Finding TDC for leak-down test
 

Hi folks,

Getting ready to do a leak-down test to check for valve issues on cylinder 2. What's the easiest way to find TDC on cylinders other than #1?

The only way I've thought of so far is to go to TDC on #1, pull the distributor cap, and crank around until the rotor has turned 90°. However, this seems.... imprecise.

Re: Finding TDC for leak-down test
 

Re: Finding TDC for leak-down test
 

How about you leave the distributor right where it is and watch the arm stop at the same place in relation to #2 contact in the body as it was on #1. You'll be within a few degrees - plenty close enough for your test.

Re: Finding TDC for leak-down test
 

So the rotation of the crankshaft pulley would be:
TDC cylinder 1: 0° (with timing gear at dimple)
TDC cylinder 2: 180°
TDC cylinder 4: 360°
TDC cylinder 3: 540° (1.5x around)

Is that right?

Re: Finding TDC for leak-down test
 

Or take a spark plug and adapt an air nipple to it,use shop air,its way more precise to check at 120 psi than 65

Re: Finding TDC for leak-down test
 

I actually bought one of the dual-gauge leak-down testers, got the adapter, the whole nine yards. Just wasn't sure how to position the piston where I need it.

Re: Finding TDC for leak-down test
 

Doesnt matter,the lower the piston in the boredoes relax the rings some but not enough to mask an issue..or just crak till rotor at #2.that more than close enough to get full ring seal from bore taper.Stethoscope is handy for this..

Re: Finding TDC for leak-down test
 

Yup.

Re: Finding TDC for leak-down test
 

Soapy water. Slacken plugs watch bubble inflate on compression and even watch it deflate once past TDC . Can be used on any vehicle and any cylinder.

Re: Finding TDC for leak-down test
 

I put my big fat thumb over the spark plug hole and feel the compression increasing as the engine is slowly turned by an assistant.

Be careful of putting air pressure in a cylinder unless the piston is exactly at top dead center. Anywhere but the top and you will have an engine trying to spin the crank or whatever right out of you hands, or worse yet is if the car is in gear.

Re: Finding TDC for leak-down test
 

Yep, put your thumb over the hole of the adaptor that screws into spark plug hole and turn crank until you feel pressure on #1. Apply some air, resistance will go away at TDC. Then apply full air pressure. Then follow firing order, 1/2 revolution to next TDC

Re: Finding TDC for leak-down test
 

Never could make an engine turn on shop air, that's an interesting point. There are other tests besides leakdown to determine a valve failure..a vacuum gauge is a very useful tool for engine diagnostics,and a wet/dry compression tests works as well,If compression is low with a cylinder squirt oil into cylinder and re test,if it comes up its rings,if it doesn't come up its either head gasket or valves.Its not prudent just to repair the cylinder with a leaky valve and neglect the others,generally they all wear evenly..find one bad do a complete valve job.

https://i.imgur.com/m0VINuF.png?1

Re: Finding TDC for leak-down test
 

Thanks, I've already done compression and vacuum tests. These tests diagnosed 45 psi compression on #2 and loose valve guides overall. The purpose of the leak-down test is to see whether #2's issues are valve-related.

Re: Finding TDC for leak-down test
 

[QUOTE=Fairview;1869004]I put my big fat thumb over the spark plug hole and feel the compression increasing as the engine is slowly turned by an assistant.

QUOTE]


Thumb method, and when on compression a dowel rod through the Spark plug hole for visual when piston is at the top of it's stroke?

Re: Finding TDC for leak-down test
 

In my opinion there is enough evidence to go inside,but that's up to you.

Re: Finding TDC for leak-down test
 

I agree, but my ability to diagnose visually is limited by experience, so I'd prefer to proceed carefully via quantitative analysis.

I feel like I got my answer with the crankshaft pulley rotation method. It appeals to my tendency to overthink things. Thanks folks.

Re: Finding TDC for leak-down test
 

I just tore an old motor down last month and set it to top dead center on #1 by the timing pin while I had the head off and the piston wasn't at the top of the stroke. Come to find out that someone 70 years or so ago had put a timing gear one tooth off. Lots of carbon in the combustion areas so they probably weren't very happy and parked the car, it hasn't been run in ages by the looks of it. Anyway, I guess you can't assume but so much sometimes.

Re: Finding TDC for leak-down test
 

That's great info. I'd love to see a list of "things you should do while you have the head off."

Re: Finding TDC for leak-down test
 

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?

Re: Finding TDC for leak-down test
 

Years ago a lot of large stationary diesel engines use air for starting, using a special valve in the head and a special lobe on the cam shaft.