M117 Crank Pully

Back in late January, I went to take my 560SEC out for a drive.  I immediately noticed that I had a very odd feeling with the power steering – it was cutting in and out.   I opened the bonnet to check the belts and fluid level and saw something rather alarming.  The engine was running, but the accessories were hardly moving.  They would stop entirely at times, and then at other times move a little.   None of the belts were damaged.  It was clear that the crank pully and harmonic balancer was not turning properly.

I obviously couldn’t drive the car in this condition.  Not only would it be dangerous with the odd power steering, but without a water pump I would cook the engine in short order.  I put the car back in the garage and then booked it in with my mechanic for March, as I was about to head overseas for work.

On the day the car was due in, I got up very early to call road service to get a tow.  I figured the wait would be shorter if I called before 6AM.   Turned out it would only be about 90 minutes, which wasn’t too bad.  And since the car could be driven short distances, I would be able to get it out onto the road for them.

When they were about 20 minutes out, I went to start the car and move it out onto the road.  I should have put the car on the trickle charger.  Having not used it properly since December, and then starting it in January without a working alternator had the battery flat.  Luckily, the 240D came to the rescue.   The 55 amp alternator on the 240D doesn’t sound all that big, but with everything turned off, the 240D doesn’t consume any power to run, so it soon had enough charging current through the jump leads to start up the 560SEC and get it out onto the road.

The tow up to the workshop went without incident and I followed in the 240D.   Being on a tow truck is probably the only time a ECE 560 is slower than a 240!

A couple of days later, I had a call with the diagnosis of the problem.  The M117 crank pully assembly bolts onto the end of the crankshaft.   It is made up of three pieces.   There is a hub (17) that goes over the end of the crankshaft (5) and is held in place with a woodruff key (11).  The harmonic balancer (26) and the actual pully (29) is bolted onto this hub.  Then the whole thing is torqued down to a high torque value.

For some reason, my pully was not on very tight.  I can only assume that in the past, somebody has changed the front crank seal and not torqued it up properly.  It’s never been off in the six years I’ve owned the car.

Not being tight enough had put too much pressure on the woodruff key, and it had been destroyed.  A combination of being beaten up and then melted.   The hub had cracked and there was wear to both the inside of the hub and the end of the crankshaft.

The ‘correct’ repair would be to replace the crankshaft, woodruff key and hub.  Neither my mechanic or I were very keen on this repair.

I sourced a good used hub, and my mechanic further disassembled the area to have room to work.   This involved removing the radiator, condenser, a/c compressor etc.   He then called in the guy who does his thread repairs to help.

The solution they came up with is something that is done regularly on American V8s when they want to add a supercharger.  In this application, the supercharger puts too much pressure on the crank and woodruff key, so they drill in and add a pin that holds it all in place.   This is a far stronger solution than the woodruff key.  The downside is the pin must be drilled out to replace the front crank seal.   We put a new seal in as part of the assembly.  He also had to make up a bracket to drill straight in.

After that everything had to be put back in place and the A/C gassed up.  Its great to have the car back.  It has always been a great car to drive.  The extra power of the ECE engine makes quite a difference.