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Jaguar Technical Articles

Article 3 – Checking Cam Tappet Clearances

Tech Article 3

Troubleshooting your XK Engine – Checking Cam Tappet Clearances

I approach the old lady hoeing weeds in her garden. “I understand you’ve got some old cars stored in that barn out back. Mind if I take a look? Go ahead Mister” she said. “I’d be glad to get rid of them so the milk cow would have more room”. I approach the barn and enter through a side door. There, in the back, what is that? I approach closer in the dim light. It looks like a 61 outside hinge flat floor E-Type, covered with hay and chicken mess! “Harvey, wake up.” (It’s my wife Kelli calling out to me as I nap on the porch swing) “It’s time to come in and eat supper.” Damn, woke me up from a most excellent dream!

So we continue with our troubleshooting guide for the XK engine. My specific example is the 4.2 litre 6 cylinder engine in my 1967 E-Type. But the concept I am going to discuss this month, the checking and adjustment of Cam Tappet Clearances, can be applied to other XK engines too.

Last time, we checked the timing of the cam with respect to the movement of the pistons. If the cam timing is off, your engine performance will be noticeably degraded. This month, we will see how you go about checking the clearance between the cam lobe and the tappet. This clearance can have an effect on your engine performance, especially if it is too tight, as this will hold the valve partially open, resulting in a loss of compression and burning of the valve seat. In the opposite direction, a clearance that is too loose will result in increased valve train noise and, if excessively large, will also degrade performance.

As we have noted in previous articles, the XK engine has overhead cams. That is, the camshafts are located in the cylinder head, very close to the valves they open and close. This is in comparison to a “push rod” engine, where the cam shaft is much lower in the engine, near the crankshaft, with the cam motion being transferred to the top of the engine via long push rods. The overhead cam configuration is a more elegant and high performance configuration.

Strictly speaking, the lobe of the cam shaft strikes the “cam bucket”, which in turn strikes the head of the valve stem, forcing the valve open. The lobe of a typical XK cam protrudes 3/8″ beyond the “basecircle” of the cam, thus opening the valve by that amount. When the lobe of the cam is away from the bucket, there is a small clearance. There are several reasons for this clearance, including the need to leave an allowance for thermal expansion of the various parts and the need to leave a running clearance so the bucket is not worn down from continuous contact. It is this clearance that we will check and, if required, adjust. You will want to perform this job on an engine that is effectively at room temperature, so that thermal expansion effects do not affect your measurements.

To check the clearance, the cam/valve covers must be removed. At any instant when you inspect the stationary engine, several of the valves will be on the cam lobe and depressed. The remainder of the valves will be closed. To keep it simple, we will check only those valves where the cam lobe is pointed straight up i.e. directly opposite of the bucket surface. On the early XKE engines, roughly corresponding to those built up to 1968, the required clearance is 0.004″ on the intake side and 0.006″ on the exhaust side. On the later “emissions” engines, the values should be 0.012″ and 0.014″ for intake and exhaust. Check a reliable Jaguar manual to be sure. You will need a feeler gauge to make the check. Start with a blade that is 1 or 2 thousands less than the expected value. It should slip into the space between the cam and bucket easily. Go to the next thicker gauge and try again. As you go to a thicker gauge, you will notice that the gauge becomes hard to insert and has a pronounced “drag” as you move it in and out. Checking for the correct drag is an acquired technique but you can’t hurt anything by experimenting with different thickness gauges as you develop your feel.

Prepare a chart listing the 6 intake and the 6 exhausts valves. Remember, Jaguar considers the forward most cylinder as number 6. After you check your first values for the cam lobes that are up, use your 1 5/16″ socket to rotate the crankshaft pulley until the next set of lobes are up. Then check those valves. You will wind up rotating the crankshaft pulley multiple times as you work through all 12 locations.

Now you will want to review your numbers. Certainly, if all the numbers are spot on then hooray, you are done. If one or two values are way off, you might want to repeat the procedure and see if the aberrant values are repeatable. Values that are too tight (less than the target value) would be unusual but would be cause for immediate action. Values that are too loose by 1 or 2 thousands are not a big deal. Some mechanics actually set the values loose by this amount to make sure that valves do not hang open. Values that are 3 or more thousands too loose will not require immediate action but might explain poor engine performance.

Unfortunately, adjusting the clearance on an XK engine is a real pain. If you unbolt the camshaft and lift it out of the engine and then use a magnet to extract one of the cam buckets, you will find a small circular shim, about the size of a nickel, inside the bucket. These shims are precisely ground to varying thicknesses. They range from 0.075″ thick to 0.105″ thick and can be purchased in 0.001″ increments of thickness. The plan is to remove the existing shim and to add a different shim that will bring the cam clearance into the correct range. This is a maddening process that involves the purchase of shims, measurements of each shim with a micrometer, and multiple removals and reinstallations of the cam shafts. If you are a novice, this is a job that you might want to pay an expert to perform. That said, I had never done the job before this year and, using the Bentley shop manual, was able to get through it without incurring any permanent disasters. The good news is that once set, the clearances are very stable and will most likely stay in spec for tens of thousands of miles.

So there you have the cam tappet clearance check in a nutshell. At this point, we have completed major checks of the cylinder head. Next time we will talk about the spark ignition system.

Disclaimer – Automotive work can be dangerous if proper safety procedures are not followed. In homage to our litigious society, I must state that I cannot be held responsible for any real or perceived mis-information that may be contained in this article. A good shop manual is mandatory before you attempt any work. Read the safety section of your manual. If you have any questions, contact me at hdferris@bellsouth.net so we can hopefully get questions worked out before a problem is created. Previous Technical Articles will be posted at my website http://www.newhillgarage.com

Specialty Tool List Feeler gauges- if you feel like splurging, buy the Starrett brand 1 5/16″ socket with rachet or 1 5/16″ wrench



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