Please read this document from beginning to end before attempting the below procedure as there are dependent steps and notes that should be in mind before beginning.
When to replace rotors:
Rotors have a long service life on a 993 lasting between 60 and 80 thousand miles in most cases. They should be replaced when they approach their wear limit (minimum thickness) or 30mm in the front and 22mm in the rear.
When one approaches these minimums the brakes can exhibit greater brake noise. With use, 993 rotors develop a ridge on the outermost circumference of the rotor faces. This is where the pads do not come in contact all the way to the outer edge of the rotor. If one can feel a substantial ridge develop here with one's finger, brake performance, brake feel or noise is present it indicates the rotors should be checked for wear. The difference between a new rotor and one worn to replacement thickness is only 2mm.
Resurfacing rotors have fallen out of favor as in the process the service life is literally cut from the rotor face and new rotors from the original suppliers to Porsche have become somewhat inexpensive as replacements if purchased outside the dealer network. Seeing that replacing them is one of the most basic and simple service items done along with pad replacement it is well suited to a Do-It-Yourself approach to replacing them. For most folks, rotors are a once in life of vehicle replacement item.
The below page is from the 993 1994/1995 Technical Specification Booklet. Specifications subject to change:
Disclaimer:
The content of this page, relates specifically to the Porsche 993 (911 1994 to1998) The authors here are not professional automotive technicians. Some of the procedures described on this page may differ from official factory recommendations and may not be the best to use in all situations. For these reasons use the information contained on this page at your own risk. Many times a lot can be learned by simply watching qualified professionals work on your 993.
Special tools needed:
- A breaker bar and socket for removing the caliper mounting capscrews;
- A manual impact screwdriver (inexpensive at Harbor Freight);
- A 5" to 8" jaw size C-Clamp;
- A click-style torque wrench. Be aware that torque wrenches when set near the ends of their torque value range are usually not very accurate so size the torque wrench you use so the required torque value falls somewhere in the middle third of its adjustable range if at all possible.
Ordering parts and supplies needed:
- Order the 4 rotors;
- Pre-order 4 replacement cap screws for all years of 993 and washers as well for 1994 for mounting the front calipers, see part numbers below. Porsche recommends replacing these whenever a front caliper is removed. It probably has to do with corrosion or debris scoring the fillet under the head during removal of the fastener significantly weakening it.
- If you live in an area where you expect to see corrosion on the mounting capscrews it is probably a good idea to order and replace the 4 used to mount the rear calipers as well.
- A brush-bottle of anti-seize
- Brake Fluid to top off the reservoir.
- A can of spray electronic cleaner.
- 2 Cans spray brake cleaner.
- Some dielectric grease.
Front Caliper mounting Capscrews and Washer:
The remarks column above refers to Model Year.
The model column refers to as below:
M493 = Narrow bodywork car;
M339 = 4 wheel drive;
M491 = Carrera 4S.
Rear Caliper Mounting Capscrews:
It is unclear to me what the "A" prefix in a part number or why two lengths of capscrews are specified for the rear calipers in the catalog. They may be interchangeable or two different lengths were used based on VIN. This is a good question for the dealer when ordering the capscrews.
Procedure steps:
- Break free and gently resnug the wheel nuts on the wheel-end you will be lifting off the ground to replace a rotor. Each wheel-end can be worked on individually lifting and lowering each wheel-ends as you work your way around the car replacing each rotor;
- Lift the wheelend off the ground. This page discusses safely putting one's car on a jack stand(s)
- Remove the wheel nuts and wheel-with-tire. Be sure to put the wheel-with-tire on the ground face-up where you will not scratch the wheel;
- Unplug the wheel brake-pad wear sensor wire pigtail if a sensor is present;
- Draw down the brake reservoir with a turkey baster or a syringe before working on the rotors. After installing each new rotor I pump the brake peddle a little pushing the caliper pistons against the pads and new rotor. In this way it avoids the cumulative effect of all the wheel ends having all their piston cups pushed in causing a good volume of brake fluid to back up into the reservoir potentially overflowing it and making a mess.
- I compress the caliper piston cups using a "C" clamp with the original pads and rotor still in place. Placing the clamp along the top edge of the caliper casting and the pad backing plate. If you have a concern about scratching the paint on the caliper face one can use a little patch of rubber to protect it from the "C" clamp's jaw. This is done so the caliper with pads are backed off to clear the outer edge of the old rotor when sliding the caliper off and allows added clearance when mounting the caliper over the new thicker rotor.
- Unbolt the caliper from the wheel carrier. This will take the use of a more robust 1/2 drive socket and breaker bar;
- Slide the caliper off the rotor.
- Use some coathanger wire to hang the caliper out of the way without excessively twisting the brake hose;
- Remove the small M6x12 Countersunk Head 900.269.047.09 hold-down screws on the face of the rotor's hat attaching it to the wheel hub. These screws are usually seized if not previously removed. They are easily removed without damaging it by using a manual impact screwdriver & hammer to pop them loose;
- Slide the rotor off of the wheel hub;
- I use some brush-bottle anti-seize and very lightly coat the hub face where it comes in contact with the rotor. Use it sparingly as otherwise, it may migrate to the face of the rotor when in use;
- If the new rotor is covered with a heavy anti-corrosion coating hose off the front and back pad contact faces with some spray brake cleaner. This can be done before or after they are mounted in position. If the faces seem bare and dry this step can be omitted;
- Slide the replacement rotor on the hub so the hold-down screw hole(s) lines up with its matching threaded hole(s) in the hub;
- Reinstall the hold-down screw(s) using a little anti-seize on its threads and the underside of its head;
- Reinstall the calipers, and torque the mounting capscrews to 85NM. Use new caliper mounting cap screws on the front calipers.
- As each wheel end is completed press the bake peddle as discussed earlier in this procedure.
- If your caliper capscrews are used in conjunction with washers be sure to use fresh capscrews AND washers with the washer's smooth side facing the underside of the capscrew's head. Check with the dealer as the later model years use a cap screw with a built-in washer face and may be a suitable supersession for the screw & separate washer. Using the washers with the roughly stamped side facing the cap screw head can damage the fillet radius under the head significantly weakening the fastener.
- If corrosion is present on the rear caliper mounting capscrews, I would consider replacing them as well.
Some use a small amount of anti-seize on the caliper mounting bolts to thwart future corrosion. Be aware this lowers the applied mounting torque required, lower than the factory specification, for these fasteners to achieve the designed clamp load. If the torque is not properly adjusted it may compromise the fastener connection creating an unsafe condition.
- Plug the brake pad wear sensor wire pigtail back in.
- Use some spray electrical cleaner to clean the plug and socket, and
- Apply a small amount of dielectric grease to water seal the plug/socket against corrosion
- Or simply tie-off the sensor pigtails as described on this page.
- Remount the wheel-with-tire and lightly snug the wheel nuts;
- I use a small amount of anti-seize applied to the rear of the wheel at its contact points to the rotor hat;
- Remove the jack stand and lower the tire onto the ground;
- Retorque the wheel nuts in a star sequence to the recommended torque (130Nm) and check the value by reapplying the torque in a star sequence;
- Before driving away pump your brakes several times to use up the space between the new rotors and the pads to avoid an initial peddle sinking to the floor braking experience. and check the level in your braking system master cylinder reservoir and top off the brake fluid as needed.
- Be aware it takes a while for the pad/rotor interface to fully mesh so don't drive aggressively until you have a few miles on your new rotors and the pads have fully bedded into them.
- If you applied a bit too much anti-seize to the wheel mating points it may become visible on the wheel after a few miles. It simply can be wiped off;
No comments:
Post a Comment