Topic: Lapping flywheel to crank taper? – Antique Outboard Motor Club,Inc

This topic has 13 replies, 10 voices, and was last updated 8 years, 3 months ago by billw.

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August 17, 2016 at 11:13 pm #5008
amuller
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In the course of fiddling my West Bend back together, I came upon this recommendation in a Chrysler service manual, with procedural details.

I can see doing this if the surfaces are roughened, or if keys have been shearing, but it seems overkill otherwise.

What say you experts?
August 17, 2016 at 11:31 pm #42271
RICHARD A. WHITE
Participant
Lifetime Member
Well let me tell ya…
YES!, by all means lap that sucker, here is why..
The taper of the crankshaft and the taper of the flywheel are BOTH made to be the same, BUT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

IF they are not within a certain amount of error, when you tighten down the flywheel one has to give and it will be the flywheel..Wanna see proof???

Here is it, the flywheel will split, and it should split right along the key way as that is the weakest point of the taper as the key is cut with what could best be called square corners which from a machining standpoint is the weakest.

Now what happens is they need only be off about maybe 1/2 a degree or less for this to be an issue, lapping the two removes that issue and literally matches the 2 tapers to create and much stronger attachment and ensures you don’t have any issues..

That being said the practice "SHOULD" be performed when using any flywheel that did not originally come from that crank shaft, and ESPECIALLY if you have had either the crank or flywheel hub machined. And to lap the two, you want a minimum of 75% contact on the lapped surfaces to consider it being within specs. 100% match is always best but 75% is considered good enough.

30 + years ago I joined the US Navy, and that specific process was taught to me…. from the picture you see above, you would think I would have remembered… well I did, AFTER I cracked this flywheel…

Good luck and lap that sucker…LOL

Regards

http://www.richardsoutboardtools.com
classicomctools@gmail.com
August 17, 2016 at 11:40 pm #42272
frankr
Participant
US Member
Even more important (critical) is tighten the nut to proper torque spec. Over torque will split the flywheel, under torque will shear the key and damage the keyways and tapers.
August 18, 2016 at 1:46 am #42282
amuller
Participant
OK! In 55 years of tinkering with engines I have never lapped a flywheel–that I can remember. I’ll have a go at it. Even know where some valve grinding compound is. Pray tell me: do any of you all actually DO this? Kinda seems like a confession of sloppy manufacturing practices to see this in a service manual.

Most of the cracked flywheels I’ve seen have been on rotary lawn mowers from the blade hitting something immovable. But greasing the taper and running the nut down with an impact wrench will certainly do it.
August 18, 2016 at 2:08 am #42285
Mumbles
Participant
I do it on almost every motor I build. It gives peace of mind knowing the tapers on the crank and flywheel fit together properly.

Get yourself some Clover Leaf or other fine lapping or valve grinding compound and apply a thin coat onto the crank taper. Lower the flywheel and turn it back and forth. Lift the flywheel and turn it 90 degrees and repeat. Don’t turn it in circles! Keep doing this until the tapers are a dull gray over their length which is an indication of the amount of contact area. Cleaning and reapplying as you go is necessary and a final clean to remove any grit and oil film is mandatory.
August 18, 2016 at 2:13 am #42287
raglover
Participant
I do it religiously on omc big twin 25-40 hp’s. Just did one last weekend. I learned the very hard way. Sheared two keys on a 57 35 while on a vacation. These guys have me the knowledge…..thought I could get away with "winging" the torque on a 1956 30 that was a Frankenstein motor. Sheared the key on first run and took the top of the crank out.

Never skip it on a new old motor now and ALWAYS torque to spec.
August 18, 2016 at 2:48 am #42290
The Boat House
Participant
- This reply was modified 6 years ago by The Boat House.
- This reply was modified 3 years, 8 months ago by The Boat House.
August 18, 2016 at 3:05 am #42293
rudderless
Participant
Well, the bell end of the taper is more elastic, at least on a typical wheel.

I used to wonder why motors did not use the square drive like on bicycle cranks.

I lap everything now. I wished I fit the wheel on my Hurricane 10…

I use the course compound as I feel the increased surface roughness lets the surfaces smash, or fit together better.
August 18, 2016 at 3:47 am #42294
amuller
Participant
Most old motors have had the crank nut beaten on to a certain extent, and this could effect the fit….

This bunch of motors is taking me a long time to get together, but I am learning a lot along the way. And when they are done I want them to be reliable enough to use on the water……
August 18, 2016 at 5:25 am #42296
garry-in-michigan
Participant
Lifetime Member
There was a service bulletin to the effect that the flywheel on the small keyed 40 horse was made with a slight difference in the taper and those models should not be lapped. It explained that the key was only their to locate the flywheel and said flywheel was held in place by the hub gripping the shaft when the proper 105 foot pounds of torque is applied by the flywheel nut. If less torque is used the key will shear requiring the flywheel and crankshaft be replaced. That said, i have lapped a few, but was careful to use more lapping compound on the upper part of the taper.
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