An easy way to check the bearing play is to jack up the corner to be checked and grab the top and bottom of the tire. Shake back and forth gently but firmly. If you feel or hear a clunk, there is play. If the clunk is more than just a wee bit, you should tighten the bearing nut. To do so, you must remove the bearing nut cap. There is a special tool, but I use a cold chisel and hit the cap from behind. It takes a few wacks on the cap at 120 degree intervals, but it will come off. Once the cap is off you will see an allen head lock nut. I think it's 6mm, but someone will correct me if I'm wrong. Loosen it and tighten the nut (clockwise rotation) until you can't anymore. The back off (counter clockwise) until the washer behind the
nut can barely be moved with a screwdriver. Tighten the lock nut and reinstall the cap. Repeat as needed on the other front wheel.
If you have play in the rear, you will need a new set of bearings, as no adjustment is possible.
Merry motoring, Ed.
A properly adjusted wheel bearing should not have any noticeable play, but there must be a little bit. Most cars have tapered roller bearings in two places: the front wheels (or more precisely, the non-driven wheels) and the transmission/differential system. Those in the power train are normally torqued to 100+ lb/ft, while those in the wheels are not torqued at all. You can't look at the bearings and tell them apart, but if you screw up the preloading/free play on the bearing, failure is certain. The only difference is a matter of a very few degrees of angle on the rollers and races.
Porsche front wheel bearings, like those in most rear-drive cars, ideally should be set with no preload, and no end play. Since this is virtually impossible to do perfectly, the safest adjustment is to have a tiny bit of end play. You do not want any preload on the bearings!
The easiest and most effective way to adjust wheel bearings on the 928 is essentially the same method used on most cars, and is as follows:
1) Lube the bearings with a soft, flowable grease. Never, ever, use what is called "wheel bearing grease" in a wheel bearing! It is a left-over from thirty years ago, and is far too thick, stringy, and heat-sensitive to work in a modern car. Use a soft black hi-temp moly grease, or a synthetic such as Mobile 1. Don't forget to grease the contact area of the grease seal.
2) Pry the brake pads away from the brake disk so the hub/disk can spin freely.
3) Ensure that the tabbed washer is in place and in good condition.
4) While rotating the hub, tighten the locking nut firmly.
5) Stop rotating the hub, and carefully loosen the locking nut until there is clearance between the nut and the tabbed washer.
6) By hand, bring the nut up against the washer so that there is no clearance, but the tabbed washer can still be moved (rotated) slightly with a screwdriver.
7) Tighten the lock on the nut.
8) Check by rotating the hub - it should turn freely and easily - and by grabbing the brake disk and wiggling it hard - there should be no end play.
If done properly, the adjustment will satisfy the pickiest tech inspector, but still will have no bearing preload, so your bearings will last forever. (If they are protected from contamination, well lubed, and properly adjusted, the projected life span of a wheel bearing is infinite!)
Wally Plumley
928 Specialists
I have learned a couple of important things after tightening my bearing a few times in preparation for Driver's Ed tech inspections:
1) Use Vice Grip tool 11R (a big "C" clamp type device) to remove the bearing cap. (Thanks, DR!)
Clamp onto the cap tightly at the 12 and 6 o'clock positions and work the tool up and down rapidly to slowly work the cap out. This method gets the cap off in less than 30 seconds with no damage to the cap. Try it, you'll like it! (Much more civilized than banging away at it it with a hammer and chisel.)
2) The factory manual says to tighten the bearing nut until it's so tight that you can barely move the thrust washer with a screwdriver. I have found that this method leaves the bearings much too loose. So, I tighten it first using the method in the manuals and then tighten the nut a bit more before locking it down. This done, you can no longer move the thrust washer with a screwdriver, but the tightness is much better.
3) You can not tell how tight the bearings are with the wheel off. I suggest that you check your work by remounting the wheel without replacing the bearing cap (to allow for easy readjustment). Then grab the wheel at the 12 and 6 o'clock positions and try to wiggle it. Experiment with different bearing tightnesses until you find the right setting. Remember, you want the bearing nut just tight enough to remove all free play but not so tight that it would cause binding.
+Greg Nichols
'87 928S4