I completed the upgrade to the 33-spline ZR1 hubs and Driveshaft Shop stub axles. I’m not sure how other builders machined the recesses for the socket head cap screws, but the approach that I took was quick and straightforward. I purchased the following counterbore and removeable pilot from McMaster (the pilot slides into the counterbore and is held in place with a set screw):
I wanted the pilot to have a close fit to ensure that the counterbore was concentric and a 15/32” pilot had too much play so I purchased a 1/2” one and used a lathe to obtain a perfect fit. This made indexing the counterbore easy… if the pilot drops in the hole, you're good to go. I used a mill, but I think a drill press with low runout would have worked fine. The picture below shows that the fit is excellent — wall clearance is about ten thousandths. The counterbore does cut into the 1/4” hole which is used for dowels that hold the triangular spacer on the other side of the upright. Since the dowel isn’t long enough to reach the machined section this isn’t an issue.
Standard washers won’t fit in the counterbored hole so I used special washers to keep the screws from galling the upright. The screws are M12 x 1.75 mm by 55 mm long. I hate iron oxide because it rusts and I couldn’t find zinc-plated ones in the correct length, so I cut longer ones down and polished and painted the tip — I think my OCD can live with that LOL.
Left to right; M12 socket head cap screw, standard M12 washer, and socket head cap washer
Most builders chamfered the ID of the upright, but Agile Automotive suggested that I machine the stub axle rather than the upright. I assumed that it would be a nightmare to machine because they were hardened, but Agile pointed out that if they were too hard they’d become brittle. Agile was correct and I didn’t have any issues machining them. This is a quicker and simpler process than machining the upright which would require it to be carefully indexed on a rotary table and cut with a decent sized chamfer end mill. To machine the stub axle you just chuck it up in a lathe and go. I took ~0.040” off of the stub axles which provides about 0.015” clearance between the stub axle and the upright.
All of the stub axles pictured below were sourced from The Driveshaft Shop. From left to right; stock 30 spline, unmodified 33 spline and modified 33-spline. The stock piece is a standard part for a Nissan. The larger cone allows it to clear the upright’s ID, but the taller spline extends past the hub and you must use a doughnut-shaped spacer between the spine and axle nut so that the axle nut binds on the hub and NOT the spline. The stock unit is fabricated by press fitting the spline into the cone and welding a nut on the backside. The 33-spline units are machined from a single piece of billet, do not require a spacer and are rated to 1,800 HP.
I forgot to take a picture before I painted the machined surface so I added a white line to illustrate the surface that was machined. Because the mounting flange is shorter I will need longer axles which will reduce the angle of the CV joints (i.e., place less stress on them).
To accommodate the sensor wire, I drilled the ID of the hub into an unused bolt hole and carefully deburred both sides of the hole. I then de-pinned the connector… well, after spending 15 minutes attempting to de-pin it I spent about 20 minutes trying to carefully cut it apart to maximize the length of wire and pinched my finger. The second connector took a second to snip off LOL. I will encase the wires with something to protect them from the edge of the hole and re-terminate them with Deutsche connectors. In addition to being superior to the OEM connectors, they are easy to de-pin.
Sensor wire routed through hole drilled in the upright’s ID