Brake disc removal - need advice...

Hi went to change the front discs of my P38 today, as they are doing a very good impression of Rizzlas, but got stuck pretty quickly.

- Can't get the brake caliper bolts out
- Can't get the posidriv screw holding the disc on out

So, my questions that I hope you may be able to help me with...

Re the caliper bolts - they are the 12-sided ones. I only have a six-sided socket in 19mm and of course this won't fit, but my 12-sided 19mm spanner fits OK but won't shift the bolt, even when hit with a hammer. Should I just be using a normal 12-sided socket on these, or is there a special socket I need to buy? I really don't want to round the bolts off.....

Re the posidriv screw - I have a large posidriv bit that fits OK but it won't shift it, and is just rounding out the screw. How do people normally get these out? If I drill it, will it be a right pain to get the bit that's left in the hub out??

Finally - any suggestions/hot tips for pushing the caliper pistons back in without the L/R special tool...?


Thanks for the advice everybody - didn't expect this to be a long job at all (changed lots of discs in my time...) but I am a bit stuck right now....

Best regards

Jerry

No special tools required apart from perhaps a breaker bar for the bolts. Use a half inch drive std 12 sided socket. The best tool for the screw is an impact driver, a. Good wack and they will be straight out.

If you are forced to drill the screw out don't worry too much as it only locates the disc, the wheel holds it in place so you're not knackered without it. As for winding the calipers back I just use a decent sized G clamp on the old pads.

yep I just done this job last week agree with the socket and I used a pole to extend the leverage, no problem.
re the screw, I had to drill out three from four. Use a nice new one, no problem.

If you realy want to get the stud out you could use a dremmel and cuta slot and try again, but again I agree with previous pint. The disc is held on by the road wheel.
Porsche do it the same way but don't bother with a retaining screw.
G-clamp for wheel cylinder, but I let the fluid out locally by undoing the bleed screw slightly and draining into a jar of brake fluid.
I hope this helps.

best of luck.

Re the screws heat them a little to expand the threads and the undo it works 95% of the time u will need a blow torch of some kind

Re the screws : Impact driver worked for me. Couldn't shift them with anything else but they came out easily with the impact driver - I just bought a cheap one.

Didn't have any trouble with the caliper bolts.

the kind of impact driver that you smack with a lump hammer is the right tool for those screws. If you drill the heads, then you should be able to take the discs off and then use grips to turn the remaining threads which will be standing proud, or weld nuts on, as long as you only drill the heads off.

Matt1978...

Not quite sure if your technique could work...


The heat will expand the Screw relative to the threaded hole it sits in, this making it tighter.

Your theory would work for a nut on a bolt, as the nut would expand due to the heat (as the bolt has more material it would take longer to heat up in relation to the much smaller nut) and as such the nut would become looser on the bolt.

In order to use heat to loosen a screw, you would have to heat up the surround material to expand the hole relative to the screw...the only trouble is the amount of material in the hub/disc would dissipate the heat quicker than the screw and as such the screw would be heated quicker than the parent material and then the screw would again expand within the hole....QED

its a well used tactic heating bolt or screw then letting it cool ,does loosen it, as like hitting repeatedly it releases the stress holding thread tight

Ahhhhh.....

Letting it cool wasn't indicated....my bad.

Every time I first get a car, I always seem to find the disk retaining screws seized on or the screw-heads already rounded.

So far, although I've had to drill the heads off 7 times out of ten, I've never not been able to use a pair of pliers to turn the remainder of the screw out once the disk is off.

The screws are tricky to unscrew when under pressure (ie. holding the disk on), but as long as the threads are not seized by rust (unlikely in my experience due to heat and grease) they will unscrew when all the stress has been released (ie. head drilled off and disk removed).

I prefer to have the screws when I fit disks just so the disk is held in place when the wheels are off. So I use new screws with a thin smear of Coppergrease on the threads and underside of the head.

Never not been able to unscrew one I've fitted...as others have said they are not "structural" so should only be nipped up tight enough to hold the disk in place.

Cheers,

99% of the time impact screw driver works

Thanks all for the advice, much appreciated

I managed to borrow the correct 19mm torx socket and this undid all four bolts with ease. It also fitted my torque wrench so was able to torque back up properly too. Very high torque figure on those bolts!!

I had to drill both screws out but was easy and as others have said, they came out straight away once the disc was off - in fact I could get them out with my fingers! The threads looked like new, so I am a bit surprised they wouldn't come out to start with but there you go... need to get an impact driver next time!

Getting old discs off was a bit of a pain but they came off after a fair bit of hitting and yanking with a crowbar!

Pistons went back in OK just by putting old pads into caliper and prising them apart. The condition of calipers suggests they have either been replaced not so long ago or been very well maintained, no sticking anywhere and four pistons that went back in no problem. Lucky!

So, job now completed - just need to road test it. Ready for the French MOT now - fingers crossed!

Thanks again for your help

Cheers

Jerry

donp38 said:

Porsche do it the same way but don't bother with a retaining screw.
.

Click to expand...

Porsche discs always have a retaining screw if your Porsche doesn't have a retaining screw it's 'cos some one has left it out.