Machining a Hub for the Sprite Motor

With the recent transition to daylight savings time and the longer spring days ahead, I can't say that I will miss working under the tent in the night and rain.  Warmer weather is around the corner.

We focus some of our attention on the Austin Healy Sprite.  We need a way to mount the flywheel to the Remy HVH 250 motor with a 25 tooth , 25 mm (1") diameter splined shaft.

We have a  coupler that has the correct splines to mate with the motor, but it is not big enough to mount to the flywheel.  A big chuck of 1018 steel will be turned to go between the coupler and flywheel.

We begin with a big cut.  This big band saw was perfect for the job.  Don't try this with a hack saw:

35 minutes later, the blank was ready to be mounted in the 4-jaw chuck on my lathe.

After a facing cut, blue Dykem really helps the scribed layout lines to stand out, especially when the work is spinning at 500 rpm.

Several hours later, removing 0.015" (380 um) of material per pass with the lathe, producing a big pile of  metal chips, the part is complete.

To join the coupler and hub, we settled on using an interference fit.  An interference fit involves making the bore smaller than the cylinder.  The resulting friction yields an extremely strong connection.


To get the bore diameter correct on the hub, I made a plug out of aluminum that is 0.004" (100 um) smaller than the spline coupler.  The bore in the hub is carefully opened up until the aluminum plug makes a slip fit in the bore.  The resulting interference between the coupler and the bore should be about 0.003" (75 um).

Absent a ginormous hydraulic press, the hub was heated up to take advantage of thermal expansion to increase the diameter of the bore.  This is Paul's oven.  We were going to use my oven, but it caught on fire earlier in the evening while my wife was cooking dinner.

The spline coupler was placed in a bowl of dry ice to shrink the diameter.

After 45 minutes of soak time, we joined the two parts.


In an ideal case, the coupler would have just dropped easily into the hub, and parts would bond as the temperature difference closed.  In reality, the interference was a bit too much - aiming for 0.002" (50 um) would have been better than our 0.003" (75 um).  The hub dropped into the bore about 1/3 of the way, and stopped.  Brief panic ensued, but we did plan for this possibility.  Paul grabbed the emergency hammer and fully seated the coupler into the bore.

The flywheel is now mounted to the motor.  Success.