My experience with C-4’s is… I started rebuilding transmissions as a career in 1978. All makes and models were included at the locations where I worked. During the first 5 years I was rebuilding between 5 and 9 C-4s a week. Those numbers gradually tapered off to the point where newer models began replacing older ones and by 1992 I was only repairing maybe 7 C-4s a month. I can honestly say I have probably rebuilt more than 2,000. I don’t do nearly as many as when Ford was mass producing them, but every once in a while I may find one on my workbench. Good hard parts for these are becoming rare and almost extinct, so unfortunately I have to charge a premium when they are severely damaged. Listed below are some things I have noticed over the years that may help some of my Bronco brothers and sisters to keep their classics alive and kickin’, and working like they should.
C-4 Transmission Pan Gasket Service for stock steel pans:
When removing the pan use a 7/8” flarenut wrench to get the dipstick tube loose first before loosening the pan bolts. Make sure all the pan bolts are the Ford flanged type. The incorrect pan bolt can also cause leaks due to improper or uneven clamp load in the areas surrounding the bolt holes. Choosing the right type of gasket is important.
PAPER GASKETS
Paper gaskets are not as friendly when used with older vehicles and usually require over-tightening of the pan bolts to make them seal. It is gaskets like these that cause steel pans to get bent and distorted in the areas surrounding the bolt holes, even to the point that they are difficult to straighten. Overtightening these bolts can also cause the aluminum threads in the transmission case to strip-out. As old as these C-4s are, you may find these aluminum threads to be pretty tired already.
RUBBER GASKETS
I have never seen a rubber pan gasket that didn’t leak on a C-4 no matter how tight it was. As far as I’m concerned, any C-4 pan gasket that comes packaged inside a box folded up, is absolutely worthless. Many auto part stores are notorious for selling them. Gaskets like these find my trash can very quickly.
CORK GASKETS
Your C-4 came with a cork gasket from the factory. It worked just fine for many years. There is no reason why it cant be the best suited material for the job as long as it is properly installed. Go to a transmission specialty repair shop or Transtar, and buy a cork gasket, and any other supplies if needed. The best way to prevent leaks is to spend some time preparing the gasket surfaces. The trick to making the pan seal is to make sure the pan surface is flat and free of any stray gasket material. I have a wire wheel on my bench grinder for getting these clean. A dry gasket surface, free of oil and dirt is important. Most all transmission pans on trucks this old will be distorted on the gasket surface. You will likely need to straighten your gasket surface on the pan before reinstallation. I usually straighten pans with a special punch that is larger diameter than the bolt hole but ground to a very shallow cone. The back of a ball peen hammer also works well for this. Brace the pan rail from behind with a straight block of hardwood, steel, or the edge of a workbench. By using a hammer with this punch you will cause the bolt hole to go beyond being flat and just slightly concave on the gasket side. But when you tighten the bolt it will draw the gasket surface on the pan flat again. Using the hammer, straighten any other distorted edges so as to make the gasket surface straight and level. When tightening the pan, the same torque spec will not work with all gaskets because of different gasket manufacturers. For this reason I tighten them by feel and by sight. When the gasket starts to bulge just slightly around the bolt that you are tightening, that is usually enough. The important part is to get them all evenly torqued. Draw the bolts down evenly, and do not over tighten. You will need to go around them 3 times or more to get them all evenly torqued. A speed handle is my favorite tool to do this. Use no sealer with a cork gasket, if you managed to get the pan surface straight, and it is clean, dry, and free of oil, it will seal properly without a problem. I have the best success installing cork gaskets dry. If you must use sealer, use Gasgasinch. Apply the sealer to both sides of the gasket as well as the sealing surfaces and let it air dry a few minutes to a haze before positioning the gasket and installing your pan. The only time I ever needed Gasgasinch on a C-4 pan gasket, was when the machined aluminum surface had some deep scratches. Chemical sealers have the ability to fill-in small voids and imperfections that sometimes gaskets can’t reach. Whatever gasket type you decide to use is up to you. Careful preparation must be taken before and during installation for the best success and quality in your finished product.
That being said, make sure your manual lever seal and kick-down O-ring are not the cause of the leak. These lever seals can drip down to the gasket and follow the pan rail, blow back in the wind etc. making it look like the gasket is faulty but not actually be the origin of the leak. A bad modulator o-ring can also cause the appearance of a pan gasket leak.
