Building Light Flywheels PDF Print E-mail
Written by BMWVMCA NEWS   
Tuesday, 05 August 2008

Building Light Flywheels
By Chris & Bobsie Betjemann

Over the past several years we have built a number of /2 era BMW specials using refurbished Bowman flywheels.  The machines having these flywheels include an R50S café racer, an R50/2 vintage racer, an R60/2 café racer and others as well.  In each case, we were very happy with the result and found that the engines revved more rapidly and transmission shifting was improved.  We are now involved in the construction of two more specials, both post-war plungers, one with an R50S motor and the other with an R60/2 motor.

However, the Bowman flywheels we have, and have recently seen in the marketplace, are increasingly shabby and often require machine work to the flywheel hub taper to make them usable again. 

It is a common misconception that the small woodruff key in the tail of the crankshaft holds the flywheel in position on the crankshaft.  Not so.  Rather that small key is only there to ensure that the flywheel is properly positioned on the crankshaft such that the markings on the flywheel can be used for ignition timing, valve adjustments and checking of piston to valve clearances. The flywheel position on the crankshaft is actually maintained by the torquing of the female flywheel taper onto the male crankshaft taper. 

With the need for more alloy flywheels we decided to fabricate our own version of a light weight flywheel for use on the /3 and /2 BMW twins.  We started with an original Bowman flywheel as a model, but soon left it behind when we found dimensional errors in the original Bowman flywheel hub and alloy portion.  Those errors tended to cancel one another out, but why repeat the error when it was just as easy to eliminate it?  Having made that decision we decided to use a NOS BMW /2 flywheel as our model.

After taking the appropriate measurements, initially from the used Bowman flywheel, and later from the NOS BMW flywheel, we fabricated the alloy portions and the hub portions of our first run of our version of the /2 alloy flywheel. Our 4 pound flywheels contrasted markedly with the 12.25 pound factory flywheel. Once the first of the steel hub portions was out of the CNC lathe we wanted to make one final check of the first of the completed flywheels on a crankshaft before hitting the “go” button on the lathe and producing the balance of the hubs.

We had an NOS R60/2 crankshaft complete with rear slinger, bearing spacer and 6207 bearing in place handy to us so we positioned it vertically in a vise using soft jaws with the rear of the crankshaft up. Once the crankshaft was securely held we presented the first of the flywheels to the crankshaft and placed the flywheel taper onto the crankshaft taper with a twisting motion.  We immediately found something that concerned us.  The gap between the forward surface of the flywheel hub seal boss was closer to the 6207 bearing on the crankshaft than we had anticipated.  At the same time, however, we had never looked at a flywheel and crankshaft in this way before. 

Out came the feeler gauges and we measured the gap to be .028”.  A measurement of the oil baffle and wavy washer that needed to fit into that gap revealed that the thickness of those two parts together to be approximately .038”.  The baffle and the wavy washer could not fit into the gap and allow the taper of the flywheel to properly seat on the taper of the crankshaft.  Furthermore, the .028” gap was without the flywheel being torqued onto the crankshaft.  How had we made this mistake in our fabrication?

In actual fact, the NOS BMW flywheel we used for our model of the hub led us to the gap measurement noted above.  In our shop we typically rebuild ten to fifteen /2 and/or /3 motors per year.

Every so often when we remove a flywheel from one of these motors to be rebuilt we find that the flywheel comes off of the crankshaft “too easily”.  The removal of a /2 or /3 flywheel should require the use of a puller and usually once the puller is exerting significant pressure onto the flywheel it is necessary to give the central puller bolt a significant rap with a hammer to free the flywheel.  But with those motors where the flywheel comes off too easily the flywheel will often essentially fall off the crankshaft once the large central bolt is removed.  And sometimes the flywheel has spun on the crankshaft leaving potentially problematic and expensive damage to the flywheel and the crankshaft tapers.  I always wondered why some of these flywheels came off so easily and our lightweight flywheel project provided the answer to the question.

Once we noted the .028” gap between our flywheel hub and the rear surface of the crankshaft 6207 rear main bearing we first looked to the NOS flywheel model to see how we had erred.  We had not.  The NOS flywheel produced an essentially identical .0285” gap when placed onto the same crankshaft.  We went to the parts room and brought out six additional original flywheels, all used.  These flywheels had come from various parted out motors and swap meets.  After cleaning the tapers of these flywheels of any oil, dirt or surface rust we placed each of them onto the same crankshaft and measured the flywheel boss to rear main bearing gaps.  The result was revealing.  Tabulating the results best demonstrates the findings:

            Reproduction flywheel                 .0280”

            NOS flywheel                             .0285”

            Used flywheel 1                         .0375”

            Used flywheel 2                         .0590”

            Used flywheel 3                         .0890”

            Used flywheel 4                         .0470”

            Used flywheel 5                         .0600”

            Used flywheel 6                         .0350”

Recalling that the thickness of the oil baffle and wavy washer that is to be inserted between the flywheel and the rear main bearing measures .038” it is clear that the two items would not fit into the gap of our initial reproduction hub and three of the seven original flywheels one of which was the NOS one. 

To digress for a moment, it is to be noted that oftentimes the NOS parts that we all periodically find may indeed be castoff parts that were rejected in the 1960’s and replaced with a more satisfactory part with the castoff being placed back on the shelf.  We have seen this with various switches, speedometers and now, we believe, with our NOS flywheel.

But what of the other flywheels?  These were probably flywheels that came off the crankshaft “too easily”.  We now believe that some flywheels are fitted to crankshafts with inadequate gap to contain the oil baffle and the wavy washer and, as a consequence, when the large central bolt that is to hold the flywheel onto the taper is brought to full torque, the flywheel hub is actually being held against the inner race of the crankshaft rear main bearing with the oil baffle and the wavy washer crushed between them.  If the flywheel stays in its assigned position on the crankshaft all is apparently well although flywheel runout may be excessive.  If it slips, however, the timing of the flywheel becomes incorrect and the owner or builder of that motor may be in for a hefty repair bill due to resultant damage to the flywheel and crankshaft.

What to do about this problem?  With our reproduction flywheels it was an easy problem to remedy.  We simply altered the forward surface of the flywheel hub to provide adequate clearance between the flywheel and the crankshaft rear main bearing. 

When we are assembling a motor in our shop we place the crankshaft to be installed vertically in a vise using soft jaws with the rear of the crankshaft up to install the rear slinger, the rear main bearing spacer and the rear main bearing.  Once that was accomplished the crankshaft was ready for installation.  Or so we thought.  We have now added an additional step to the process.  And that is to place the flywheel to be installed in that motor onto the now fully assembled crankshaft and measure the flywheel boss to rear main bearing gap.  If the gap is less than .060” we feel that it is necessary to remove enough material from the crankshaft hub to provide that gap and allow the flywheel taper to properly seat on the crankshaft taper. That .060” is only a guesstimate admittedly and could, perhaps be reduced.  The removal of material can easily be accomplished using a mill or lathe.

We welcome your experience, observations and comments on this interesting issue. 

Chris & Bobsie Betjemann
Barrington Motor Works

Last Updated ( Monday, 08 June 2009 )
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