EL Bonnie - Flywheel Balancing Act

Yeah, OK, I know Bonneville is over, but I am going to go ahead and write this just as I would have had I not run seriously short on time leading up to it.  So with that in mind, here is the next installment.

Balance the flywheels. Simple enough.  In fact if you are familiar with the process you know its normally only about a two hour process. The key word here being normally. Really nothing normal about this motor though. Turning a 1939 engine of any type into a Bonneville racer is not a simple task, but add to the antiquated design, we are also dealing with a small (by Harley standards) displacement which needs to stay that way to remain in the chosen class. In this case, what I am trying to do is make up for displacement with RPMs, and RPMs call for strong and light parts.

Well, we already had some strong parts in the S and S 3-1/2 inch stroke flywheels, and while these flywheels start out considerably lighter than their OEM counterparts at 26-1/2 pounds for the pair of bare wheels, they still needed to be put on a serious diet for this special application.  So... to the lathe!
I lightened the shiny new flywheels - a lot!  In fact, as sometimes happens, I got a little carried away.  Knowing that I was dealing with these tiny little 3-5/16" bore pistons, I "mis-underestimated" how much of the counterweight I could remove.  Oops.  Well, I had planned to make the pistons as light as possible anyway, so....

First thing was to track down a pair of .080" wall tool steel wrist pins.  It seems that I obtained the last set on the planet from Axtell by way of Zippers.  Of course they were for a Evo so I had to shorten them as well as hone the pistons for the .001" larger diameter pins.  Weight savings on the pins alone was nearly 50 grams.  The pistons also went on an extreme diet, with enough material removed from the insides to make any piston manufacturer cringe.  I was careful to try to leave at least .180" thickness in the dome though. 

I went with 55% for the flywheel balance factor, chosen because that is what the factory used for the XR750.  Small motor, short stroke, and high revving - those were the characteristics I hoped EL Bonnie would share with an XR. To get there, not only did the pistons go on a diet, but even such small things as the crankpin nuts were cut down, not to mention adding an internal bevel to the ends of the pin.  Before all was said and done, the finished crankshaft assembly (with rods and shafts, ready to run) weighed but 20 Lbs 5 Oz.   obviously this motor borders on anorexic.

Before final assemble and true, the flywheels received the same oil shedding, heat dispersing coating as the interior of the crankcases.  The underside of the pistons also received the same coating (the idea is to keep the oil moving off the inside of the piston to aid in cooling it.  The domes got a ceramic coating as well as an oil retaining skirt coating. 

Oil pumps on these old motors are becoming a little problematic.  Back in the good old days, it seemed that a guy could always sort through a drawer full of spare oil pump gears to find pairs that would give acceptable protrusion from the body.  As time goes on though, the pickings are getting pretty slim.  The solution actually results in a better pump than what one might come up with if the drawer of spares was still well stocked.  By surface grinding the oil pump gears so that the pairs are of matched thickness, and then surface grinding the pump body, the protrusion of the gears from the body can be "blueprinted" while compensating for the thickness of reproduction gaskets.  Bear in mind that we are talking about doing this on a precision surface grinder, not a belt sander.  EL Bonnie's pump was in medium poor shape, but the surface grinding treatment brought it back - that and some welding and re-machining on the cover.

That pretty well wraps up the story on the bottom end. The heads had been re-worked by Ron Adamson before the motor came to me, so I'll just comment on the rocker arms.  They too received the lightening treatment; or maybe we should call it the "lightning" treatment, since we are dealing with an early Knuck.  Once lightened and spec'd for shaft clearance, all that was left was to resurface the pads and treat them with the same oil shedding coating as the bottom end parts.

After all the trials and tribulations which accompanied prepping the engine parts, final assembly was somewhat anticlimactic - in other words it went smoothly.  And all finished with nearly a week left for the team to install the engine in the bike and dyno it before leaving for Bonneville!