Alpha 1 lower internal corrosion repair with pics

Ted Russell

Member
Sep 13, 2020
74
Penetanguishene ON Canada
Boat Info
1998 400 DA
Engines
Twin 350 hp 3116 Cats
Hey all,

I pulled apart my drive to put in a new pump and found some corrosion between the exhaust and cooling water chambers. It appears the drive had never been apart (1998 with 950 hrs).

Here’s the damage:

IMG_2771.jpg

UNADJUSTEDNONRAW_thumb_b6.jpg


IMG_2771.jpg

The wall is very thin here, and even in freshwater, I was told the combination of stray currents from shorepower and the stainless water pump causes the aluminum to corrode away. I spoke to a Mercruiser mechanic and he said at the marina they add beads of silicone and slam it back together but I thought that wasn’t really the best way. I also had seen some rebuild the wall with JB weld, but I was afraid a piece could break off and wedge in the impeller, which would consequently overheat the engine. So I opted to replace the metal itself and make the wall thicker.

First step is to take a dremel and remove the damaged material.

UNADJUSTEDNONRAW_thumb_b2.jpg


I bought a piece of 1.5" x 1/8" thick aluminum stock and cut two pieces, one to fill the damaged area and one to go behind it on the exhaust side.

UNADJUSTEDNONRAW_thumb_af.jpg

IMG_2810.jpg


UNADJUSTEDNONRAW_thumb_b0.jpg


UNADJUSTEDNONRAW_thumb_b1.jpg


I drilled and tapped with 4 mm stainless steel screws to attach the two plates together and attach the plate to the lower unit.

UNADJUSTEDNONRAW_thumb_b3.jpg


UNADJUSTEDNONRAW_thumb_ad.jpg


After that, I ground a notch on the exhaust side plate to ensure the relief hole from the pump wasn't partially blocked.

UNADJUSTEDNONRAW_thumb_b5.jpg


UNADJUSTEDNONRAW_thumb_b4.jpg



The rubber block was cut down so it would fit with the thicker plate in place. I used JB Marine weld to coat behind the plates, in the screw holes, and in all the gaps to make sure everything was sealed and would stay firmly in place. After that it was a coat of zinc chromate primer and black epoxy paint.

UNADJUSTEDNONRAW_thumb_ae.jpg


Then, when I installed the pump on the bottom side of the steel gasket I used some silicone to take up any small irregularities as the surface wasn't 100% like the factory machined surface. Other then that I followed the regular procedure for the pump.

All this cost less then $30 in material, and I have enough to do two drives. Much better then replacing a lower!
 
Last edited:
Hey all,

I pulled apart my drive to put in a new pump and found some corrosion between the exhaust and cooling water chambers. It appears the drive had never been apart (1998 with 950 hrs).

Here’s the damage:

IMG_2771.jpg

The wall is very thin here, and even though my boat is freshwater, I was told the combination of stray currents from shorepower and the stainless housing causes the aluminum to corrode away. I spoke to a Mercruiser mechanic and he said at the marina they add beads of silicone and slam it back together but I thought that wasn’t really the best way. Another idea I had seen was to rebuild the wal with JB Weld, but my fear with that was the aluminum could continue to corrode, then a big chunk of JB weld could dislodge and jam the pump and take out an engine. So I opted to replace the metal itself and make the wall thicker.

First step is to take a dremel and remove the damaged material.

IMG_2809.jpg


I bought a piece of 1.5" x 1/8" thick aluminum stock and cut two pieces, one to fill the damaged area and one to go behind it on the exhaust side.

IMG_2810.jpg

IMG_2811.jpg

IMG_2812.jpg


I drilled and tapped with 4 mm stainless steel screws to attach the two plates together and attach the plate to the lower unit.

IMG_2813.jpg


After that, I ground a notch in the longer plate to ensure the relief hole wasn't partially blocked.

IMG_2815.jpg


IMG_2817.jpg


The rubber block was cut down so it would fit with the thicker plate in place. I used JB Marine weld to coat behind the plates, in the screw holes, and in all the gaps to make sure everything was sealed and would stay firmly in place. After that it was a coat of zinc chromate primer and black epoxy paint.

IMG_2825.jpg


Then, when I installed the pump on the bottom side of the steel gasket I used some silicone to take up any small irregularities as the surface wasn't 100% like the factory machined surface. Other then that I followed the regular procedure for the pump.

I have been told that this is a fairly common failure, and thought some may find this handy to make the repair instead of opting for a new lower. Materials for all this was less than $30 and I have enough to do both my drives. We will see what the other drive looks like when I open it up.
Nice work!
 
I can't see the pics for some reason. Alpha 1 on a 290? Was it factory?
 

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