Article 35 Engine Rebuild Part 2
Last month’s article addressed rebuilding the Jaguar XK engine. Since there is a lot to discuss regarding engine rebuilds we are continuing with a second article. As we discussed last month, my engine is a 4.2 litre XK engine for my 1967 E-Type 2+2. Dis-assembly of the engine revealed damage to the combustion chambers, rings, pistons, and cylinder walls. I decided to send the engine to Dick Maury at Coventry West for a professional rebuild, not to mention repair of the damaged parts so that I could continue with a “numbers matching” engine combination. When we left off in last month’s article, Dick had finished with work to the block, which involved replacement of two cylinder liner sleeves and machining of all the cylinder bores to a consistent size. The final step is to apply the correct finish to the metal surface of the cylinder bores using a “hone” which preps the cylinders to break in properly with respect to the new rings that will be installed on the new pistons. More on that later.
One reason that causes folks to consider an engine rebuild can be low oil pressure. The engine oiling system on an XK engine is very straight forward. The oil pressure starts out high at the exit of the oil pump and it drifts downward as it squirts out at each of the various bearing surfaces. That would be the bearings on the crankshaft, the piston connecting rods, and the camshafts. As the bearings wear over time, the gaps widen and the oil can run out more freely, which unfortunately reduces overall system pressure. With the rebuild, all of the bearing surfaces are reset to their original clearances, restoring oil pressure. The oil
pump is also replaced to ensure that it is in tip top shape. At Coventry West, a new oil pump is used as part of the rebuild. The bearing surfaces on the crankshaft are all machined to a slightly smaller but consistently uniform size. Oversized bearings just for this purpose are used to complement the small crankshaft surfaces to provide a gap that is correct for good oil pressure. Machining of the crankshaft and its mounting points in the block also ensures that the crankshaft can rotate freely without binding or vibration.
Of course, your engine won’t be very happy if the oil can’t even reach the bearing surfaces requiring oil. There is a passageway drilled
down the center of the crankshaft that moves the oil to the various oil points. Over the years, this passageway can become clogged. Coventry removes access plugs in the crank and uses various cleaning tools and solvents to get all of the gunk out of the crankshaft internal passageways.
Engine builders know that power is derived from the head but smooth running is obtained via the major moving parts including the crankshaft, the pistons, the connecting rods, and the flywheel. A key element of the engine rebuild strongly recommended by Dick is the balancing process. The pistons and the
connecting rods are each weighed individually. Small amounts of material are removed from these parts until all six assembles weigh the same. This weight is noted and comparable temporary “bob weights” are mounted on the crankshaft to simulate the effect of the rods and pistons. The crankshaft and flywheel are then mounted in a special fixture that spins the crankshaft through all ranges of engine speed or rpm. Special sensors note any imbalance and direct a skilled operator where to remove or add weight to obtain a precisely balanced assembly. Weight can be removed by grinding off small amounts of material. Weight
can also be added via the insertion of special high density metal into machined pockets. In both cases, the result is a crankshaft that can spin up to redline speed in a smooth vibration free manner.
Some people will ask about the harmonic balancer. A harmonic balancer is used on most engines including the XK engine. From the name, you might expect this part to accomplish the same balancing function as the process just described. Unfortunately the harmonic balancer doesn’t really cause your engine to be vibration free, at least as far as what you might feel from the cockpit. The harmonic balancer is designed to absorb the small but persistent bursts of twisting force applied to the crankshaft each time a piston is loaded downward on its power stroke. These forces in the worst case can cause crankshafts to fail prematurely. The harmonic balancer on an XK engine looks like a solid metal disk mounted on the front end of the crankshaft but upon closer inspection it is really an inner and outer disk separated by a rubber section. The bursts of twisting force are absorbed by the rubber/metal assembly. Coventry will inspect the harmonic balancer and replace it if it is not in good condition. Note that a “high performance” harmonic balancer is really only necessary if you are building a race engine.
Once all of this prep work is done, it is time to start re-assembly of the engine. The crankshaft has bearings that hold it securely in the block yet allow it to spin. These bearings are designed to be replaced. They come in two halves that form a full circle. They are relatively thin but as they are backed by the substantial material of the crank bearing surfaces, called journals, and the bearing surfaces on the block, called bearing caps, they can be thin yet will not deform under the tremendous loads exerted during operation. The bearings may or may not have a groove and/or a hole to allow movement of oil. Here is a picture that Dick took of the old bearings. They are showing some wear, as might be expected.
Finally, the crankshaft is mounted into the block, which is turned upside down for this part of the assembly process. A key check at this point is to make sure that the crankshaft “end play” is within specs. This is the amount of movement that the crankshaft exhibits in a fore and aft direction. Too much is obviously bad. Too tight can also cause problems. The amount of movement is checked with a dial indicator and shims as added or removed as required.
One area that concerns many folks, including me, is the leak integrity of the rear seal on the crankshaft. If this seal fails, oil seeps into the bellhousing and potentially onto the clutch material. Repair involves engine/transmission removal. There are after market kits available from Terry’s and others that upgrade the seal from the factory OEM rope type to a seal that uses modern materials. This upgrade requires irreversible machining of the seal location and is somewhat costly. Dick convinced me that he and his crew are Masters of the Rope Seal and, installed properly, the seal is not a weak link in the engine. I went with a rope seal. We will see. Here is a picture of the seal in place. Dick uses a special tool to help place the seal in just the right position. Next month we will look at more of the engine rebuilding process.