Flywheel Balancing and Tapered Bore Repair  

By Paul Greenberg   

Having recently spent what turned out to represent an embarrassingly long time refurbishing the flywheel on my prewar R51, I offer a few comments.  In particular, I will address the re-machining of the tapered center hole.    

Despite several attempts, I had very poor success in adding metal to the interior of the tapered hole. This occurred, of course, after the damage that had been inflicted on this hole had first been ground out.  This renders the hole oversize, which leads to the need to build the surface back up (e.g. adding new metal).  While anyone can try their hand at this, I will note that attempts to add new metal were performed by colleagues here at our NASA Center.  These people are very smart, very experienced, and have very fancy equipment, yet were unable to keep the added material  from peeling when it was reground.  They mentioned that the metallurgy of certain materials can be problematic.  So, this technique probably could be made to work, but we encountered difficulties.  If anyone has better success, I would appreciate learning more about whatever technique was employed.

I ended up machining a tapered insert (see attached photos).  









It required  some patience, but was otherwise straightforward.  The wall thickness was 0.030" (apologies for my use of English units), and it has a cutout to  accept the woodruff key.  Also visible in the first photo is the Chicago  Speedy Sleeve installed to cover the Archimedes oil spiral cut in to the original flywheel. This needed to be covered in converting over to a new, more modern lip seal.

It should also be noted that the tolerance required to establish a good  locking taper is very strict. Both grinding the center hole and machining  this insert necessitates matching the taper of the crankshaft extremely  accurately. This was first done with an indicator, then refined by iteratively machining a tapered blank, and checking it against the crankshaft with Prussian blue.  Extremely time consuming, but important. The final assembly locks up beautifully (as it should).

Since the tolerance for the axial location of the flywheel is also very  tight, the final diameter of the re-machined center hole must be accurate to the order of approximately 0.001".  Not impossible, but it requires patience.    

While I had the compound rest of the lathe set to match the taper of the crankshaft (roughly six degrees half angle, for reference), I machined a matching tapered mandrel. This allowed me to then have the crankshaft and clutch assembly balanced.    

Lots of time was consumed in the process, but in the end, it did work.  

Paul S. Greenberg
NASA-Glenn Research Center
MS110-3
Cleveland, OH 44135
(216) 433-3621
(216) 322-3793 FAX

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