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.

Motor Mount for the Doka

It is time to fit the motor and transmission into the Doka.  If we had a transmission jack, sliding the assembly under the rear bumper might have been easy.  We have an engine hoist, so through the engine access hatch we will go.

It almost fits.  We just need a few more inches of clearance. 

We used a floor jack to lift the rear bumper a couple of inches and the motor is positioned snugly into its new home. 

The mount on the nose of the transmission will remain stock, but the engine mounts will not work for the electric motor.  We will re-use the mounting holes in the frame of the Doka, but we need to fabricate a new steel structure.


Here we are laying out the miter angles for a few cuts.

And we arrive at this shape, a cradle of sorts, to attach to the bottom of the electric motor.  The goals of the design were to use a pair of Volkswagen motor mounts to minimize drive train noise and shock from transferring to the frame, and to maintain as much ground clearance as possible. 

Next we need a pair of ears to stick out from the frame rails, to support the cradle arms from below.  To make a proper fit with the motor mounts, we need to thin the ends of our 1.5" (38 mm) square tube down to 1 inch (25 mm).  We used a grinder to section out 0.5" (12.7 mm) of material, squeeze it in the vise, and weld the material back together.

Some day I will remember that welded and freshly ground upon material is quite hot.  Here the motor mount arm is enjoying a mild spring day, cooling to a more manageable temperature.

After a bit of grinding, the arm looks quite good, and more importantly, it fits the motor mount isolator.

And here is the competed motor mount assembly, ready to be attached to the vehicle.

Joining the Motor and Transmission, Part 3

We last left off with bolt trouble, while trying to join the Doka transmission and motor.  The difficulty was in determining the correct size of bolts to use.  Bolts are specified by diameter and the thread pitch (and length and material and grade and head type and... the list goes on).  The diameter of our tapped holes for mounting the flywheel looked like they could be either 7/16", M12, or 1/2" (all within 60 thousandths or 1.5 mm of each other).  7/16" had the right thread pitch, but was a rattle fit.  1/2" was just a bit too big.  The thread pitch of the 7/16" bolt was the imperial 20 threads per inch, within 1.5% of the metric thread pitch of 1.25 mm per thread.  There is an M10 bolt with 1.25 mm pitch, but the fine threaded M12 bolt is 1.5 mm.  Nothing seemed to make sense until we found out they make metric bolts in extra fine pitch.  Of course they do.  Our solution was an M12 bolt with 1.25 mm pitch.

Here is the face of the motor fitted with the hub and adapter plate.

After bolting on the flywheel, the clutch disk is centered using a nice alignment tool. Do not even attempt to replace a clutch disk without an alignment tool. The transmission and motor need to line up at the same time that the transmission input shaft lines up with the splines in the center of the clutch disk.  Once the pressure plate is bolted on, the clutch disk will be fixed, and the clutch alignment tool is the best way to ensure that the disk is centered.

Now that the pressure plate is bolted to the flywheel, the alignment tool can be removed - the clutch disk is secured and centered.

After all of that, it really is as simple as bolting two halves together.