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  40 HP Mercury OB and Stator replacement

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Author Topic:   40 HP Mercury OB and Stator replacement
Wayneman posted 07-16-2002 09:47 AM ET (US)   Profile for Wayneman   Send Email to Wayneman  
Hello there. I am the victim of a disreputable repair facility who has kept my boat for 2 months and not done the repair. I need to replace the stator which I understand is located under the flywheel. My brother and I have the wheel-puller and all needed tools. Could you tell me the degree of difficulty involved? Thank you for any tips!
dangc78 posted 07-16-2002 10:07 AM ET (US)     Profile for dangc78  Send Email to dangc78     
it is relatively a simple job, but you will need to have access to a flywheel puller to remove the flywheel. after that is removed, a few simple screws and wire connectors connect the stator to the engine. If you haven't already bought a new stator plate which can be very expensive, check arround to see if anyone in your area rebuilds them. I know of a few places i have dealt with in MA.
Bigshot posted 07-16-2002 11:17 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Make sure you mark EXACTLY how the old one was set up....including where the harness fits, etc. Easy job once flywheel is removed. If you have a digital camera....take pics so you know what to do.
Wayneman posted 07-16-2002 11:23 AM ET (US)     Profile for Wayneman  Send Email to Wayneman     
Thank you all but somone mentioned the "Stator-plate." This sounds scary. How do I know that it is shot or not?
jimh posted 07-16-2002 10:22 PM ET (US)     Profile for jimh  Send Email to jimh     
I don't know exactly what is under the hood of your Merc, but I do know in general what you'll find.

The first problem in this repair will be to get the flywheel nut loose. The flywheel circumference is cut with gear teeth for the starter. You have to retain the flywheel, usually done with a specially designed tool that fits some holes or slots in the flywheel, then break loose the flywheel nut. The nut will have been torqued to a very high setting, so it will be a bear to loosen. An impact wrench is probably manditory.

With the nut loose you have to pull the flywheel off. Be careful because you will be working against the crankshaft bearings. Don't whack the shaft to loosen the flywheel or you might damage the bearings.

Once you get the flywheel off you will find the underside of it has magnets. These should all be firmly secured in place. The flywheel has probably a keyway or some other means of indexing its position on the top of the drive shaft.

Fitted into the top of the engine will be several coil assemblies. The coils generate electrical pulses when the magnets rotate above them. There are typically three sets of coils:
--ignition primary current
--ignition timing pulse
--battery charging current

Each has a different function.

The IGNITION PRIMARY CURRENT coil supplies current to the low voltage part of the spark voltage generating unit. Most often this is another coil and a transistorized device. This voltage creates the spark.

The IGNITION TIMING PULSE coil is very important. It generates the pulse which ultimately triggers the spark plug firing. Its physical relationship to the the magnets on the flywheel is crucial, but not absolute. The ignition timing is adjustable, but on your engine it has already been adjusted with respect to the position of this coil. If this coil is moved in any way, you will probably have to re-adjust the ignition timing on the engine.

The BATTERY CHARGING (or LIGHTING) CURRENT coil supplies current for charging the battery and to run other engine accessories, like the trim pump motors, etc. This coil has the largest current output.

On some engines all three coils are assembled into a stator assemby and you must replace the whole assembly if any one of them is bad. Other engines have the three coils as separate coils, and each can be replaced individually.

The bad thing with 3-in-1 assemblies is the cost (usually over $200) and the need to retime the engine (because of the new timing coil).

Since these coils are located at the top of the engine and are contained under the flywheel they are exposed to a great deal of heat. In addition, the generation of current, expecially in the charging coil, also produced heat. Thus it is common for these coils to get hot and suffer damage from heat.

If the insulation melts away and a coil shorts out a turn or two, the shorted turn causes electrical losses which also generate heat, leading to more damage.

As mentioned, the lead dress when installing these parts is crucial. The flywheel will be rotating above them at 6,000 RPM and any stray wire will be ruined instantly.

And it goes without saying that you must reconnect all these coils to their proper circuits--I guess on continuousWave we take it for granted that when you take something apart you make notes so you can put it back together.

After you get everything back together you'll have to tighten the flywheel nut to the specified torque. This usually specifies having the threads clean and dry--no lubricants--and setting the torque to over 100-foot/pounds. See you manual for the numbers and any notes. Again, don't damage the crankshaft bearings while you are pulling in that torque, and don't damage the flywheel, either.

Also, the flywheel was probably balanced, so be sure you get it all back together just like it was.

Now, if you had the 3-in-1 stator you'll have to retime the engine. You'll need a timing light and a good tune up procedure to follow. See manual.

If you had the three separate coils and you did not change either the primary current or the timing coil, your engine may not need to be re-tuned, but it might pay to check it anyway.

Wayneman posted 07-17-2002 08:39 PM ET (US)     Profile for Wayneman  Send Email to Wayneman     
Thank you Jim for the splendid and detailed account concerning my stator replacement. I think I will be in luck in terms of the 3in 1 stator and coil assembly. Wish me luck.
jimh posted 07-17-2002 09:09 PM ET (US)     Profile for jimh  Send Email to jimh     
Good luck--last time I tried to get the flywheel nut loose I couldn't! I ended up having my mechanic to do it for me.

One alternative, if you don't own an impact wrench, is to drive the boat (on its trailer) over to a local gas station with a mechanic. Have him use his air-driven impact wrench to break the nut loose for you. I wish I had thought of that before I paid the mechanic at the boat yard to do it for me!


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