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Here's a very easy repair that can keep you out of all sorts of trouble. Stock, stamped valve covers are easily distorted by overtightening the four hold-down screws. Attempting to stop oil leaks from the bottom of a bent cover by further tightening just makes the problem worse.
If you are having oil leaks from the cover, it is pretty simple to straighten it. Remove the cover and remove the gasket. If the gasket was glued on with some sort of sealant, first scrape off every trace of goop from the cover without leaving any gouges.
Holding the cover upside down, examine the area around the four hold-down screw holes. You'll probably find the metal around the holes dimpled from the pressure exerted by tight screws. That's not good. In serious cases, overtightening the screws can warp the front and back of the cover upwards, away from the head. That's even worse.
Find something solid with a long, flat edge like a large vise or the edge of a heavy work bench. A length of wooden 2x4 will do if you stand it on edge. Lay the edge of the cover, still upside down, on this straight surface (if the ends of the cover are warped, this should now become evident). Get an old socket and a hammer, and use these to pound the dimples flat. In many cases, this will suffice to correct the end warpage as well.
If not, you'll need to find some long, straight strip of metal or wood to pound on -- it needs to fit between the lip on the edge of the cover and the clips that hold the gasket in, and should be about half the length of the cover long. Work on each area outboard of each hole to the center of the cover's edge individually. Go around the cover a few times, and you'll get it nice and straight.
Of course, what you don't want to do is beat with a hammer directly on the cover. We don't really need to tell you that, though, do we?
Obtain a new cover gasket (preferably from one of our sponsors). We favor the cork ones over the rubber ones for no real objective reason. If the four retaining clips are intact on your cover, there's no need for sealant. You will need to work with the cork and the clips a bit to get the new gasket properly seated in the cover.
Here's the part where we put the cover back on the car without reintroducing the damage we just corrected. If you have the original Phillips head hold-down screws, hold the screwdriver with your fingers from the end of the handle -- don't palm it. If you have bolts and a socket driver, hold it the same way -- don't grab the handle and jerk it around. Tighten the first fastener (your choice) so it just contacts the cover's edge. Do the same for the second fastener in the hole diagonal to the first, the third in the other hole on the same side as the second and the fourth in the remaining hole. Go around in the same order, gradually tightening each fastener until they feel just good and snug to your fingers.
The cork gasket will compress over the course of the next few days as you drive the car. Retighten the fasteners a few times as this happens -- but no tighter than the first time. The bottom of the gasket will take an impression of the head surface; in subsequent installations with the same gasket, be sure the gasket seats into these impressions (it will naturally want to do this on its own).
Aftermarket stuff: ipd offers a dandy hardware upgrade that greatly reduced valve cover dimpling. The core of this is a piece of flat metal that fits into the shape of the cover around each hole and extends a bit along the edge, thus spreading the pressure of each fastener (a bolt and lockwasher, in their kit) over a greater area. It's safe to torque these down tighter, but you'll still want to go in the same order and follow up for a few days.
Cast valve covers: We think these are good-looking and really don't suffer from warpage at all. However, there are no clips to hold the gasket on. Invariably, the gasket between the holes on each side will want to creep inwards as you attempt to install the cover. This is your chance to finally get out the gasket sealant and glue the new cork gasket to the clean and dry mating surface on the cast cover. We've had very good results with a thin layer of Yamabond evenly applied to the cover. Obviously, you will need to time when to mate cork to cover according to the tackiness of the sealant. Install the cover before the sealant cures fully, then let it cure before running the motor. As before, tighten in the same order and do the follow-ups.
Get this right the first time, and you can use the same gasket without further bother for many valve adjustment over years of driving with zero leakage.
One final note: The rubber gasket that fits between the valve cover and oil filler cap is also subject to leakage as it gets old and hard with age. Replacements are readily available and inexpensive. As a guideline, the filler cap should go on and off stiffly -- if it turns easily in your hand, get a new gasket for it.