Mercury 90-HP: Shearing Flywheel Key

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
Hippo
Posts: 5
Joined: Sun Aug 19, 2018 5:53 pm

Mercury 90-HP: Shearing Flywheel Key

Postby Hippo » Tue Aug 28, 2018 1:22 pm

I replaced a bad stator on my 2001 Mercury 90-HP two-stroke, not realizing that my lower unit had seized up and the gears were frozen. As a result, when I started the engine up, the flywheel key sheared.

I replaced the lower unit with an aftermarket SEI product,and installed a new flywheel key.

Starting up the engine was very hard--lots of smoke and it didn't sound like it usually does. When it eventually got going, it performed really well. I took it a couple of miles and shut off for an hour. When it came time to leave, the engine started immediately, but when I tried to power up, the engine refused to do so. After a couple of times of my giving it throttle, the engine quit entirely. When I tried to restart it, it caught briefly but kept dying.

Then, the flywheel key sheared again and I had to be towed into harbor.

I have replaced the flywheel key again and the engine sounds fine, but I don't know what is causing the key to shear in the first place. Does anyone have any ideas about what is happening? Thanks.

jimh
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Location: Michigan, Lower Peninsula
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Re: Mercury 90-HP: Shearing Flywheel Key

Postby jimh » Tue Aug 28, 2018 1:49 pm

Typically the flywheel on a classic two-stroke-power-cycle engine will contain permanent magnets that are part of the magneto ignition system. The orientation of the flywheel relative to the crankshaft position is critical, as this will affect the engine ignition timing.

Generally any time the flywheel is replaced or moved or altered in any way, you should verify the engine timing, and also verify that the timing marker (that denotes the dead-top center position of the primary cylinder) is properly oriented to point to the 0-degree mark on the flywheel.

If you did not perform this re-calibration of the flywheel timing mark and then check and verify the engine timing, the actual engine timing could be off. Poor engine timing could account for the poor running characteristics you describe.

As for the flywheel having a tendency to break the key and become dislodged, this is probably a mechanical problem. I would GUESS that perhaps the initial breakage of the key may have caused a bit of damage to the flywheel or the crankshaft, resulting in more play in their interlocking arrangement that was there originally. This play--if it exists--may be a factor in the repeated breakage of the flywheel key.

Also, the torque specifications for tightening the flywheel retainer nut usually calls for a lot of pound-feet of torque on the nut. To exert that much force without causing the engine to rotate, a special tool is normally used to hold the flywheel in place while torquing the nut.

When you applied the flywheel retainer nut, did you follow the specific procedures?

Did you have the flywheel tool to hold it in place while tightening the flywheel retainer nut?

Yellowjacket
Posts: 24
Joined: Mon Jul 31, 2017 9:33 am

Re: Mercury 90-HP: Shearing Flywheel Key

Postby Yellowjacket » Tue Aug 28, 2018 9:26 pm

Keyed flywheels use a tapered crank. Any time you shear a key you need to carefully inspect the shaft and flywheel and if there is no damage to the crank or flywheel, the key can be replaced and the timing will be fine.

Tapered cranks fit tightly and, when properly torqued, the taper is what transfers the torque to the flywheel. While the key is there and you can't do without it, the taper is what transmits the torque, so they require a smooth fit on the crank and flywheel to properly seat and if the shaft or flywheel is damaged it won't seat properly and you'll just keep shearing the key. What often happens when the key on a tapered crank is sheared is that the slot can be damaged, or sloppy or the crank or flywheel can get scored and the mating surface on the parts won't seat properly. Either of these things will cause the next key installed to break in short order.

First you need to pull the flywheel and carefully inspect the crank and flywheel and determine if it is damaged. If the key slot is damaged you'll have to have it repaired and that likely means an engine tear down. If the key is loose in the slot on the crank you need to take it to a machine shop and get them to make a key for you that fits tight in the slot and you may need to dress the edges of the slot if they were damaged or flare out from the key being sheared. Look very carefully at the edges of the slot and make sure they are not flared out. Oversize keys are available, but it may take some custom work to get it properly fitted. That's not really a big deal. If the flywheel is damaged you can get a new center mount flange. On Merc's those are removeable, you can get another one or just get another flywheel, used ones are not expensive and there should be plenty around. Be careful if you replace the center flange on some Merc's this sets the timing, so make sure it is oriented the same as the one removed or you won't be timed correctly. If the crank is damaged or shows signs of the flywheel spinning on the shaft you can polish the shaft with crokis cloth and then, again, providing the damage isn't major, you can get some grinding compound, put some on the the shaft (with no key) and lap the flywheel on the shaft until the parts are smooth and the flywheel and crank appear polished. You can get some bluing and put it on both parts and then make sure they are lapped and have good contact.

Finally, as noted above proper torque on the flywheel nut is critical. If it's not properly torqued it'll shear the key every time too.

If you lap the shaft and flywheel and install a new key then torque the flywheel to the rated torque. Then remove the nut and make sure that the flywheel is tight on the shaft and you need a puller to get it off. Make sure it takes a good bit of torque on the puller to pop off the flywheel after it has been seated and torqued. If it come off easily without a good bit of torque on the puller, it's not going to stay tight and you need to go back an lap it again.

dtmackey
Posts: 380
Joined: Thu Sep 28, 2017 9:29 pm

Re: Mercury 90-HP: Shearing Flywheel Key

Postby dtmackey » Tue Aug 28, 2018 9:50 pm

There are performance products for 2 stroke motors called offset flywheel keys that offer up to 3 degrees of flywheel advance from a machined key that advances the flywheel 3 degrees ahead of the intended crank position. I doubt this is what you installed and if installing a stock key it will match the original setup the day the motor was built, unless there other factors involved (crank twist, elongated flywheel slot, etc). You should hit the motor with a timing light, but a flywheel key replacement should have NO impact on timing.

Your motor may benefit from a link and sync to adjust the timing and carbs, but would not be the result of replacing a flywheel key.

D-

Hippo
Posts: 5
Joined: Sun Aug 19, 2018 5:53 pm

Re: Mercury 90-HP: Shearing Flywheel Key

Postby Hippo » Sat Sep 01, 2018 11:45 am

Thanks for all the feedback. If it happens again, I at least know what to look for. I tested the boat (with new flywheel key installed) extensively yesterday for an hour, and it went fine. I'm hoping that the first time the key sheared it was because the flywheel nut was not sufficiently torqued. Thanks again for all the great feedback. Much appreciated.

dtmackey
Posts: 380
Joined: Thu Sep 28, 2017 9:29 pm

Re: Mercury 90-HP: Shearing Flywheel Key

Postby dtmackey » Sat Sep 01, 2018 7:18 pm

Hippo wrote:Thanks for all the feedback. If it happens again, I at least know what to look for. I tested the boat (with new flywheel key installed) extensively yesterday for an hour, and it went fine. I'm hoping that the first time the key sheared it was because the flywheel nut was not sufficiently torqued. Thanks again for all the great feedback. Much appreciated.


A loose flywheel will surely cause a sheared flywheel key.

D-