351 Windsor Leak Down Test

The '68 has been making a LOT of noise lately - most specifically an awful popping sound when idling and accelerating. It was particularly noticeable at the Goodguys show and on the way home.

A friend from BAMA lent me a leak down test kit. Unfortunately, both adapters in the kit have a 14 mm thread, while my spark plugs have an 18 mm thread....Grrrr...a problem that only the internet could solve. This was particularly irritating because I had spent twenty minutes warming up the engine and another ten or so taking out all the plugs.

A week later I received the 14 mm to 18 mm adapter in the mail. This part is designed to allow the installation of 14 mm spark plugs into an engine block with 18 mm threaded holes. But it still didn't fit! The depth of thread (about 10 mm) is not long enough to engage in the block before the bolt head hits the top of the recess around the spark plug hole. Grrrr!

So....I wrapped some electrical tape around the thread...

...and introduced the adapter to my grinding wheel setup. I ground the hexagon shaped head into a circle, and the smaller flange now fits inside the spark plug recess:

From above: 

Now we can do the leak down test. The test is a little bit tedious but goes like this:

1. Get the car up to operating temp. (I skipped this step since I pulled all the spark plugs out last weekend and I couldn't be bothered to put them back in etc.)

2. Take out all spark plugs, remove radiator cap, dipstick and oil cap. (See above).

3. Find Top Dead Center (TDC) on cylinder #1. (I did this by placing a piece of narrow-gauge stainless steel wire in the spark plug hole and turning the crank with a socket wrench until it peaked. Some people use the dipstick instead of a piece of wire).

4. Now that I have the correct adapter, I can use the Lang test kit I borrowed from a mate to check the pressure on each cylinder. (Note: If the pressure drops to 100 % immediately, it's probable that the cylinder is aligned at the exhaust cycle; so TDC +180 deg. If you rotate the crank through 180 deg. and get the same result, then an exhaust valve leak is the diagnosis). 

5. Repeat from #3 another seven times to cover all the other cylinders.

The biggest issue, literally, with doing this, is that the 351 Winsor motor is a bit too big for the engine bay, so access to the spark plugs is difficult. This particular engine was not an option in the '68 Mustang; mine was salvaged from a '69 Cougar which has more space between the shock towers. Luckily Amy Sheep came down to the garage on her birthday, to help with the bits that were most difficult. It didn't do much for her manicure, but it was super helpful.

The pressure fell instantly, and air was blowing through the tail pipe at the driver side on #5. We got the same result repeatedly in spite of re-finding and re-re-finding TDC multiple times....conclusion: A leaking exhaust valve at cylinder #5.  

All of the other seven cylinders held between 40% and 60% and any leakage was passed the rings which manifests as air flowing from the valve cover at the breather cap opening. The variation in percentage on these cylinders is most likely the result of variations in TDC alignment - it's hard to be accurate with the method I was using.