Merc 20H Flywheel shear pin

[pburgford]

If anyone out there is going to Silver Lk. this weekend and has the bar stock shear pin for the flywheel and crank look me up R5 Hal Kelly round nose vintage with H20 B and B mod. with the quincy pipe. Thanx to all. Glen

[A/B Speedliner]

I believe you are asking about a flywheel key not a shear pin which is used to pin the prop to the prop shaft.

[Smitty]

Did you get your key?

If not, and if those flywheel keys are rare and valuable (??), maybe someone who has one can take measurements and post a dimensioned drawing so you can make one yourself, even by hand if you can do close work. You can buy key stock, probably from Tacoma Screw Products. Generic key stock usually is square, so it would have to be ground down to size, say .001-.002" smaller than the tighter of the keyways in the flywheel and the crank. Keys seem to vary in hardness from mild steel to medium carbon, and I don't know about the 20H key, but likely any steel you can work with would be sufficient.

Before making a key, somebody here should know whether the 20H key was common to a lot of other old models; maybe the antiquers have them. Give the guy a little money, not so much for the key as for the hassle of digging through dusty boxes of old motor parts trying to find you one!


The problem is that a lot of old cranks and flywheels no longer have good matching tapered surfaces, so look at those closely for signs of spinning, hardened areas, etc. If you have good conformity in the tapers, and the flywheel nut is properly torqued down, you shouldn't break even a soft key. Talk to a machinist if you don't like what you see. Also, you don't know how far out of balance the flywheel might be without taking it to a balance shop. When 20H's were raced in BU/BSH, there was a stupid rule against rebalancing flywheels. You can and should ignore that now.

Got a photo of your retro boat? Some of us old guys would like to see that. Two of my neighborhood pals and I built Hal Kelly boats right out of high school in 1964-65.

[dwhitford]

The key should be easy to find because the same crankshaft taper was in use for at least 3 years of Mark 25 production, perhaps also for the Mark 28, 28A and Merc 200 follow-on powerheads.

Lap the flywheel thoroughly to the crankshaft taper with plenty of valve grinding compound. The key is just a locating device, not at all intended to keep the flywheel from spinning on the crankshaft. It is the the good fit of the taper and 60 foot-pounds of torque on the flywheel nut that keeps things together in the proper orientation.

[Smitty]

Lapping was standard practice in the day, and the fairly uniform matte finish it left behind did look to be an improvement. But I did wonder about what appeared (?) to be hard spots, and wondered if the conformity of the tapers was really all that much better after lapping. But I never sheared a key or spun a flywheel, so I guess it must have been okay. I still think checking balance might be a good idea.

[rumleyfips]

A KG4 key measures .222 x .127 x 1.250 .If I remember correctly, they are the same key.

1/4 " keystock can be found at tractor and agricultural dealerships. The power take off shafts transmit a lot of torque. A bit of work with a hacksaw, a file and 220 cloth on plate glass always worked for me .

Dave is right ; we were always told that the key just indexes. The taper fit is critical. I always lapped mine in but never had the courage to run without a key just to see.

[krazy karl]

When I tried running a Merc in Cmod I had a large taper crank in the motor. I had to cut the 30H flywheel to fit the large taper crank. Of course the keyway went away, so I couldn't run a key. Worked just fine but the 30H flywheel didn't weigh very much. The fit between the two is the important part.

[bill van steenwyk]

Would have to think that the fit on the taper is the most important as PRO motors such as the Konig and Yamato (don't know about the current crop of motors) have run without a key for 40 years at 40% more RPM. Perhaps the keyway is just there for location purposes of the flywheel. but don't know for sure about that.

Also when Harry ZAK made the conversion on my Model 80's 30+ years ago to Konig style ignitions, no key was used and it worked fine. That leads me to believe that the taper fit is the most important, unless as previous mentioned location of the flywheel is important for other reasons. I remember when one flywheel slipped a little which changed the timing, I used some valve lapping compound as Smitty mentioned in an earlier post and no more problems.

In the stock Category, it might be mandatory for rule purposes, but not necessary for technical purposes to secure the flywheel if it fits correctly.