Paddcomp

One of the front caliper hex bolts is completely rounded.

I've tried using an impact driver but it's too far gone. There doesn't seem to be much room around there for an easy out or vice grip socket either.

I'm sure someone else will have been on this position before, so any suggestions appreciated.

dr bob

Drill the head off. Then the caliper should lift off the shaft of the bolt that remains. Some penetrant where the bolt threads into the knuckle might help some. Then Vise-Grips, and --maybe-- some heat on the knuckle if it still doesn't want to come out come out.

Generally, if one bolt is that corroded, they all are pretty close. Replace all the bolts with new, considering high strength and cad plating if you source locally. Regardless, they get some Nevr-Seeze on the threads and on the shaft so you don't have to do this again. Chase the threads in the knuckle too if you have the tools available.

New Brembo bleeders are available from our preferred 928 parts vendors too. Yours look corroded/tired, so get some penetrant on them now and let them soak for a few days, or risk breaking them off in the aluminum. If you are doing a full rebuild on the calipers (new seals and dust boots) it's OK to use heat on the area around the bleeders and the crossover pipe connections to help free the fittings.

Those thru-bolts look pretty sad, but they are very tight and secured with LocTite. The aluminum needs to be heated to soften the LocTite, but that expands the aluminum up into the bolt head faster than it expands the bolts, so the bolts actually get tighter at the heads while they get looser at the threads. Use your best judgement WRT even trying to get those out.

SQLGuy

I had the same problem on one of my front caliper bolts. I tried slotting it with a grinder and using a slotted impact driver, but still no luck. Had to cut drill the head off. Once the caliper was out, the remainder of the bolt came out with hand torque. Roger has the correct bolts. In most cases, I wouldn't expect to find suitable bolts locally... I did try.

Rob Edwards

I thought the conventional wisdom was to never split the caliper thru-bolts, no?

Bertrand Daoust

I had this same problem with one caliper bolt few years ago.

I removed the caliper with the hub and had a big bolt welded on top of the rounded one.
Put everything in a vise and use my ratchet with a pipe as a lever and unscrewed it.
I think that the heat when they welded it helped too.

Good luck.

Paddcomp

Drilling off it is then. I'm expecting these to be high tensile steel bolts, so better charge my drill battery and sharpen a few bits.
I've already purchased M12x70mm 1.5P high tensile allen bolts, 4off so replacing all of them. I know the calipers would benefit from a refurb, but it will have to go on the to do list for now.
Porsche WSM say under no circumstances break the caliper in half and undo the thru bolts, so I won't.

Thanks.

Rob Edwards

What year is your S4? If it's '87 or '88 and the calipers are original, AFAIK there are no replacement parts for the style of dust seal used on them. That may have changed but make sure you can get the rebuild parts before venturing down that path.

StratfordShark

I had this situation on my first S4. What I did was file couple of flats on the rounded bolt head, which allowed enough grip to use bolt removers similar to these in photo. No need to drill anything. I find these removers highly reliable.

http://m.screwfix.com/p/irwin-bolt-g...5-pieces/96028

dr bob

The warning on splitting the calipers is liability/defensive. There are no hydraulic passages inside, it's just a metal face-to-face there. The bolts are held in with some pretty hardy LocTite, but they do come out. When I did a full refurb, I took them apart. Turns out there was no good reason, since I decided not to refinish.

The original black finishing doesn't include the bolts, the crossover pipe and fittings, or the bleeders. To duplicate the original surfaces, the halves must be split. Since you can see the ends of the cad-plated through-bolts through the wheels, they can't be painted or PC'd in place. A lot depends on how much you treasure originality I guess. Green Loc-Tite (sleeve retainer) isn't that hard to find. I used red (stud and bearing mount), on the off chance I'll want to take them apart again some day.

The pad hold-down spring cleans up nicely with a little Scotch-Brite, does better if electropolished. The cross-pipe cleans OK too, but needs clear to maintain the looks.

Hilton

Here is the easy answer, albeit pricy.

M. Requin

I bought a set of these, both inch and metric, to remove a socket head cap screw when I did my CV/rear axle project last year, worked like a charm. And just a couple of days ago used them again on a motor mount bolt. The only problem might be the clearance issue the OP noted.

Paddcomp

I brought some TCT drill bits and drilled off the head. Caliper came off easily leaving a stud about 2" long, I slotted this and used an impact driver to remove.
Only advice I would offer to anyone else new to this is it takes time. The drilling is slower than most people are used to, and even impacting the remaining stud takes patience. For a while I thought it wasn't going to budge, also the angle you're working at isn't great. The hammer comes close to the wheel arch bodywork, which isn't a job I wanted to add to my list.

Now I can access the plates, but I couldn't get a good grip with my 4mm Allen key. Need to rethink.

dr bob

Those little button-head screws also have LocTite keeping them in place. IIRC they are SS so the heads are soft too. Those factors and the odd angle can make them a chore to get out, especially with the corrosion factor. Heat is your friend, right up until it gets so hot the seals get damaged. If you can get the pistons and seals out first, go ahead and warm the housing up a bunch to at least soften the LocTite. Make sure the head of the screw is clean so your wrench will fit all the way in. If your wrench is at all rounded now, grind the tip back so you have new sharp corners there at the tip. A few taps with a small punch might help, just enough to disturb the bond but not enough to spread the end of the screw in the aluminum. When you reassemble, new screws are in order, and you'll want to very carefully clean/chase the threads in the caliper with a bottoming tap. Yes, they'll need LocTite again.

And, remind yourself (and me too...) why you are doing this. The '86.5 to '88 piston seals and dust boots are not available as separate parts anymore, so removing and putting the existing ones back is a crapshoot as far as sealing again. You may be able to make them prettier, but there's no good benefit to taking them apart IMO. I spend a few minutes with needle files to detail the notches in new pads for the pad wear sensors. A couple more swipes with a flat file helps get just the right tension/clearance at the ends of the pads where they tuck into the frame against those plates.

And now's a great time to start a casual search for a set of later rebuildable calipers. They will only get more expensive and harder to find as time goes on. "Get all you can before the hoarders get there!" is my motto on stuff like this.

fraggle

drill off the top, replace them all.

use caution if you have an early S4. Already mentioned here, no boots are available. Only other options is all new pistons at $50+ each.

Paddcomp

Scraper and pressure seals available individually from
http://store.zeckhausen.com/catalog/...aa2iurv058pvf2