Lapping tapered crankshaft and flywheel surfaces for good no slip bond

While an a long ride to Sacramento recently I had some trouble with my Peugeot 103 stalling and giving me a "hard to start" problem. After checking the plug and checking the air cleaner for flow I discovered that the flywheel had slipped on the crank taper and the timing was way retarded. This flywheel is for the Motobecane La Parte CDI that I had modified to run on the large taper"points style" Peugeot stock crank. I had my flywheel puller and timing tools in my bag and quickly reset my timing to 1.2 BTDC. On the long ride home the bike developed the issue again and I had to once again effect the same repair.
Turns out that a shattered star spring washer in my clutch was causing a unbalanced vibration that helped loosen the flywheel. Still the vibration in the clutch should not have been enough to affect the crank/flywheel taper fit. I decided that the aftermarket flywheel taper and the stock crank needed a more refined dressing to keep them married and not slipping.

This can also be used to bind a flywheel onto a crank without needing a woodruff key, allowing complete control over setting timing, for example if you wanted to swap ignitions between various makes of bikes.

pulling tool

I start the teardown to reveal the crank taper in my normal manner with the same tools I had on the road. Being able to pull your flywheel on the side of the road has saved me the call of shame 5 or 6 times.

CDI uncovered

Uncovered is the affordable Motobecane La Partie CDI unit. I was so happy to learn how to mount this to my Peugeot and do away with my points.

remove CDI

I will remove the CDI from the case and let it dangle from its spare grounding strap so I can get a clear view of the crank taper.

minimal damage

A close up shot reveals minimal score damage, just a couple of small buildups. Had this been a sheared key way, I would be dressing the taper for 20 minutes or so.

Lapping compound

I have always used this brand of lapping compound for this task not because of its fabulous reputation but because I found it under my sink along with some Coppercoat spray gasket stuff. This is more compound than a ped builder will use in a lifetime.

apply dab

Just a little here and there on the taper.

twist and advance

Now simply reinstall the flywheel hand tight and start the twisting process 1/4 twists back and forth to a count of ten. At ten, advance the flywheel through your hand 1/4 turn and give it ten more twists. Keep this up till the large feel of the grit in the compound smooths out to more pasty feel. You can then add more if you feel that the damaged taper is not flat again.

check work

I did this process the first time a few years ago on a Honda crank that had sheared the key. Another MA poster Rebel Moby advised that I should check my progress as to how well the two tapers were conforming by coloring the taper with a sharpie to show high and low areas. In the picture I only made a line with the marker. It is best to just color the whole surface being worked to give you a better overall visual of your progress.

more compound

Another dab of compound and another session to the twists

nicly cut

Wow, my sharpie line is gone in no time and I am confidant that the two tapers are close to perfect in their marriage. Well. close enough for this little issue I had. I have read some research about "lapping" and it is a very complex engineering field.

set timing

All that's left is to use my homemade timing tools and re position my flywheel with the crank bolt tight. Now I have one less thing to worry about when heading out on those long rides. I am certain that the flywheel is dead solid and I plan to do this process to all the bikes I build or modify.

Below are some pictures that I found of the Honda crank that I had mentioned I had sheared the key way and really buttered up the taper. It really took a long time to clear this one up. The pics date to 2011 and I still blast this bike after the repair.

smeared key

Smeared key

Laping crankshaft 002.JPG

Laping crankshaft 004.JPG

Laping crankshaft 006.JPG

Hope this thread helps as I do recommend that when you assemble aftermarket parts onto cranks and such, this process is good assurance that the flywheel will stay locked.

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