posted 05-04-2009 12:54 PM ET (US)        

I've read a few other threads here that suggest this isn't an unusual problem, so I thought I'd post my fix in the hope it would be useful for someone else. The whole job was about 4 hours. The procedure should be the same for the Mercury 75 and 90 4-strokes using the Yamaha powerhead - not necessarily the newer all-Mercury "Veradito" engines.

At first I was just going to leave it until it failed, then replace it. But that's sloppy, so I had to do something about it. I was concerned that the casing might merely be sheet metal and would soon admit water into the motor, which of course would ruin it in a hurry.

Here is the removal and fix procedure:

Wiring: Trace the trim/tilt wires (blue, green) up into the powerhead and disconnect the bullet connectors. Pop out the rubber grommet on the underside of the powerhead and thread the wires through. Remove the wire clamp on the right side of the engine mount so the wires are completely free.

Dismounting: Raise the engine and support it on the tilt lock to inspect the upper mount of the trim/tilt rod. The rod ends in a cross-tube, and a large pivot pin (approx 3" long, 3/4" dia) goes through the cross-tube and through holes in the engine mount. A small press-fit pin (approx 1-1/4" long and 3/16" dia) fits through the cross-tube and pivot pin to hold it in place. Use a punch to drive out the press-fit pin. In my case, I had to lower the engine to get access from the front, then raise and re-lock to finish removing the pin. Tap out the pivot pin. Remove the screw that connects the grounding wire to the anode on the bottom of the motor mount so that the grounding wire is free. Use a punch to drive out the lower retaining pin and tap out the lower pivot pin. Remove the trim/tilt unit.

Disassembly: On the top of the trim/tilt motor are the heads of two long hex-head bolts that hold the motor unit onto the pump/valve unit below. Remove the two bolts. DO NOT REMOVE THE 4 ALLEN BOLTS AT THE BASE OF THE MOTOR. Gently remove the cap on the top of the motor. As you remove it, the motor brushes will pop off the shaft and may fall out of their retaining clips, though mine did not. Remove the motor casing - the armature will come with the casing. Carefully but very firmly remove the armature from the casing - the motor magnets will try very hard to resist.

Cleanup: Contrary to my fears, the motor casing is not thin sheet metal. It's about 1/8" thick, with two very strong permanent magnets attached to the inside. Apparently it's made of a rather substandard grade of steel, and it was extensively rusted and pitted. Cover all of the other parts so they don't get full of rust, then sand the motor casing with coarse sandpaper to remove the loose paint and rust. Use a Dremel with a sanding band or flapwheel to remove the remaining paint and most of the surface rust. Finish with the Dremel and a wire brush to remove most of the rust from the surface pits. Be sure to carefully polish the ends of the casing so it will form a watertight seal with the gaskets in the base and cap. I suppose you could sandblast or acid-bath to get things even cleaner, but this was good enough for me. Clean the motor casing thoroughly, and carefully remove all bits of rust and metal dust from the magnets on the inside of the casing (no easy task). Apply several coats of Rustoleum spray paint to protect the surface.

Reassembly: Lightly oil the armature shaft on each end, but not so much that the commutator gets oily. Clean the gasket on the motor cap. Gently pull back the motor brushes into their brackets and slide the cap onto the commutator end of the armature. This can be a little tricky, but the brushes need to sit evenly on the commutator. Carefully insert the armature back into the casing. Be sure that the armature windings are on the end of the casing with the magnets - the commutator should be on the opposite end of the casing from the magnets. Clean the gasket on the pump/valve base and place the motor onto the base, twisting gently to make the motor shaft engage the pump. Turn the motor so that the gap between the magnets lines up with the mounting bolts - you can't see the gaps anymore, but it'll become obvious when you re-insert the long bolts. Insert and tighten the motor bolts, making sure that the rubber gaskets are on the bolts. There's probably a torque spec for this, but I don't know it - you want it tight enough to seal the gaskets on each end, but not so tight as to distort the motor cap.

Remounting: Clean and/or polish the pivot pins and retaining pins. Grease the lower pivot pin, align the trim/tilt unit, and insert the pivot pin. Tap in the retaining pin from the bottom upward. Reconnect the trim/tilt wiring, running through the hole on the starboard side of the motor mount, re-installing the wire hold-down bracket, re-installing the powerhead grommet, and reconnecting the bullet connectors. Grease the upper pivot pin, align the trim/tilt shaft, and insert the upper pivot pin and retaining pin. You may need to adjust the trim up/down to get everything aligned. Reinstall the grounding wire on the anode.

Job is complete! It might not last forever, but it should last a couple of years before the rust returns and the job needs to be done again.