Hi All,

I’m posting this as a for info post in case it might help someone with similar problems.

I’ve had my 1974 V12 for a couple of years and although it ran reasonably well (after solving several oil leaks and balancing the carbs properly) it had always had a strange tapping from the top end. It sounded like the valve clearances needed sorting, but when checked they were all there or thereabouts within tolerance (a few were if anything a bit tight at 11 thou, but none were over the 14 thou recommended)

The tapping seemed to come and go in severity and sometimes seemed to be on the left and sometimes on the right, usually from the rear of each bank. Recently the auto gearbox finally started making horrible noises engaging 3rd gear so I decided it was time to have the whole lot out and get the gearbox reconditioned and properly investigate the tapping. I set the clearances absolutely spot on and left it for a week while waiting for the gearbox to come back and decided to double double check the clearances to find that 2 of them (rear left inlet and rear right exhaust, were now a bit off from where I’d carefully set them! After long discussions with several very helpful expert garages I was recommended to do a “draw down” test. For anyone (like me) who has never heard of them, they are available for less than £20 and is one of the best investments I have made. Screwing it into the spark plug hole (when each piston is at TDC) and introducing compressed air it not only gives a reading of how leaky each cylinder is, you can actually hear where the air is going. If you can hear it coming out of the exhaust then it’s an exhaust valve problem. If you hear it out the carb it’s an inlet valve, if from the crank case breather it’s the piston rings and if it pressurises the radiator then you have a blown cylinder head gasket. All from a simple tool!

In my case the left hand rear exhaust valve and the right hand rear inlet valve were both poor (20% leak on the gauge). So both heads had to come off (quite a job involving a very big special tool to wind the heads up the corroded studs!) and I sent them to a specialist to be refurbished. When he stripped them he found that 17 of the 24 valves were very slightly bent and the 2 mentioned above were so bad that when I roll the stem along a flat surface the valve head wobbles really badly! Astonishingly the engine seemed to run really well despite this! While cleaning up everything before reassembly it was clear that the engine had reasonably recently been rebuilt as the liners were absolutely like new. There was only one very slight mark on the top of the piston with the bet exhaust valve but it dressed out easily, so I was fortunate not to have to do anything to the bottom end. I suspect that whoever rebuilt it got the cam chain timing wrong and turned it over before realising and correcting the error. They probably thought they had got away with it when they got it running and it seemed basically OK. I’m told that the engine is designed to slowly rotate the valves which helps to keep them sealed, so the tapping noise on mine was obviously changing the clearances as the valve rotated.

Anyway I now have it all back together and it sounds beautiful, tapping noise all gone.

Hope this tale helps someone else out there.

Rick

 

Congratulations on finding and correcting the problem and great information for the group.
Neil

 

In the states it’s called a leak down test.
Unlike a compression test which only tells you how much compression you have. A leak down test tells you where the fault is. Rings? Valves ? Which valve? Plus a percentage of leakage.
It’s best done on a warm engine.
Amateur racers use it to check condition of the engine post race. Our goal is to retain % of leakage we went to the race with.
One more point. . Next time you think you need to remove the head, Before renting/ buying that head removal tool. Clean the studs off. Doing so makes it possible to remove heads without that tool!!!
You need a complete radiator flush. Not just one. Often several. What has happened is some person failed to change the coolant every 2 years as the factory requires. It doesn’t matter how many miles you drive. If you’re going to leave it for a while drain the coolant system. ( and put a note on the instrument panel that it doesn’t have any coolant).
I’m annal so I always keep a couple of gallons of anti freeze and distilled water. ( if the water freezes the plastic bottle it’s in expands enough so they don’t break)
You have dissimilar metals in your coolant system. Steel, aluminum, copper, & iron.
Then you try to use tap water instead of distilled water. Nope not drinking water. Distilled like your wife buys for her iron. ( no it’s not expensive). Don’t buy the premixed type either. Some are just made with tap water.
If you buy a used engine, pull the water pump off and pour radiator flush directly in each side. ( you’ll have to set the engine up on the flex late to do that.
Let it sit for a few days. Flush it out and repeat until the water comes out clean.
See the studs that sit in the coolant? They build up calcium and other crud and you are trying to scrape off that buildup as you pull the heads off.