Shearing Crank Key in Old 'Rude Twin?

[PS94]

I have a nice old 1968 40HP Evinrude with selectric shift, model 40872D.
I got her with the key sheared, and points installed wrong. Fixed that..
She ran ok in the bucket, then sheared.
I put in a hardened key and she ran great....but I made a mark across the crank to flywheel...she's still running ok, but now I see she has moved again...
I searched the forums, and found "lapping crank to flywheel"...does that just entail some lapping compound and spinning the flywheel about on the taper?
What is the correct torque for the flywheel?
I had an issue with a spun prop....would that be like the old push mowers hitting a stump and shearing a key and that was the root of my evil?

 

[F_R]

Happens so often. People don't install the flywheel correctly and it shears the key. And does damage to the flywheel and crankshaft. So it shears again, and every time it happens, more damage. Enough on that. What you need to do now is inspect the parts and see what you have left. If you are lucky, you may get by with lapping the tapers with valve grinding compound as you suggested. If the keyways are battered, well not so lucky. You should be using the correct oem key, it is not a standard Woodruff key.

At any rate, the tapers must be clean and dry. And you MUST use a torque wrench to tighten the nut to 100-105 foot pounds.

Good luck.

 

[PS94]

100-105? Therein may lay the issue..If I recall, all I could find for torque specs on the lil guy was 80 I believe..You're 100% on that spec?.the tapers looked good when I had it apart, I cleaned them as well..
It hasn't 'spun' per se, but crept; and relying on the key to stop an undertorqued flywheel is asking for trouble.

 

[racerone]

????----105 is the torque.---------The FACTORY key positions the flywheel for proper ignition timing.-----The flywheel is driven by the tapers when all is assembled correctly !!!------Do not use a standard woodruff key from the hardware store.

 

[PS94]

Roger that. Key has been ordered. When I re assembled her I google torque and if I remember it correct, I went to 80. With the correct key and higher torque I should have her licked!! I'll update when key arrives.

 

[F_R]

100-105? Therein may lay the issue..If I recall, all I could find for torque specs on the lil guy was 80 I believe..You're 100% on that spec?.the tapers looked good when I had it apart, I cleaned them as well..
It hasn't 'spun' per se, but crept; and relying on the key to stop an undertorqued flywheel is asking for trouble.

Yes, I am sure, 100-105 ft lbs for the 40 starting about 1960, up through? 80 would have been correct for the older V-4 models.

 

[Tim Frank]

Yet another reason why an OEM manual is worth every penny.

 

[gm280]

While that sounds like a huge amount of torque, you really haven't seated the flywheel until you get to that much torque applied. And that subtle movement of the flywheel can reek havoc on both the crankshaft and the flywheel. And I can tell you if you have to buy and replace those two parts, you might as well throw the engine away...

 

[PS94]

Still waiting on my key to arrive.

 

[PS94]

Key arrived, flywheel and crank tapers lapped in... was pretty rough the first bit, but got smoother and I now have nice grey tapers to seat against each other. Torqued to 105ft-lbs, and runs again....
The extra ~20lb-ft; and lapping netted me another 1/4 turn out of the nut from where it was previous...Hope this fixes the lil gal..I'll have her out this weekend and will report back!

 

[PS94]

Lastly, before I take her out this weekend; What is my correct mixture? I've seen 25:1 and 50:1

 

[Willyclay]

50:1 is okay for 1964 and later JohnnyRudes. Enjoy that old beauty!