1971 9.5hp Johnson crankshaft key keeps breaking

[gmarshall43]

Hello all

I have a 1971 9.5hp Johnson. Some call it a squat motor. I couldn't get it to start. I found that the crankshaft key had broken and the flywheel must have been spinning around the shaft. I thought it would be an easy fix. I got another key and it snapped off after I got the motor running for a few minutes. I tired another and it snapped off as well. Does anyone have any ideas why this is happenning?

Thank,
Gary

 

[CATransplant]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

That's easy. First, next time you have the flywheel off, inspect the crankshaft taper and the internal taper in the flywheel very carefully. If there are any defects, then you need to lap the two together. You can use valve-grinding compound on the parts then put them together and rotate the flywheel to lap the two until you have a good mating surface.

Second, it's absolutely critical to torque the nut on the flywheel to the specifications for that engine. Anything less than the specification will cause your symptom and you'll continue to break keys.

I do not have the torque spec for that particular engine, but someone will come along and provide it shortly.

 

[JB]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

Either your crank taper or the flywheel may be damaged, preventing a good fit, Gary. Try dressing both with some fine emery cloth/wetordry paper.

You also may not be putting enough torque on the flywheel nut. Not sure what spec is for the 9.5, but no less than 25lb.ft.

Hope someone will come along with the exact torque spec.

 

[JB]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

CAT types faster than I do. It is comforting that his diagnosis is pretty much the same as mine, though.

 

[CATransplant]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

CAT types faster than I do. It is comforting that his diagnosis is pretty much the same as mine, though.

Yeah, but you have way more posts under your belt.

 

[ezeke]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

Flywheel torque is 40-45 ft lbs. according to your 71 service manual pg 2-3.

 

[Dhadley]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

Also make sure the bottom of the nut is smooth and doesn't have a burr on it.

 

[Joe Reeves]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

Also, be sure to use only the factory designed flywheel key which is part number 120395 and align it so that it is perfectly vertical on the outside edge, not aligned with the crankshaft taper. DO NOT use the run of the mill keys that are available in various hardware shops.

 

[hayshaker]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

also on the subject of your flywheel key It's Extremely Important to make sure that the single upset mark on the side of the key (the long notch at the bottom of the key) is facing down. Incorrect installation of the key will affect the cam position and will result in retarded ignition timing!

 

[tashasdaddy]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

standard hardware store keys will not hold up. the shaft and flywheel has to be perfectly clean. if you do not torque, as above, it WILL shear again.

 

[F_R]

Re: 1971 9.5hp Johnson crankshaft key keeps breaking

The first time it shears is because of improper installation by the last guy that was in there. But with the first shearing, damage to the flywheel and crankshaft begins. Each following shearing increases the damage. It will get to the point that the motor is junk. Lapping the parts may help on minor defects. but won't help if it is very bad. Not only that, the flywheel flopping back and forth before it shears will wallow out the key slots. If the key is not a snug fit in both slots, (did I mention junk?)

For all you other guys reading this out there, pay attention. The flywheel and shaft must be clean and dry. Then use a torque wrench to tighten the nut to proper spec, which is 40-45 ft/lbs on a 9.5. If you don't have a torque wrench and refuse to get one, you shouldn't be working on it. I challenge anybody to accuratly guess the difference between 40-45 and 30 or 50. Yes, it is that important.