[dv657172]

Trying to do a leak down test for the first time to confirm if I have a head gasket leak. Bought a tool from Summit, all plugs are out, air cleaner off, pvc disconnected, radiator cap off.

Lined up balancer to TDC tested #1 and 9% leak down, so far so good. Cant seem to find TDC on any others as air leaking out of carburetor and >30% leakdown. Car ran ok when shutoff but with possible head gasket leak (discussed on forum earlier - white smoke, water found in oil, but only ran that way for about 10 minutes at idle)

Question 1) When #1 cylinder is at TDC is #6 also at TDC with both valves closed? Same for cylinders 8 & 5, 4 & 7, 3 & 2?

Question 2) If #1 cylinder is at TDC and I turn the crankbolt 90 degrees will I be at TDC on cylinder #8?

Thanks
DV

[25stevem]

I would look at it this way, and I look at it this way because I have been there before.

Since you had water in a cylinder or also in the
Crankcase, in the least your going to need to take the motor apart enough to remove one head to fix the issue.

Once your to that point pulling the other head is only 15 minutes more work.

In short, ditch the leak down tester, bite the Bullet and take both heads off.

[dv657172]

Steve, you are likely correct in that I will have to remove the heads however I would like to learn something through this experience otherwise its just a waste. So any answers to my questions would be greatly appreciated by you or anyone else.

[70gtojosh]

In order to find tdc on your other cylinders one option would be to get a small long screwdriver or piece of wire. (anything that can fit down spark plug hole) Put your finger over spark plug hole spin engine by hand until compression is felt. Put said tool down spark plug hole to touch top of piston. Continue to turn engine by hand and watch/feel tool come to tdc. Then you can install you gauge.It's possible to turn engine so many degrees or turns to find tdc in the firing order or companion cylinder but as much as I do those tests and procedures I never took the time to really get good and master those techniques. If you go down 1 cylinder bank You just have to turn to engine more times than would if you followed the firing order.

[chiphead]

I use the dizzy rotor. When the rotor is pointing to a plug wire, that cyl is near tdc with valves closed.

[Jack Gifford]

"near" TDC usually isn't good enough with 100 PSI in the cylinder.

[242177P]

Originally Posted by dv657172

Question 1) When #1 cylinder is at TDC is #6 also at TDC with both valves closed? Same for cylinders 8 & 5, 4 & 7, 3 & 2?

Uses the same mark, but #6 will be on the next revolution.

Originally Posted by dv657172

Question 2) If #1 cylinder is at TDC and I turn the crankbolt 90 degrees will I be at TDC on cylinder #8?

Correct.

[Schurkey]

IF (big IF) you have a degreed torsional damper, you can count off the 90 degree intervals in firing order, and leakdown test each cylinder 1--8--4--3...and so on through the firing order.

Most engines won't have a torsional damper marked at 90 degree intervals. In that case, I just use the leakdown tester itself to find TDC.

Set the leakdown tester to supply 10--20 psi when the output hose is blocked/plugged. Connect it to the cylinder. "Bump" the crankshaft in the normal direction of rotation, while watching the leakdown pressure gauge. If the valve(s) are open, there'll be no pressure showing on the gauge. If you're bumping the crank, and the gauge pressure jumps "up", and then returns to whatever you set the regulator to, the piston is going up with the valves closed. You should be able to feel the piston as it nears TDC because the crank turns easier, and once past TDC the pressure will drop as you bump the crank--the piston is going down. Turn the crank backwards, past TDC, and then go slower in the normal direction until the gauge barely moves as you bump the crank--that's as close to TDC as matters.

Then adjust your leakdown pressure to actually perform the leakdown test. Don't wrap your necktie around the fan blades, just in case the crank does spin.

Remember that leakdown testers have NO legal or "industry standard" for calibration in the automotive world. The closest we get is through Aircraft use--the FAA has guidelines which automotive tool makers may or may not follow. The leakdown tester itself has as much influence on the indicated leakage as the actual leakage of the cylinder.

Which means you either need experience with YOUR leakdown tester on known-good cylinders of similar bore diameter to the engine being tested; or you need to calibrate your expectations with a "Master Orifice". A "Master Orifice" is just a calibrated leak of known size, used to simulate the maximum allowable leakage a cylinder should experience. Some guys have brazed or screwed a Holley jet of a particular size into an air-chuck quick-coupler plug, which then plugs into the leakdown tester. If the cylinder under test leaks LESS past the rings than the Master Orifice (and little or no leakage past the intake or exhaust valves, and zero leakage into the water jacket) the cylinder is considered sealed well-enough to pass the test.

I bought a leakdown tester that includes a built-in Master Orifice through Aircraft Tool Supply. They used to sell through Amazon, but apparently not any more. Amazon now has thirty different "brands" of leakdown testers, most of them made in the same Chinese factory to unknown quality standards.

When I ordered mine, I requested a 14mm spark-plug adapter instead of the 18mm adapter it normally ships with.
https://www.aircraft-tool.com/shop/detail.aspx?id=2EM

The only downside to the ATS master orifice is that it's too "generous" with the leakage. A smaller Master Orifice would suit me better. The Master Orifice size on this tester complies with Continental/Teledyne specs for their aircraft engines, and they allow more leakage than I do.

Official aircraft procedure is to test at 90 psi. Everyone I know of tests at 100 psi as it makes the math easier. Of course, with a single-gauge leakdown tester like the Snap-On MT324, you don't really have a choice of test pressure--but with one gauge, there's no math.

[Schurkey]

Originally Posted by dv657172

Question 1) When #1 cylinder is at TDC is #6 also at TDC with both valves closed? Same for cylinders 8 & 5, 4 & 7, 3 & 2?

When #1 is at TDC-Compression (both valves closed), #6 is at TDC-Exhaust, both valves open on overlap. Crank has to turn 360 degrees to get #6 to TDC-Compression.

Originally Posted by dv657172

Question 2) If #1 cylinder is at TDC and I turn the crankbolt 90 degrees will I be at TDC on cylinder #8?

Yes, that works. Follow the firing order each time you turn the crank 90 degrees. Keep in mind that on a 4-stroke engine, there's "two" TDCs: TDC-Compression, and TDC-Exhaust.

[dv657172]

Thanks to all for the info. Very helpful and informative!

[242177P]

Originally Posted by dv657172

Thanks to all for the info. Very helpful and informative!

Want an accurate 90 degrees without any fancy equipment? Grab a marker,
and count teeth on the flexplate/flywheel. 166 divided by 4. So you're looking
for 41-and-a-half after whichever was TDC. Repeat again and again, or keep
counting higher. Next is 83 (even) and then 124-and-a-half and back to your
starting point.