Outboard Motor Flush Kit

Celtic Myst is a 1987 Capri 22, #239

This project started out easy enough and then turned into executing Plan "E" as I failed to follow the old adage "If it ain't broke...".

Plan A -- Follow the Manual.  I have a Nissan Marine 6 hp Sailpro, which is a rebranded Tohatsu 6 hp outboard.  The  Owner's Operating Manual stated:

After operating the motor in sea water or polluted water, or if it will be stored for a long period, wash all enclosures and the water cooling system with fresh water.

• Remove the water plug from the motor, and screw in the flushing attachment.  Connect a hose to the flushing attachment. 
• With the shift lever in Neutral (N), run the engine at a low speed while flushing the cooling system to ensure all sea water and mud are removed.

The "water plug" was nothing more than a bolt and a fiber washer threaded into a hole at the top of the lower unit where it is attached to the engine.  The "Flushing Attachment" was a short piece of hose with a 3/4" water hose connector at one end and a brass fitting at the other end that screwed into the bolt hole.

Flushing Attachment.

Because the Flushing Attachment was connected when I acquired my Capri 22, it wasn't until I read the owner's manual that I realized Nissan/Tohatsu intended the flush kit to be removed after each use and the "water plug" replaced.  I checked my collection of parts that came with the boat, but there was no "water plug" so I ordered one online.

Dissimilar Metals.  The Flushing Attachment uses a brass fitting threaded into the cast-aluminum lower unit.  This is a recipe for galvanic corrosion when two different metals are in contact in a corrosive environment.  I was currently sailing in freshwater which slows the corrosion process, but the boat had spent time on San Francisco Bay with one of its previous owners.  Mindful of the outboard's history, I sprayed the components with penetrating oil in hopes that it might be drawn into any corrosion.  In the meantime, I ground a #M13 metric wrench thinner so it would fit onto the brass fitting threaded into the cast-aluminum.

Because my initial attempts to unscrew the Flushing Attachment failed, I followed up by gently tapping my wrench in one direction, and then doing the same in the opposite direction.  Still, no movement until the brass fitting sheared off.  Suddenly the project was going to get a lot harder.

Plan B -- Restore Flushing Capability.  The outboard was now unuseable until the Flushing Attachment could be sealed off.  I resigned myself to pulling the outboard off the boat and taking it home.  Once home, the plan was to soak the area with penetrating oil and then use an EZ-Out to extract the remnants of the brass fitting from the bolt hole.  The EZ-Out and a left-handed drill bit both failed to dislodge the brass, so it was onto Plan C.

Plan C -- Just Seal Off the Hole.  I was in the middle of race season when I ignored the wisdom of "If it ain't broke...", so while I sought a solution that addressed the issues that prompted the project in the first place, my goal now was to just get the outboard useable.  Plan C involved drilling out the brass and the corrosion so that new threads could be cut into the cast-aluminum and the next size up bolt could be utilized as a water plug.  This part of the project went as expected.  The new threads were in, and the bolt sealed everything off in time for the next race.  While I wanted to restore the ability to attach a flush kit, I was in freshwater, so it was a pretty low priority.

Plan D -- Install a New Flush Kit.  Fast forward three years and Celtic Myst is no longer in freshwater.  Faced with the need to flush the engine after each use, I decided to build a new flush kit based on the M10x1.25 threads I had cut into the cast-aluminum.  The kit would be made up of a brass connector that exited with a barbed hose ID connection of 10mm (3/8"), an 18" length of vinyl reinforced clear tubing, a female garden hose thread withy 3/8" barb, and sealed with a 3/4" host plug when not in use.

Much to my horror, the brass barbed fitting wasn't making a solid connection to the threads in the cast-aluminum.  The reality was that the thickness of the part just didn't support enough threads.  Faced with another failure, I devised a Plan "E" to salvage Plan "D".

Plan E -- Rebuild the Flush Kit Attachment Point in the Cast-Aluminum.  In examining the flush kit attachment point on the top of the lower unit, it was clear that the original design was destined to fail.  The first contributor was the dissimilar metals, followed by the need for users to install and remove the flush kit in order to keep the threads in the cast-aluminum free from corrosion.  The second contributor was the wall thickness of the cast-aluminum lower unit.  Even if users had been rigorous about not leaving the flush kit attached, the threads in the cast-aluminum would have eventually been stripped out due to use because there weren't enough threads.

The irony is that I decided to go with a permanently installed flush kit, which is effectively what I had when I started the project before I sheared off the brass fitting.  This is where the project gets complicated because I needed to epoxy the brass fitting in the cast-aluminum.  This meant removing the lower unit from the engine so that I had access to both sides of the repair.

Engine removed from the lower unit.

Top of the lower unit with the brass fitting epoxied into the cast-aluminum.

Brass barb for flushing.

New gasket to fit between the engine and the lower unit.

Following the service manual, the outboard came apart and went back together as described.  I did have to replace the gasket between the engine and the lower unit.  With the outboard back together, I took my flush kit components from Plan "D" and completed the install.

Flush kit installed.

Nylon tie-wraps secure the garden hose end of the flush kit.

My final solution is pretty similar to what Tohatsu ships today.

Tohatsu flush kit installed.

The Bucket Alternative.  I could have foregone the flush kit and gone with a bucket, but in the end, I felt it was better to restore the flush kit as the primary method and use the bucket as a backup.

Flushing with a bucket.