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of Cases, Studs, Cams and Cranks By Bob Donalds |
| So, you're ready to strip your engine?
Or at least do a top end overhaul? Here are a few steps to keep in mind
while working on your engine. First, check the cylinder head studs. Which ones do you have: 8 or 10 mm? The 8 mm head studs will be the same size as the exhaust studs on the head. (Don't confuse this with nut size, all head nuts use a 15mm socket.) If your head studs are 10 mm, it is very important that you test them before removing the heads. As you can imagine, the higher the engine temperature, the greater the expansion of the cast iron cylinder. The cylinder head studs will increase to 45 or more pounds of torque in the normal temperature range. The weak point of this is the head studs threaded directly into the aluminum case. At times, the studs will pull out or at least become lose torque. Torque-testing is done by torquing the studs up to 45 pounds to see if they hold or pull out. Doing this check now will help avoid pulling the studs out as you reinstall the heads later, or worse, have the heads loosen after just a few miles. You don't want to do the stud repairs on a freshly closed case after new bearings were installed, and all the parts were cleaned and oiled. Metal flakes from drilling and tapping for the stud repair go everywhere, and it sticks to the freshly oiled parts. Before you close the case, all machining, testing and stud repair should have been done. 8 mm studs are called stretch studs. They stretch when the cylinder grows and only see about 35 pounds when hot. You cannot torque-test these because they will actually break off. They also have inserts in the case giving them more surface to hold so they ever pull out. They became standard items after 1972 as a way to deal with higher engine temperatures. After-market cylinder head stud inserts will in time distort the cylinder bore. Trimming the cylinder bore at the insert is necessary, more on this later. Now that the 10 mm studs have been torque-tested, you are ready to remove the heads. Sometimes, they are stuck to the cylinder. Rather than risk braking a head fin when prying them off with a screwdriver or hammer, remove the spark plugs to let in air. This will make it easier to pull the heads off together with the cylinders. I then tap the cylinders off the heads with a hammer. . Next, remove the pistons by drifting out the wrist pin, being careful not to scratch the wrist pin bushing. You are now ready to inspect the lower end. Start with the end play. The wear limit here is five and a half thousandths. Anything more than this tells me that the thrust bearing is loose in the case. Trust me, it's the bearing and not the crankshaft play. It is time to align bore the case. Do not try to correct this by changing end play shims. Do this and the next time that the engine gets hot, the bearing will seize and you will be going nowhere. Next I check the torque on the six large case nuts. Bring them up to 25 pounds. You may find them loose. I am not concerned about loose nuts, but rather about the crank or cam shaft binding after they have been retorqued. After you get the nuts to pounds, turn the crankshaft with your hands and if you After you get the nuts to 25 pounds, turn the crankshaft with your hands and if you feel any resistance or binding, it is time to disassemble and align bore the case. The cases are actually rubbing against each other as you drive along making the holes for the crank and cam shafts too small. Not only do the two halves rubbing together at the mating surfaces actually remove metal, but the bearings pound the webs so you might have an oval crank bore and a worn thrust bearing surface. These require align boring and thrust cutting for oversized OD main bearings. As you might suspect, I don't select the main bearings until I know the outside AND inside diameter. Remember, you might have to grind the crank, too. If these two tests turn out OK, you can visually inspect the cam lobes and lifters for excessive wear. Do the stud repairs if any needed, and then reuse the lower end. (Let me repeat what I've recommended in previous articles. When putting the top end back together, please put new pistons and cylinders in the engine. You will restore the power and prevent oil consumption within a few thousand miles. Also, rebuilt cylinder heads with good exhaust valves and a lowered compression ratio will make the engine last longer and run cooler.) If, on the other hand, these tests on the low end show you need to do more work, here are a few steps to keep in mind. The first step I take is to take it all apart, degrease everything and inspect each part. I check the case for cracks and wear, and have the crank measured to see if it needs to be ground. Next, clean and prepare for reassembly. I remove the pressure relief plungers. I then ball hone the pressure relief, lifter, fuel pump pedestal and distributor drive bores, all the gasket surfaces including the push rod tubes and cylinder bores. Before I assemble the case halves, I check the cylinder bores in the case with a new cylinder and grind the high spots off the inside of the cylinder bore if needed. You can often see where the cylinders have rubbed on the bore by the shiny spots where the studs are. In extreme cases the cylinder will not seat completely into the case. Next, I do any cylinder head stud repair. I also inspect the drain plate studs. If they are stripped, I drill and tap for an 8 x 6 mm repair stud. Don't go past the taper in the tap, but leave some taper in the new threads. This grabs and locks the replacement stud. A final washing in the jet wash tank removes the metal flake and machining oils. Then it's ready for the last tests and assembly. If you don't have the proper tool to true the cam bore, then there is one very important test to see if the cam shaft is binding. Try this test. Install the new cam bearings and cam shaft. Bolt the two halves together and torque the six large nuts to 25 pounds checking to see if the cam shaft still turns. If it does, go to 45 pounds on the lower middle nut. Does the cam still turn? If not, back the nut off just until the shaft turns. Turn it a couple of times and disassemble the halves. It may be time to chuck the case but first, I try this little trick. You will notice shiny spots on the cam bearings. This shows where the cam shaft is rubbing against the bearing. Since you cannot make the cam shaft smaller, the idea is to make more room for the bearing so it will sit deeper in the web. This will give the cam shaft more room. Use a fine sand paper, 300 or finer, and clean the web. Notice I am not saying, "Sand the hell out of it," but rather a nice touch to CLEAN the surface of the web. There are limits to how much you can do this! You cannot true the cam bore with sand paper. |
~Boston Bob
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