Finding TDC for leak-down test

You will find that when the pistons are at TDC and BDC, the crank will turn back and forth a bit easily, not so when not on TDC
 
Fairview said:
Hi Jack,

I don’t know if a Model A engine will start making complete revolutions with compressed air piped into one cylinder, but when doing leak down tests on airplanes, an assistant has to hold the propeller for dear life as it tries to turn. If the piston moves off TDC it takes a lot of strength to hold the prop. If it gets too far down the stroke, it can’t be held in place.

I would think a piston not exactly at TDC will rapidly blow to the bottom of the stroke and catch the tester and assistant by surprise, sending a handheld starting crank or wrench flying, or a lurch forward or backward if in gear. Whether or not it will continue to make complete revolutions I just don’t know.

I could be wrong, but I’d be extra careful.

What do you think?
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Put it on top dead center, high gear, parking brake on. Maybe block the wheels. It isn’t going anywhere.

There is nothing restraining an aircraft propellor. The car is restrained. If the airplane prop is on top dead center when you apply the air pressure it will not move.
 
If the engine gets away i doubt it will travel to BDC. At most just surprise those doing the testing and possibly cause physical harm. If you have exact TDC nothing should happen. I have done this many times alone and was able to leave the engine to listen at the tail pipe, Crankcase breather and radiator without problems.
 
TDC doesn't have to be perfect for this test. Close is good. Just make sure its on compression stroke. Put transmitter in gear, apply park brake. It only takes a little air pressure to perform this test. If the crank position is off a bit then the air pressure can certainly cause the crank to turn, reason for putting the trans in gear and applying the brake.
 
I would like to disagree with much of the information that suggests a cylinder does not have to be exactly at TDC to perform a leak-down test.

First, there are A) leak-down tests and then there are B) so-called leak down tests and then C) just air pressure tests, often of a single cylinder, that some erroneously call 'leak down tests'.

 Actual leak down tests were standardized for aircraft cylinder comparisons and evaluation with a 0.040" orifice and manifold and the entire assembly, including gauges, feed fittings and hoses is calibrated for a specific indicated flow rate drop. Those instruments are certified as to calibration and not chinese junk. Nice, but overkill for the casual user.

Now, in the case of this thread the O.P. is only looking for a valve leak so yes, a hose of shop air through a single cylinder not exactly at TDC may give you an audible hiss, but that is NOT a true leak down test. That would be like calling a farmer-fix with bailing wire a factory authorized and certified procedure.

OK, who cares and so what if I do either A,B, or C above not exactly at TDC?:

First, all used engines develop a ridge above the top ring position. Only at true TDC is the top ring tightly pressed against that ridge which may result in intermittent ring cocking and contact loss resulting in leakage not seen 1/16" down. A cracked top compression ring may only show excessive loss at this position. Also, only at true TDC can you detect any interference problems with piston/ gasket/ head that may cause a piston to tilt and break ring seal.

You only really need to do what you have to do to look for a specific fault, like a valve leak. But please do not refer to an un-metered pressurization of a single cylinder at any old position as a leak-down test. Call it what it is- a listen for some kinda hiss test. A true leak down test is quantitative, a hiss test qualitative at best.
 
I have to agree. Having worked in Aviation for 50 years. On your A this is a good test for valve leakage. It can also verify blo-by and help find possible head gasket problems. Yes you can hear air in the radiator.
 
I have an OTC 5609 cylinder leak tester, and the plan was to go to TDC and use regulated airflow to find out not only where the leak is but how severe. This will help me determine the cost/benefit for attempting a repair. As seen in another current thread, replacing valve guides is not easy and I don't have any local resources to help me out.
 
Have not done a leak down myself, have questions.


Instead of listening in the radiator, can you just overfill the radiator and look for coolant bubbles? Other causes besides a bad head gasket?


If you hear hissing in the crank case, other causes than rings?


Instead of listening, can you seal the breather tube with the palm of your hand to see if there is pressure from the air hooked up to the spark plug hole?
Thanks.
 
30 Closed Cab PU said:
Have not done a leak down myself, have questions.


Instead of listening in the radiator, can you just overfill the radiator and look for coolant bubbles? Other causes besides a bad head gasket?


If you hear hissing in the crank case, other causes than rings?


Instead of listening, can you seal the breather tube with the palm of your hand to see if there is pressure from the air hooked up to the spark plug hole?
Thanks.
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Just for context, are you asking these questions because there's something about your situation that would make hearing these sounds difficult?
 
alexiskai said:
Just for context, are you asking these questions because there's something about your situation that would make hearing these sounds difficult?
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Asking for info only, do not have a problem,. Would like to try it sometime this year as educating myself. Just exploring the possibilities.
 
You can fill the radiator if you want to deal with the mess. Do not hold your hand over the oil fill tube. You will force oil out the rear main. With experience you will be able to determine what is acceptable and what is not. The point is there is a lot of trouble shooting that you can do with a leak tester.
 
All that is needed for an 'A' engine is the 'hiss' test. Its still a leak test no matter how its looked at. You're looking to see/hear where the cylinder air is escaping and if its more than normal.
 
Posting #26 described positioning the piston at TDC on the compression stroke.

How do you know you are on the compression stroke?

If one turns the crank until the pin falls into the timing gear dimple, cylinder #1 should be at TDC. But, which upstroke is this on? Obviously, the leakdown test will leak massively out the open valve, but if you are not on the compression stroke with a stuck or leaking valve the cam may have opened the valve on purpose.

Translating post #4:
When the pin falls in the dimple on the timing gear, cylinder #1 is at TDC, is cylinder #4 also at TDC?
Then 180 degrees advancement is #2 and #3 at TDC?

Diastole
 
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Diastole said:
Translating post #4:
When the pin falls in the dimple on the timing gear, cylinder #1 is at TDC, is cylinder #3 also at TDC?
Then 180 degrees advancement is #2 and #4 at TDC?

Diastole
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W/the pin in the dimple, #1 will be at TDC of the compression stroke, #3 will be at TDC of the exhaust stroke. Advancing the crankshaft 180 degrees will place both #2 and #4 at TDC, #2 on the compression stroke and #4 on the Exhaust stroke. Simples.
 
Diastole said:
Posting #26 described positioning the piston at TDC on the compression stroke.

How do you know you are on the compression stroke?

If one turns the crank until the pin falls into the timing gear dimple, cylinder #1 should be at TDC. But, which upstroke is this on? Obviously, the leakdown test will leak massively out the open valve, but if you are not on the compression stroke with a stuck or leaking valve the cam may have opened the valve on purpose.

Translating post #4:
When the pin falls in the dimple on the timing gear, cylinder #1 is at TDC, is cylinder #3 also at TDC?
Then 180 degrees advancement is #2 and #4 at TDC?

Diastole
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The cam gear is twice the size of the crank gear, so when the pin falls into the dimple, you can be sure that #1 is on the compression stroke and #3 will be on its exhaust stroke. The exhaust stroke for #1 has the dimple on the exact opposite side.
 
I never circled back to this thread, but it was in fact a loose exhaust valve seat. The leakdown test revealed a leak at the exhaust valve and the loose seat was evident when I removed the head. I had the seat replaced and put everything back together and the car was fine.
 
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