The Ford Barn

old31 · 05-08-2026, 02:32 PM

FYI

I ran a wet compression test on #1 and it only went up by 1 lb. I also checked the valve clearances on #1 and they were ok.

I should have it running again next week.

rotorwrench · 05-10-2026, 11:07 AM

Having worked in aviation for over 40 years, I don't quite get the leak down numbers listed so I'll relate the method used in aviation. I use a differential pressure test rig which is a control manifold with two gauges, a pressure regulator and a balance orifice in the manifold. The cylinder to be tested is first set to top dead center on compression stroke with the input air fitting installed in the test cylinder's spark plug boss. I use a low enough input air pressure so that the crank can be turned while watching the pressure gauge for slight rises in pressure while bringing it up to TDC. At TDC the pressure reading stops increasing with movement. If it decreases with movement then the technician went too far. It takes a while to get used to getting a cylinder exactly at TDC so that the piston will not move as pressure is increased. Once at TDC, the technician brings the regulated input air slowly up to 80 PSI. Once at 80 PSI, the tech reads the differential gauge PSI. On a good cylinder, the differential pressure should give a reading of 65 psi or better. A good cylinder will read from 70 to 79 psi. If it reads 80 or more then the gauges should be calibrated since it can not read accurately over the input pressure reading of 80 PSI. If the readings are less than acceptable, the tech then listens at the intake, exhaust, and vent locations to hear where the leakage going. Hissing at a valve port indicates that the respective intake or exhaust valve is leaking. Hissing or building pressure at the vent tube is indicative of piston ring leakage.

All rings leak a little and that's normal. The rings move some in the piston grooves and can move enough to align ring gaps so one with ring leakage can be rechecked after a run with power being pulled to get them to move out of alignment. Squirting oil in the cylinder will generally increase the differential reading but usually not by more than 5 PSI or so but it does help to confirm ring leakage.

The intake or exhaust valves can have carbon on the valve seat which can be blown off by running the engine for a while and then rechecking the differential pressure. A person has to figure that an engine with a lot of carbon in the cylinders is likely starting to burn more oil whether its coming around the rings or valve guides. Flathead motors generally don't leak as much around valve stems but they can leak some when the clearance is worn a lot in there.

old31 · 05-10-2026, 05:15 PM

That's interesting, thanks.

AKJurnee · 05-11-2026, 05:17 AM

Get or borrow a good boroscope and look into the cylinder. Scratches and grooves will be obvious.

BTW how much air is blowing out the oil fill when engine is running?

old31 · 05-11-2026, 04:41 PM

BK, I never noticed the blowby from the oil fill tube. I have a draft tube attached.

I actually have a good boroscope, had not thought of it.

05-08-2026, 02:32 PM	#21
old31 Senior Member Join Date: Jan 2016 Posts: 2,686	Re: Leak down test and a compression test FYI I ran a wet compression test on #1 and it only went up by 1 lb. I also checked the valve clearances on #1 and they were ok. I should have it running again next week. Last edited by old31; 05-10-2026 at 05:14 PM.

05-10-2026, 11:07 AM	#22
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 18,006	Re: Leak down test and a compression test Having worked in aviation for over 40 years, I don't quite get the leak down numbers listed so I'll relate the method used in aviation. I use a differential pressure test rig which is a control manifold with two gauges, a pressure regulator and a balance orifice in the manifold. The cylinder to be tested is first set to top dead center on compression stroke with the input air fitting installed in the test cylinder's spark plug boss. I use a low enough input air pressure so that the crank can be turned while watching the pressure gauge for slight rises in pressure while bringing it up to TDC. At TDC the pressure reading stops increasing with movement. If it decreases with movement then the technician went too far. It takes a while to get used to getting a cylinder exactly at TDC so that the piston will not move as pressure is increased. Once at TDC, the technician brings the regulated input air slowly up to 80 PSI. Once at 80 PSI, the tech reads the differential gauge PSI. On a good cylinder, the differential pressure should give a reading of 65 psi or better. A good cylinder will read from 70 to 79 psi. If it reads 80 or more then the gauges should be calibrated since it can not read accurately over the input pressure reading of 80 PSI. If the readings are less than acceptable, the tech then listens at the intake, exhaust, and vent locations to hear where the leakage going. Hissing at a valve port indicates that the respective intake or exhaust valve is leaking. Hissing or building pressure at the vent tube is indicative of piston ring leakage. All rings leak a little and that's normal. The rings move some in the piston grooves and can move enough to align ring gaps so one with ring leakage can be rechecked after a run with power being pulled to get them to move out of alignment. Squirting oil in the cylinder will generally increase the differential reading but usually not by more than 5 PSI or so but it does help to confirm ring leakage. The intake or exhaust valves can have carbon on the valve seat which can be blown off by running the engine for a while and then rechecking the differential pressure. A person has to figure that an engine with a lot of carbon in the cylinders is likely starting to burn more oil whether its coming around the rings or valve guides. Flathead motors generally don't leak as much around valve stems but they can leak some when the clearance is worn a lot in there. Last edited by rotorwrench; 05-10-2026 at 11:14 AM.

05-10-2026, 05:15 PM	#23
old31 Senior Member Join Date: Jan 2016 Posts: 2,686	Re: Leak down test and a compression test That's interesting, thanks.

05-11-2026, 05:17 AM	#24
AKJurnee Member Join Date: Jun 2025 Posts: 32	Re: Leak down test and a compression test Get or borrow a good boroscope and look into the cylinder. Scratches and grooves will be obvious. BTW how much air is blowing out the oil fill when engine is running?

05-11-2026, 04:41 PM	#25
old31 Senior Member Join Date: Jan 2016 Posts: 2,686	Re: Leak down test and a compression test BK, I never noticed the blowby from the oil fill tube. I have a draft tube attached. I actually have a good boroscope, had not thought of it.