun]

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

Well,

Just to close out the thread, I put put the power head back together Friday and drove it up to the lake on Saturday. Took most of the day to get the entire engine put back together. My rational was that the worst that would happen is that I would have to bore the cylinder and my time/labor to put it back together and tear it back down is basically free - learning experience and all.

Anyway, it started right up and there are no banging noises. I took it down to the ramp and ran it on the trailer for about 20 minutes between idle and 2500 rpm. So far, so good. I potentially had gotten some bad advice on another forum that suggested running it at the same speed for a long time as opposed to constantly changing the RPM which seems to be the preferred break-in method. It is running rough and misses now and then. Forgot to throw the timing light in the car so I have no idea if I changed the timing any when I put it back together.

It is running off a small tank with 50:1 oil mix as well as the injected oil with the old plugs.

I checked the compression afterwards when it was cold. The plugs on all three cylinders are just dripping with oil. The results are 115-115-115 with the HF tester. Actually the dial jumps up to 115 but reads 110 until I release the pressure.

The oil warning module or the float switch in the oil tank seems to have failed beep-beep-beep, and when I put it in gear at idle on the trailer in the water the tach jumps up to 2k or so momentarily from the 800 idle speed - I've never seen that before.

Thank you all very much for your help!

 

[CharlieB]

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

Does this mean you replaced just the one piston and all the piston rings?

Many do not realize that the break-in isn't just the 'wearing-in' of parts but the complete heat cycling of the piston from stone cold to full temp then back to stone cold. That's one thermal cycle. I like to see 6 to 10 thermal cycles to 'normalize' the piston to the bore. After that the piston is capable of handling full throttle loads.

You still haven't mentioned if you rebuilt the carbs, fuel pump, and water pump. If not, you may not have cured the root cause of the last failure.

 

[propgun]

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

Does this mean you replaced just the one piston and all the piston rings?

Many do not realize that the break-in isn't just the 'wearing-in' of parts but the complete heat cycling of the piston from stone cold to full temp then back to stone cold. That's one thermal cycle. I like to see 6 to 10 thermal cycles to 'normalize' the piston to the bore. After that the piston is capable of handling full throttle loads.

You still haven't mentioned if you rebuilt the carbs, fuel pump, and water pump. If not, you may not have cured the root cause of the last failure.

This may have been a bad move, but I put it back together with the old rings on the two good cylinders. I did clean the carbon off the pistons and the ring grooves. I was not sure that I really knew what I was doing with the honing so I decided to not take a chance on the other two cylinders. Wish I had honed and re-ringed them now.

I re-built the fuel pump which had bad check valves - I have a picture of them in another thread, and checked the WP impeller (which I replaced 2 years ago and have probably only put about 12 hours on the engine in fresh water since then). I did not rebuild the carbs - I am hoping that the seafoam cleaned out anything that got stuck in them. So, the only thing I know as far as a cause is concerned is the fuel pump.

I just got back from running another two gallons of mixed gas along with the injected oil through the engine - cycling from off/idle up to 2500 rpm with a few excursions to 3000. Still the old fouled plugs. Seems to be running pretty well today - no misses and the tach is not jumping anymore.

I'll run it one more time - partly to get stabilizer in the carbs in a couple of weeks and then fog it and put it up for the winter.

I'll post if it anything bad happens, of course.

Thanks again for all your help.

 

[CharlieB]

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 weakening fuel pump can cause the upper carb to go lean, same with air leaking into the fuel system.

A aged water pump impeller can cause a reduced cooling flow, toasting the upper cyl by failing to keep the block full.

This is why you really need to KNOW what caused the failure, was it lean fuel or lack of cooling, or a combination?

A two year old impeller installed yet never run will still dry, age, and take a 'set' reducing its ability to move water just as much as it it had been used almost daily. Change it in the Spring.

Chemical additives can help extend the time between carb cleanings, but do NOT trust your motor to it, chemicals can never get all passages near as clean as disassembling ans scrubbing/soaking in dedicated cleaner. Spring cleaning extends to more than your house, it should apply to all carbs to ensure they will have a flawless season.