Damaged Flywheel???

[jsmendzi]

Here's the thing, I have a 2004 40hp Johnson motor with about 10hrs on it. I bought it used, took it out to the lake, it ran ok for about 20mins, then it died. Brought it home, took the flywheel off (after replacing the plugs, cleaning the fuel filter, and refitting the carbs etc.), and the key is sheared, so I replaced it, and it ran ok for a little while, the coughed out (like it wasn’t getting enough gas) and died again. <br /><br />However when the original flywheel key sheared, it pretty much ground into a powder, some of which fused to the crankshaft, which I was able to grind off. The flywheel key also dug out a few millimeters of metal from the inside of the flywheel (where they fit into each other) when it sheared. I took it to the mechanic and they said I need to replace the entire flywheel (around $400 new). Apparently this causes the flywheel to rock a very minute amount, which causes the motor to not run properly. He says that it starts, but then dies out a few seconds later because of the flywheel. Anyways my question is: Is the flywheel the problem and do I need to replace it? Thanks to anyone who might have some information.

 

[CATransplant]

Re: Damaged Flywheel???

Sadly, it's likely that the mechanic is right. You probably will have to replace the flywheel.<br /><br />The troubling thing here is that the engine had only 10 hours on it when the key sheared. That should not have happened. There's no way the key should have sheared after 10 hours of operation unless it was incorrectly assembled.<br /><br />You bought it used, you said. I wonder if this problem was a pre-existing one. A good case could be made that such a fault should never have occured on that new an engine.<br /><br />Depending on who you bought it from, and when you may have some rights with the seller.

 

[jsmendzi]

Re: Damaged Flywheel???

Thanks CAT, that baffles me as well, I don't know what could have caused the key to shear in the first place on such a new motor. Unfortunately the seller lives 10hrs away in the US and I am in Canada, so I don't know if there's much I can do there. Is there anyway to tell whether the problem was a pre-existing one? Thanks.

 

[CATransplant]

Re: Damaged Flywheel???

I don't know of any way to tell for sure, but I'd wonder if that wasn't the reason for selling it in the first place.<br /><br />The problem with taper-fit things like this is that it's really the taper that holds the flywheel in position. Anything that messes up the taper prevents the two parts from matching. The key is really just a locator and adds little strenght to the assembly. It's designed to shear as a protective device.<br /><br />I had to file key material off the crankshaft of my old 35 hp unit after the key sheared, too. It worked OK, and the shear was caused by the previous owner not properly torquing the crankshaft nut.<br /><br />One possibility for you is to check that tapered bore on the flywheel. What you're looking for is a raised irregularity on the bore that could cause a poor fit. If you can locate and correct that raised spot, you'd have a fair chance of getting a clean enough fit not to have this slip and shear the key again.<br /><br />For $400, I think I'd give it a try. Get some machinist's bluing and paint the inside of the flywheel taper, then fit them together without the key and rotate the flywheel on the crankshaft. If there's a high spot, the bluing will be removed.<br /><br />How to dress it down? Well, very carefully, I'd think. a fine abrasive drum on a Dremel or other similar tool, applied very carefully. Then back to the bluing and test. Once the high spot is just gone, apply some valve grinding compound to the taper and rotate the flywheel on the shaft again to lap the parts together. You'll see the progress. Repeat until you get the best result you can. Then, reassemble the unit, with the key, torque it properly and give it a try. Since this is a 2-stroke motor, no harm is done if it shears the key again. No valves to hit or anything like that.<br /><br />If it doesn't work, well, then it's a new flywheel. There you go.<br /><br />I'd give it a try. You have nothing to lose.

 

[R.Johnson]

Re: Damaged Flywheel???

We are talking about mating surface's here! Crank, and flywheel hub, and both are a precision fit. What kind of a finish does the crank have, after you took a grinder to it? You may be able to bring these surface's back into truth by lapping, depending on the damage. Start with a medium compound, and finish with fine. Clean away all lapping compound, and check surface fit. Th check the fit, smoke the inside of the flywheel hub with a candle flame which will leave a flat black soot. Carefully place the flywheel on crank, and rotate back, and forth. The soot will rub off, showing the truth of fit. Torgue the flywheel to 105 lbs. Don't use an impact wrench, and use a proper holding device "flywheel wrench". You can either salvage these parts, or you could be looking at a flywheel, and crankshaft. There is more than one way to shear a flywheel key, A high speed impact to the gearcase will do it.

 

[CATransplant]

Re: Damaged Flywheel???

Not the crank, the flywheel. You don't grind the crank. That way madness lies.<br /><br />I removed the spalled key material from my crankshaft with a fine mill file. The key material is much softer than the crank material, so it's easy to avoid cutting the crank material.<br /><br />But we're talking about removing a high spot from the flywheel's taper. That part's replaceable, and may have to be replaced in any case.<br /><br />Once the high spot is identified, a careful hand with a small grinder should be able to remove it, with constant checking. It's hard to take off much of that metal quickly. <br /><br />Yes, it should then be lapped to the crank, as I suggested. Lapping with the high spot remaining might just lead to grooving the crankshaft. With the high spot just removed, lapping should true the mating tapers just fine.<br /><br />I'm not talking about high pressure grinding here. I'm talking about using machinists bluing to identify the high spot, then carefully removing material from the flywheel to dress that down, repeating with frequent mating of the parts.<br /><br />Yes, they're a taper fit. A high spot will definitely prevent proper mating. A slight low spot will not. You'll still have almost 100% contact.<br /><br />This is not a rocket engine, frankly, and lots worse has been done. Since the only part being messed with is the flywheel, which might have to be replaced, anyhow, it's a safe enough operation.<br /><br />I'd sure try it on my engine.

 

[jsmendzi]

Re: Damaged Flywheel???

Thanks guys, the problem with the flywheel is not a high spot, it's that when the key sheared it ground out the inside part of the flywheel (so there is metal missing on the flywheel), and it is not just in one small spot. I'd say about 30-40% of the part where they connect has been partially ground away, so it fits loosely on the crankshaft. Is there any way to build that back up so I don’t need to replace the flywheel?

 

[CATransplant]

Re: Damaged Flywheel???

Nope. It's hoszed, I'm afraid, if it has that much damage. You're stuck with getting a new flywheel, it appears. Minor stuff can sometimes be corrected, but not major damage.<br /><br />You might try the outboard junkyards, like tcoutboard.com. There's a chance they might have a flywheel from a motor that was badly damaged somehow and ended up there. It's sure worth a try. You'd probably save about half the cost. <br /><br />Bummer. The cost of boating always seems to go up.

 

[jsmendzi]

Re: Damaged Flywheel???

Thanks, I'll give them a try...

 

[clanton]

Re: Damaged Flywheel???

Lap the flywheel to crankshaft with valve lapping compound, finish with fine grade, you need at least 85% contact. All surfaces must be clean and dry. The nut must be 105 ft lbs, OMC sealer on nut threads.

 

[rodbolt]

Re: Damaged Flywheel???

I am with the lapping<br /> its cheap and easy<br /> use a coarse and finnish with fine. to check the fit use a #2 pencil, draw 3 lines 120* apart down the crank taper. place the flywheel on the taper without the key. rotate the flywheel 90* and remove it. when all 3 pencil marks are distorted clean everything carefully. clean the tapers with alcohol and install the flywheel nut to the proper torque.<br /> to lap it place the compound on the flywheel taper. set the wheel on the shaft and rock it back and forth 90-180* a few times. lift the wheel rotate it 90* and do it again. dont just lapp in one position. your attempting to make the two surfaces mate . the taper actually holds the flywheel to the crank. the key just indexs it.<br /> if it still wont work try another flywheel. if that wont work your crank may be hosed as well. but the pencil marks will tell the truth the whole truth and nothing but the truth.