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Importance of Sixth Hole and the Screw in the Rotors

XJ40 ( XJ81 ) 1986 - 1994

Importance of Sixth Hole and the Screw in the Rotors

 
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Old 04-16-2019, 10:14 AM
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Default Importance of Sixth Hole and the Screw in the Rotors

How important is the sixth hole in each of the rotors that is countersunk and allows for a Phillips head screw to be inserted then tightened into the threaded "mate" hole on the hub? I am about to undertake the replacement of my rotors and am very concerned about my success with untightening the Phillips head bolt. I've unsuccessfully attempted to remove a few at the junk yard. I have been spraying the screws with PB Blaster in preparation for my eventual attempt to remove. Should I also apply heat? If I strip the head of the screw and have to resort to drilling them out in order to remove the rotors, can I install the rotors without using the screw? Every car I've ever owned has had the rotor held fast by the lug nuts.
 
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Old 04-16-2019, 11:35 PM
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Originally Posted by retroren View Post
How important is the sixth hole in each of the rotors that is countersunk and allows for a Phillips head screw to be inserted then tightened into the threaded "mate" hole on the hub? I am about to undertake the replacement of my rotors and am very concerned about my success with untightening the Phillips head bolt. I've unsuccessfully attempted to remove a few at the junk yard. I have been spraying the screws with PB Blaster in preparation for my eventual attempt to remove. Should I also apply heat? If I strip the head of the screw and have to resort to drilling them out in order to remove the rotors, can I install the rotors without using the screw? Every car I've ever owned has had the rotor held fast by the lug nuts.
Hi Ren,

Those screws simply hold the rotors in place in the factory until the calipers are installed somewhere farther down the assembly line, so if you have to drill one out, no problem. I generally have the best success getting them out by using a hammer-struck impact screwdriver with the largest bit that will fit the screw. I've had several of these tools over the years but my current favorite looks like this:



https://www.ebay.com/itm/Annex-ANEX-...4AAOSwfwFcZn-G

You can leave the screws out when you reinstall the rotors, though they do serve their intended purpose of holding the rotors in place while you install the calipers.

Cheers,

Don
 

Last edited by Don B; 04-16-2019 at 11:39 PM.
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Old 04-17-2019, 09:42 AM
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Thanks for the explanation Don. Unfortunately I don't own that tool so I will wing it with what I have. I'll use a lug nut or two to mimic what the tapered screw does.
 
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Old 04-17-2019, 01:42 PM
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It's been a while since I had an XJ40 so my memory may be inaccurate.......

1. we must be talking rears because the front discs don't have the CS fixing screw
2. the fixing screw is used to ensure correct orientation of the disc

The disc will fit on any five studs because they are symetrical BUT only the correct orientation will leave the knurled handrake shoe adjuster accessible through the hole in the disc. Fitting the CS screw ensures this. Not essential to actually fit it (checking is sufficient) but it does make assembly idiot proof.

Graham
 
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Old 04-17-2019, 03:17 PM
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I don't recall those windows on either of our XJ40s - I wonder if the aftermarket rotors did away with them?
 
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Old 04-17-2019, 04:13 PM
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Good point Don - It's been so long since I had the back wheels off I can't remember if those access holes are there or not

Don't think I have aftermarket rotors though ..

Larry
 
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Old 04-17-2019, 05:50 PM
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This was my 1989 XJ40 with OE rear brake disc showing the CS screw fitted and the access hole for handbrake adjustment at the top.




With the disc removed you can see the semi-circular cutout in the hub for adjuster access (arrowed in blue):




The disc will fit any way round on the 5 studs but if the adjuster hole isn't aligned with the cutout, it's impossible to get to the knurled adjuster.

Graham
 

Last edited by GGG; 04-17-2019 at 05:53 PM.
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Old 04-17-2019, 09:10 PM
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Graham: Thanks for the explanation and supporting photos.
 
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Old 04-17-2019, 09:12 PM
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Don: I'm having difficulty understanding the usage of this tool, specifically the hammer set element?
 
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Old 04-17-2019, 09:22 PM
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Originally Posted by retroren View Post
Don: I'm having difficulty understanding the usage of this tool, specifically the hammer set element?
Hi Ren,

An impact screwdriver rotates the bit slightly when the rear end of the tool is struck with a hammer. The tool can be set to rotate clockwise or counterclockwise. To remove brake rotor screws, I first apply penetrating oil to the screw and allow to soak as you have already done.

I set the impact screwdriver to rotate counterclockwise, install the largest bit that will fit in the head of the screw, insert the bit into the tool, place the bit in the screw, and while firmly pressing the tool/bit into the screw, strike the rear end of the tool with a good hammer. A steel-head hammer with good mass works better than a dead blow hammer, because you want a good striking impact.

The combined force of the bit pushing and twisting the screw will usually release the grip of corrosion on the threads and the head's countersink bevel so the screw can be twisted out. It sometimes helps to twist the impact screwdriver counterclockwise with your wrist while pressing the tool into the screw and striking the tool with the hammer.

Cheers,

Don
 

Last edited by Don B; 04-18-2019 at 04:02 PM.
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Old 04-18-2019, 02:53 AM
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Here's a video explanation:


Graham
 
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Old 04-18-2019, 10:08 AM
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Graham & Don: Thank you so much. I'm off to go buy one. Ren
 
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Old 04-19-2019, 07:14 PM
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Originally Posted by Don B View Post
Hi Ren,

Those screws simply hold the rotors in place in the factory until the calipers are installed somewhere farther down the assembly line, so if you have to drill one out, no problem. I generally have the best success getting them out by using a hammer-struck impact screwdriver with the largest bit that will fit the screw. I've had several of these tools over the years but my current favorite looks like this:



https://www.ebay.com/itm/Annex-ANEX-...4AAOSwfwFcZn-G

You can leave the screws out when you reinstall the rotors, though they do serve their intended purpose of holding the rotors in place while you install the calipers.

Cheers,

Don
Completely agree, this is my experience as well.
 
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Old 04-19-2019, 10:14 PM
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Thanks Roger95. I now own a screw driver that I get to whack with a hammer. I'm supposed to receive the rotors and pads this coming Tuesday. I'll have the calipers and brackets painted by tomorrow. I had tires mounted on my 5 spoke Cragar look-a-likes. So with any luck, I'll have the old Jag fitted with new shoes next week. Pictures will follow. Come May 4th, I will fly to Albany, NY to drive back to Detroit my new '92 Oldsmobile Toronado Trofeo. Anybody remember those?
 
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