Everyone knows that rotors are sometimes hard to remove if they are rusted onto the hub. So there are two threaded holes, and you can grab the right bolts from somewhere else, and thread them into those holes to pop off the rotor. The correct size is M8 x 1.25.

So on my 1995 VX, there were old rusty rotors from some unknown source, and the right bolt (M8 x 1.25) definitely does not fit those holes. It starts to thread and then jams, like the thread pitch is wrong.

No problem, I used my BFH to remove the stuck rotor. But now I really want to know why that thread pitch is wrong on that old rotor. I took the rotor to Home Depot and tried really hard to find a bolt that would fit. I'm pretty sure I tried all of these:

M8 x 1.0 (fine)
5/16" - 18 tpi (coarse)
5/16" - 24 tpi (fine)

(5/16 is the closest Imperial equivalent to 8 mm. 8mm is 0.314961 inches, and 5/16 is 0.3125 inches.)

I was really surprised that I could find nothing that would fit, even though I think I tried every possible bolt with the correct diameter. The diameter (8mm or 5/16") is correct, but the thread pitch is wrong. WTF?

Not really a problem, since the old rotors are off, and the threaded holes in the new rotors are definitely the normal M8 x 1.25. But the mystery still bothers me, and I wonder if anyone has an answer. Thanks!

(A minor observation I might as well pass along: M8 x 1.25 is found all over the car, with a 12mm head. But when you buy M8 x 1.25 in a hardware store, it will have a 13mm head.)

 

The rust clogs up the threads, if you chase it with M8 x 1.25 it will clean out the rust and the OEM valve cap bolts will fit once again.

 

Thanks for the quick response, and you are 100% correct. I see that with force I can drive a M8 x 1.25 bolt in there.

Now I realize that a key issue is that I'm not using a quality bolt from somewhere else on the car. I'm using a cheap bolt from Home Depot. When I force the bolt in and then back out and then inspect it, it looks like the bolt threads are damaged. When I noticed this earlier, I thought it meant the pitch was wrong. Now I realize it just means this cheap bolt isn't strong enough to act as a chaser and cut through the rust properly.

I'm not surprised to see a lot of rust on those threads, since I'm pretty sure these rotors have been on the car for at least 140,000 miles.

Anyway, I was not going to figure this out on my own, so I appreciate your help! The mystery was bothering me.

 

A bolt, even a quality one won't be a good chaser as it has nowhere for the rust or debris to go. If you cut small grooves perpendicular to the threads on the bolt then it could be a chaser.

I usually just use a tap and clean out the threads that way. It will only cut out the garbage as long as it's started properly.

 

Excellent point about using a proper tap or chaser instead of a plain bolt.

If anyone is curious about this here are some suggestions for what to google:

ARP 912-0001 Thread Cleaning Chaser M8 x 1.25" (expensive)

Bolt Motorcyle Hardware Thread Chasers M8X1.25 (not so expensive)

I can never have enough tools, so now I think I know what to tell my wife to get me for my birthday!