The Stovebolt Site Forums The Shop Area Engine Shop Leak Down Test

I did a cylinder leak down test on my 261 today and the results were good. Every cylinder showed less than 10% leakage. On three different cylinders I could hear a very slight leak of air past the exhuast valve but those cylinders still showed less than 10 % loss.

So my question is seeing as this is the first leak down test I've done the motor passes right, even with the slight leak past the three exhaust valves seeing as all the cylinders show exceptable loss?


-mike-

Uh oh, 33 hits and no replies. Even WAG's are welcome. :>) Seeing as the motor ran so well for the previous owner it'll probably go in as is.

I would be a little biased, but those numbers sound good. Run it. LOL Get it running and warm, then check the valve adjustment to make sure it's still okay.

Is the engine in regular service, or has it been sitting awhile? A leakdown test is only effective if the engine has been run recently, otherwise there can be minor leaks caused by rust buildup on the face of a valve that's been sitting open during storage. I'd say a 90% + reading is excellent. Once you get it installed and running, do a cylinder balance test where you short out one spark plug at a time and record the RPM drop and manifold vacuum drop.
Jerry

Jerry,
Yes it has been running up until recently. Thanks for the balance test advice, I'll do that when the motor's in and thanks for the input in general.

-Mike-

Roy,

I did have a couple of tight valves so I'm setting them all at three thousands over the recommended clearances until I can do that "warm" setting.

The leak tester is a new toy given to me by a friend, a dangerous tool in the hands of someone like me with limited knowlege. LOL

Agree with all the previous Mike, especially that if getting the engine hot to do the test. I
might add the even if your valves are hand lapped and seal perfectly and hold water you
will still have some leakage past the rings, no cylinder seals perfectly. Cold the ring gap
isn't closed up and you will have more leakage past the rings, warm and there will be a
minimum of leakage. Also when the engine is running the combustion pressures force the
ring out against the cylinder wall for a better seal so you will never get a perfect seal
during a leak down test.
As Jerry posted, a 90% reading would be excellent. 8-10% on a normally aspirated engine
knocks it out of the ball park and variation of 4-5% cylinder to cylinder is normal, but
even a 10% variation isn't anything to worry about unless your building an engine to run
at Indy.

Denny Graham
Sandwich, IL

Denny Graham
Sadnwich, IL

My "leakdown tester" is nothing but a pressure regulator installed in an air line. With the hose plugged off and no air flowing, adjust the pressure to 100 PSI, and then connect the coupler to a quick-connect fitting brazed into an old spark plug shell. If the pressure drops to 89 PSI, for instance, that's 11% leakage. Listening at the intake, exhaust, and oil filler for airflow will tell where the pressure is escaping. Bubbles in the radiator indicate a leaky head gasket or a cracked head.
Jerry

i've done the same as Hotrod but i just adapted my compression tester to a quick connect for the compressor.

Looks like this is gonna be a you show me yours and I’ll show you mine
discussion so……here ya go, mine is just a home made job also, put it
together when I was doing the annual inspections on the IO-320 Lycoming
that I had in my Pitts.
http://www.pbase.com/dennygraham/image/141718796/large
http://www.pbase.com/dennygraham/image/141718797/large

Denny G

Mike, Got thinking about this part. Where are you listening, tail pipe, exhaust manifold off, etc. Is it possible that the air noise you are hearing is not comming out the exahust valve of the pressurized cylinder but out an open exhaust valve of another cylinder via the crankcase?

OK Denny, looks good. Now I'll show you mine.

Here is the leak-down tester I made to diagnose a problem in the engine I rebuilt for my 1937 John Deere Model B tractor. I used an old sparkplug as an adapter. Turns out the plug shell (after busting out the porcelain) was exactly the right size to tap for the pipe. Lucked out.

After all this discussion, I'm going to make an adapter for my stovebolt engine too. Anyway, thought this might be of interest.

Leak down tester

Kurt

BC,
I had the exhaust/intake manifold off the motor, I used a piece of rubber heater hose as a stethiscope and I was able to listen into the exhaust ports and intake ports individualy. I did make note of the air noise at the dipstick tube which from what I have learned is normal as all motor will lose some air to the crankcase past the rings. I listened at each manifold port when I did the test so was able to detrmine no air leak in any of the intake valves and three slight barely audable leaks from the exhuast which by the way were not adjacent to each other if this makes clearer what I did.

-Mike-