How to fit 108 PCD rims to a 98 PCD hub - Alfa Romeo Bulletin Board & Forums
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post #1 of 3 (permalink) Old 09-14-2018, 09:18 AM Thread Starter
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How to fit 108 PCD rims to a 98 PCD hub

Last week I posted a question as to whether it is possible to re-drill 98PCD hubs ex 116 series cars to fit 108PCD GTA rims for a project I'm working on.

No positive feedback was forthcoming, so decided to give it a shot as I am trying to minimise the extent of the mods required by utilising the standard parts over hunting for suitable alternatives.

The De Dion rear hubs are dead easy as there are no disks outboard, so a simple re-drill is all that's needed. The fronts are a little more difficult as there is little room inside the rear of the disk to refit the wheel studs, but as can be seen, a satisfactory fit is possible (if only just)!

Using an old Alfetta brake disk and hub as a trial case, I first turned up a drill jig. The large centre hole has a snug fit onto the wheel boss on the hub to ensure perfect concentricity and the smaller hole is to centre the jig onto the rotary table axial shaft.

I drilled 4 x 12mm holes @ 49mm from centre to fit over the existing 98mm PCD studs (two holes opposing would've been fine). I then drilled 8 x 10mm holes @ 54mm from centre (108 PCD); four offset at 45 degrees and four offset at 22,5 degrees, as per the picture. The 45 degree hole placement is centered between the existing studs and the 22,5 offset to avoid the 2 countersunk retaining screws as seen in the pics. The rear hubs will be drilled using the 45 deg. offset.

This completed, I then bolted the jig to the hub/disk using the existing studs and placed it in the mill. Using 10mm ground bar in a collet chuck, I then aligned the bar into a hole to be drilled, clamped the disk to the bed and drilled a 10mm hole through the hub flange and disk assembly. Exchanging the 10mm drill for a 12 mm slot drill, I then finished off the hole. Repeating this 3 more times, the hub was complete with 4 x 108 PCD holes. The holes on the hub (not the disk), were then increased to 12,5mm in the drill press to allow clearance for fitting as per original.

Lastly, I machined out the small step on the backside of the disk to create a flush seat for the studs. The studs needed to have their flats on their heads cut in closer, and also slightly rounded to fit against the inner wall of the disk, but as this didn't alter the existing contact patch on the bolt head, due to them originally being angle-cut to the shank, had no concerns doing this.

To ensure the wheels run concentrically true, I will machine up acetyl rings which will centre the mags onto the hubs. Voila!

Now I can order my 15"GTA rims with delight and some confidence!

Manny
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post #2 of 3 (permalink) Old 09-17-2018, 03:35 PM
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This is a clever piece of work. You should keep this as a template for others. Maybe rent it to them, or offer to drill theirs for a fee.

I had a shop drill my wheels to fit the 98PCD instead, but my wheels were not as precious as yours. I can see why you would not want to change the wheels.

A couple of questions - why do you need to drill holes at 22.5 degree offset? I know there are set screws there, but aren't they flush? Does your gig not sit on top of these without interference? However, it looks like you drilled on the 22.5 degree offset. Did you mean to do that?

Also - I haven't inspected the hubs on my Alfa, but I changed the discs on my wife's Japanese car and the discs just slid off the studs. Are the hubs in the Alfa in front of the discs? If so, are the studs driven into the discs instead of the hub? I can't seem to see from the pictures what is the disc and what is the hub.

Thanks,
Stefano

Stefano
Concord, CA
iachella [at] gmail [dot] com
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post #3 of 3 (permalink) Old 09-18-2018, 05:25 AM Thread Starter
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Quote:
Originally Posted by iachella View Post
This is a clever piece of work. You should keep this as a template for others. Maybe rent it to them, or offer to drill theirs for a fee.



A couple of questions - why do you need to drill holes at 22.5 degree offset? I know there are set screws there, but aren't they flush? Does your gig not sit on top of these without interference? However, it looks like you drilled on the 22.5 degree offset. Did you mean to do that?

Also - I haven't inspected the hubs on my Alfa, but I changed the discs on my wife's Japanese car and the discs just slid off the studs. Are the hubs in the Alfa in front of the discs? If so, are the studs driven into the discs instead of the hub? I can't seem to see from the pictures what is the disc and what is the hub.
Thanks, Stefano

The set screw holes would be in the way of the drill, as they too are on 98PCD, and I had to drill a mere 5mm off their centres.
Would have preferred drilling at 45 degrees for aesthetics, but chose 22.5 degrees (could have been 30 or 35deg.) to avoid drilling into the set screw holes, so as to keep the drill from possibly wandering off-centre when drilling partially into set screw thread. Saving the set screws was never my concern.

The disk is mounted at rear of hub and centred on a boss. The studs are pressed into the 12mm holes on the disk and have a loose fit through the hub. Set screws keep assembly together when wheel is removed, but this is not in any way essential.
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