Nightmare: seized gear shifter shaft on Yamaha 6hp outboard

(March 2024)


Gradually, the forward/neutral/reverse shift lever on my ten year old Yamaha F6C outboard has been getting stiffer and it felt like the plastic handle was about to break. Losing the ability to change gear out at sea would be pretty unpleasant, so I decided to fix this. Surely a simple task? Perhaps undo a few bits, regrease the shaft, all good? No chance, this is a nighmare job!

The basic plan

Looking at the Yamaha service manual, it became clear that the only way to get at the “shift link lever” was to remove the whole power unit. I spent a few days in denial, rereading the manual to see if there was any way around this - but alas no! The shifter lever moves the shift rod, and the shift rod sits in the middle of the engine exhaust which comes out of the bottom of the power unit. Pretty much all 4-6hp outboards follow the same design of having the shift lever under the power unit. Sadly this means removing the power unit, and requires a new gasket at huge price from Yamaha.

Still, hopefully not too bad? Follow the ten steps to unhook fuel lines etc, Undo about 13 bolts, then lift the power unit off - sounds simple?

Problem 1: seized bolts

There are nine bolts which hold the power unit to the case. Mr Yamaha did not expect you to disassemble his lovely engine, and so he did not protect the threads during assembly. Ten years in a salt-water environment essentially turns the aluminium casing and steel bolt into a bettery, making copious amounts of white powdery aluminium oxide corrosion which clamps the threads tightly together with a force that’s hard to believe until you try to remove the bolts.

I’m no stranger to working with corroded bolts (mostly on motorbikes) so took care when removing the nine bolts - penetrating oil, tapping with hammer, working them back+forth gently. But even with this care, I still managed to shear off two of the bolts.

So now I have two problems to solve: 1) the original gear shift problem, 2) two bolts broken off flush in the power unit.

Problem 2: lifting the power unit out

Once unbolted, the service manual suggests you use a ‘special tool’ screwed into the flywheel to lift the power unit up since “it will be well adhered to the gasket”. I wasn’t sure how heavy the power unit was, nor whether it might be damaged if not lifted vertically, so decided to fabricate my own ‘lifting tool’ out of a square of 6mm plate steel. With hindsight, next time I wouldn’t bother - it’s probably liftable by hand, although the lifting tool does make things easier.

Nonetheless, even with my flywheel-lifting tool and a jury-rigged block-and-tackle acting as a crane, I couldn’t get the power unit to part company with the rest of the outboard. I pulled upwards, I levered in various places, I squirted pentrating oil around the visible gasket line. Nothing helped. I even left the outboard hanging from the power unit overnight (with a second rope ready to catch it) but that didn’t work. In the end, it took nearly 7 days of occasional prodding and levering to finally get it off. What worked was using a car-trim lifter tool, resting on top of the rubberised bolt on the port side just behind where the fuel tank lives, and levering upwards on a bolt on the underside of the power unit. I’ve no idea if that hit the “magic spot”, or if it was just the culmination of days of levering - but that’s what finally got the power unit separated from the casing.

Problem 3: the sheared bolts

The two bolts I’d broken had sheared slightly recessed below the surface of the power unit. They were presumably locked in there tight - the bolt had sheared because the bottom part just wasn’t turning because it was locked in place with white powder corrosion.

There’s only really two ways to deal with these - welding or drilling out. I went for MIG welding, because the power unit is an awkward shape to mount in a dril press, and drilling by hand was almost certainly going to damage the threads.

There’s a big different between the theory and practise of welding broken bolts out. In theory, you deposit weld to build up the bolt until it’s above the surface, then weld a nut to it and use a spanner to loosen the whole thing. In practise, I failed maybe fifteen times on each bolt before I got them out.

What I learned, After all these failed attempts, was:

  1. You need the top of the bolt to be clean and flat. I used a dremel with a burr tool for this.
  2. You need a really solid first weld. You’ll do pulses so you build up a little vertical tower of weld. But that first pulse needs to be long enough to make a solid connection with the bolt. Otherwise, it’ll be the weakest link and shear off.
  3. When welding the nut on, do continuous steady circles to fill up the inside of the bolt. Don’t try to do tacks, just fill it all in one go.
  4. Even though the nut will be red hot then cool, that heat/cool cycle by itself was not enough to loosen the bolt if it’s stuck in with aluminium corrosion.
  5. The extra magic/critical step was a propane blowtorch to heat the area around the bolt. I had to get it really pretty hot, but then the bolt finally would move a tiny bit.
  6. Then it was a case of working it loose-then-tight little by little, stopping several times to reheat things.

It took me perhaps 7 hours to get these bolts out. Now that I know the above steps, it would probably go faster, but there’s a lot of learning. These bolts were REALLY stuck!

Even the clearence holes in the lower casing were clogged with white powder corrosion. I noticed this when putting everything back together - the new bolts just wouldn’t go in some of the holes. I was able to get a 6.5mm drill bit down through the holes which were uncorroded, but couldn’t even push a 6mm drill bit down the clogged one. A quick blast with the drill cleared this out.

After all that, I could get back to original problem. The service manual implies that you “just” remove a circlip and the gear link shaft will slide right out. Ha! No chance!

The gear link shaft was absolutely not ‘sliding out’. It would rotate, eg. to change into forward/reverse, albeit with some resistance. But it would not slide out at all.

The inside end has very limited access, and it’s hard to get any leverage to ‘push’ it out that way (the car trim tool was the best thing I found, but even that did nothing).

I ended up removing the lower ‘swivel’ part of the outboard to get rid of the mounting clamp etc. Then I was able to put the casing into a vice (with soft jaws) and try pulling on the shift link shaft with vice grip. Still no joy.

Then I tried penetrating oil overnight - nope! Then I tried blowtorch to heat the casting and hope it frees up - still nope! I managed to get it out maybe 2mm by wedging a screwdriver then the car trim tool under the detent-plate and wiggling it back and foward but it would not move beyond those 2mm. But I also bent the detent plate a bit, so gave up on this approach.

In the end, I jury-rigged a slide hammer. I bought some 8mm threaded rod to match the M8 thread in the shift lever, and welded an M8 rod-connecting-nut to a piece of 6mm plate to act as a stop. Then I used a piece of heavy pipe I had lying around as the ‘hammer’. With the outboard case clamped in a vice, I heated the area with blow torch again, and then gave some huge whacks with this improv slide hammer. After about 5 hits, the shifter shaft had moved by about 10mm, and after a few more it came all the way out.

I think there’s just corrosion between the alu case and the bushing which gradually compresses the shift shaft. When you change gear, it presumably gouges out a region that allows the shaft to rotate, but there’s no longer enough space for the shaft to slide out.

I used a 12mm drill bit to ‘ream’ out the bushing, and this allowed the shaft to slide in and out freely. Once I reinstalled the o-rings, it was a bit more of a snug fit, but changing into forward/reverse gear was now very easy.

Problem 5: Cleaning the old gasket

A gasket seals the surface between the power unit and the lower case. This ensures the that incoming cooling water stays separate from the outgoing cooling water and exhaust, and nothing leaks out into the upper casing. Gaskets want to have a nice clean flat surface to sit on. Whilst this one isn’t as critical as a head gasket, I did take some effort to clean everything away.

I used some plastic razor blades, and wd40 as a solvent, and scraped away for a while. It’s worth spreading this job over a few sessions, since the solvent soaks in and frees up the remaining gasket.