FAQ - Cam timing, degree wheels, true TDC......

Timing cams, finding true TDC

Timing cams on a stock Jaguar XK engine does not require the use of a degree wheel. What is necessary is indexing the crankshaft to true top dead centre (TDC). Because of the location of the timing pointer beneath the crankshaft damper on E Types, using a degree wheel to put things in a manageable position is a practical advantage. If the engine is late enough to have a pointer in the three o'clock position, the degree wheel is superflous.

Briefly, a degree wheel is installed in front of the damper (harmonic balancer) and a dial indicator is set up on the front piston. The crank is fiddled back and forth until the highest point of travel is found. The degree wheel is then set up with TDC under the pointer. The engine is then rotated backwards a little, before being brough forward until the dial indicator is 0.010" short of the highest point. The indication on the degree wheel is noted. Rotate the crank again until the point 0.010" before the highest point is found again, and note this reading. True TDC is precisely midway between these two points. It is important that the crank is rotated in its normal direction during this operation and throughout the rest of the proceedure. (This ensures the lower chain is kept under proper tension).

Say that the first reading was 8 degrees before and the second reading was 6 degrees after. This equals a total of 14 degrees. Half of this figure is 7 degrees, so 7 degrees from the first reading (or 1 degree before) is true TDC. Rotate the crank to this point and reset the degree wheel to 0 degrees. You have now found true TDC and can set the pointer on the sump to zero as well.

Once true TDC is found, there is no further need for the degree wheel. In fact, there never was any real need except to save working upside down with the normal pointer. Bring the crank forward until it is again at TDC and check that the distributor is pointing to # 6 (the front) cylinder - or that the groove in the drive is oriented with the larger side up and forward. Refer to the manual if this is not clear.

Now, with the head on the bench, using the Jaguar timing gauge, the cams can be set to their indexes. Be careful when torquing the cam caps down that they are coming down in index to prevent interference between the exhaust and intake valves. I normally take a strip of business card and loosen the second cam cap enough to slip it under the bearing and lightly snug the cap up. This act holds the cams on index whilst the head is being installed and torqued down.

The sprockets are slipped onto the cams, with the snap rings removed and the centres disengaged, then the upper chain is tensioned. Now the centres are pushed home. If the bolt holes fail to line up, rotating the centres 180 degrees will bring them into position in all but the rarest of cases. Remove the strips before rotating the engine to install the second cam wheel bolt. Safety wire the bolts and you've finished the ticklish part.

That is basically the drill. Although the degree wheel saves your back, it is hardly a necessity as some would have you believe.

When looking for TDC when the cylinder head is installed to the engine block (i.e with a fully assembled engine), a useful tool to have in your armory is a 'dead stop' (see pictures below).

The degree wheel method of finding TDC described above is obviously for 'head off' applications where the pistons are standing there winking at you. The principals, however, remain the same when dealing with an assembled engine.

As you know by now, TDC stands for 'Top Dead Centre'. Dead is the operative word here. When the piston reaches its highest point there is a 'dead' period when crankshaft rotation does not move the piston. This is why we use a point below highest (before and after) to interpolate true TDC. How we set these points is open to numerous proceedures - there is no one true way to find true TDC!

Other than a dial indicator reading directly from the piston top, the other commonly used method is called a 'dead stop'. Some means of stopping the piston in the same point in its travel before and after TDC is used. A bar across the bore bolted down to a couple of head bolts is one. However, with the cylinder head removed, the dial indicator method is probably the easiest - assuming you own such a tool.

When working with the head installed, the dead stop method is probably the most accurate. A two stroke timing gauge will work if the spark plug hole is relatively central to the piston. They are available from Outboard Marine Corp. and several motorcycle manufacturers. In the pictures above you can see a dead stop that you could easily fabricate for yourself. It will work equally well on vee and in-line engines.

From your local parts store purchase an 'air jack'. This is a tool that is used to hold valves closed while changing valve stem seals. Thread inside the lower part with a convenient bolt size - generally a 5/16" SAE will be right. Take a 5/16" bolt and grind the head into a ball. Cut a slot in the tip. Now thread the bolt in from the piston side until it is butted against the air jack.

Using some sort of general method, approximate TDC. 1/4 drive extension, screwdriver, long fingernail - or whatever - just something you can stick down the spark plug hole to feel the piston. Alternatively, you could simply look at the timing pointer. Remember you are only looking for the approximate position of TDC at this point. Screw your dead stop into the spark plug hole until it touches the piston. If it screws all the way in without contacting the piston, so much the better. If your dead stop did not contact the piston, take your screw driver and turn the captive bolt down until it does contact the piston. In either case, move the crank forward about 20 degrees then screw the bolt - or the entire tool, in further until it again contacts the piston. Now gently reverse the rotation until the crank stops dead. Mark this point relative to the timing pointer, or note the degree wheel reading if you are using one. Gently rotate the crank forward 300 plus degrees until it again stops dead. Mark this point or note the degree wheel reading. Midway between the two marks is true TDC.

This proceedure is useful when installing an uprated harmonic balancer.

If you have any questions relating to the above described proceedures, or any other technical issues, feel free to contact me at techsupport@classicjaguar.com

Lee Merrifield
Master Mechanic
Team CJ