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Jaguar Repair/1967 Jaguar 420 stuck valve


I have a 1967 Jaguar 420 in my garage, which is a "project" car, but I am enjoying the challenge.
I have gotten it running, and just had a local English/German restoration mechanic run a compression test on it. Five of the cylinders read 160-170 psi, and one read ZERO. He removed the intake and exhaust valve covers, and felt he noticed a slight hesitation in the intake valve of the bad cylinder, when cranking the engine.
I bought it for $1,000, and have another $1,000 in transporting from Maine, and miner parts. My mechanic inspected the underside of the car, and felt it was worth holding on to it.
He believes the cost to repair the stuck valve will run about $1,000.
My question to you is whether it makes sense to continue, and also if there is perhaps a quick way to try to free up the stuck valve?
I look forward to hearing your thoughts,  Bill

ANSWER: Hi Bill,

If you bought the car for a $1,000. I can't see where you can possibly loose no matter what is wrong. As for a stuck valve it is possible, if the car has been setting for a long time the stem of a valve can get rusted and thus stick when it tried to operate. However, a sticking valve on a 6 cylinder jag is very likely to then get bent by most likely the other valve or the piston. And the following is a test for what happened.

Here is what you should do if you want to take on the project. All you need is a standard set of mechanics tools (most all the bolts and nuts are USA.) And I will walk you through the process. You do not need to be highly skilled mechanic to fix the car.  

First, be sure you get a clear title. Then remove the cam covers. It would be good to order a cam alignment tool from Moss Motors or Engel Imports (not expensive) but that is not 100% necessary.

Also get a large socket the size of the front crank bolt in the center. I don't remember the size but it is close to 1-5/16 inch socket. This is so you can hand rotate the crank small amounts.

Blow out the spark plug wells in the head and remove all the spark plugs. This area MUST be kept very clean !!!!

You can use the starter for this first test. Remove the SMALL wires off of either side of the ignition coil. This disables the ignition. Bump the starter until you have "Both" cam lobes of the bad cylinder pointing away from their respective lifter. Now check the valve clearance of the intake and the exhaust valves on that bad cylinder and let me know what the clearance is with a mechanics feeler gauge. (not expensive if you don't have one) I can then tell you what your next move is. It depends on what the clearance is to make the next move.


---------- FOLLOW-UP ----------

QUESTION: Thanks Howard. I will get back to you with the valve clearances. Bill

ANSWER: OK Bill, Let me know.


---------- FOLLOW-UP ----------

Just got back from vacation in Florida, the reason for not getting back sooner with valve clearance info.
Here's what I came up with:
         Intake valve clearance:  .016 inches.
         Exhaust valve clearance: .006 inches.
So, what do you think? I would love it if there would be some way to free it without disassembly.

Hi Bill,

The 420 came with either standard transmission or automatic. If it is a standard transmission, the next test is  easy but more difficult if it is an automatic.

First clean the spark plug valley out (with shop air if you have it) then remove all the sparkplugs and rotate the engine so that the zero compression cylinder is at top dead center (TDC) of the compression stroke. (make sure it is TDC of the compression stroke not TDC of the exhaust stroke.)

At that point put the car in 4th gear and put the hand brake on. Now you need two tools, a 3 foot or so piece of rubber hose either vacuum hose or fuel hose size to use as a stethoscope. (unless you have a stethoscope) If you have an air compressor or shop air get one of the fittings that adapts a shop air hose to a sparkplug hole. (not expensive) If you don't have shop air available go to a store like Radio Shack or a computer store and get a can of compressed air used to clean keyboards.

With the spray can you are not going to try to pressurize the combustion chamber, you need it for the noise it makes. This will work for an automatic transmission car too.

Remove the car air filter to gain access to the inlet to the carburetors. Wedge the throttles open and push the rubber hose into the intake and put the other end of the hose in your ear. Now have someone spray the nozzle of the spray can down into the plug hole of the bad cylinder that you put on TDC of the compression stroke. If the car was a standard trans and you have shop air, connect up the shop air hose to the spark plug fitting.

If the intake valve is not seated for any reason you will easily hear the hissing of the spray can in the rubber hose. If you don't hear any hissing then remove the oil filled cap and put the end of your hose in there to listen then go back to the tail pipe and listen in the tail pipe for the hiss. Where ever you hear the air hissing is where the compression if lost on that cylinder.

To have zero compression in a cylinder you must have a valve partly open, either an intake or an exhaust valve or a hole in the piston. With shop air it is very loud and clear but with the spray can you will need a quiet place and put a finger in your other ear so as not the hear the person spraying the canned air from the engine compartment.

Let me know.


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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