LZ V12-Finding Bottom Dead Center for a Leak Down Test

[Guest friscozephyr]

Does anyone know how to find bottom dead center (bdc)on a LZ V12 ? I am going to perform a leak down test and for safety reasons have to set each cylinder to bdc prior to injecting pressurized air into the cylinder.

I am hoping that that leak down test will help me identify if I am losing pressure through the rings or valves.

[peecher]

As there are no index marks on the crank, flywheel or front damper I think finding BDC is total guess work. If the cylinder heads were removed you might have a better chance but that would defeat your purpose?

[Guest friscozephyr]

They only reason I am considering finding bdc is that the leak down test test kit warned that the cylinder being test should be at bdc because the compressed air being inserted into the cylinder could "turn the engine over suddenly and cause great bodily harm.

I am not sure how the engine would turn over if all the spark plugs are removed.

Have any of you over performed a leak down test on an Lincoln V12 ?

[Guest DADDIO65]

Frisco, as long as you are as close to BDC as possible, with the compressed air, the engine will only move a fraction of an inch...just keep you hands clear of belts, fan blades etc. and gradually increase the air pressure to your cylinder being tested.... no different than a V8.

Again, I still think that the coil AND the distributor should both be looked at while you are doing your testing on other areas....remove all doubt from all major factors.

Compression, fuel, spark, timing, etc. I think it will pay off for you in the long run. I have gone through the same things as you have on my V12...unfortunately for me, My "good" engine was toast.... but without going through the various tests, and eliminating every possible cause, I would have been chasing my tail. Good luck, you have a wealth of knowledge in this forum... you are starting at the right place, I have learned alot from these gentlemen.

Daddio

[peecher]

Put the transmission in high gear when you get ready to apply air pressue...the engine will not turn over. Not sure about valve timing so BDC may not be the best position to check for leaks? 1/2 way between compression or firing stroke might be best?

[Guest friscozephyr]

Peecher,

I read some articles on the Internet regarding leak down testing and many are suggestion testing a top dead center so that the crank shop is vertical and would have a lot less mechanical advantage to help it turn over.

In addition a top dead center both the intake and exhaust valves would be completely closed and would give a more accurate reading.

The articles also say to make sure to "lock" the motor so the engine does not turn so putting it in third gear is a great suggestion.

I am not sure how to hand crank the engine to get it to tdc. Do you know the easiest way to crank the engine and get it to tdc ?

[peecher]

The only sure way I know would be to remove the coil ( for vision access) and line up the rotor with one of the spark plug stanchions on the terminal plate(s) facing the rotor. The front part of the rotor fires the cylinders on the left bank and the back part of the rotor fires the right blank. You will have to do a little research to determine which terminal stanchion belong to which cylinder. You may also have to loosen the distributor caps to varify position of the rotor to the terminal stanchion, especially on the bottom ones.

Here are the locations for the cylinders on the terminal plates.

Left side,front; The bottom is #1, the middle is #9 and the top is #5. Right side, front; top is #11, middle is #3 and the bottom is #7

Now for the back part of the rotor. Left side, rear; bottom is #10,the middle is #6 and the top is #12. Right side rear;top is #4, middle is #8 and the bottom is #2.

Using the firing order you should be able go to each cylinder in sequence and "hit' them all in 2 turns of the crank.

This will locate the cylinders pretty close to TDC. Good luck and have fun.