home made cylinder pressure tester

[FRANK PKNY]

This is a home made pressure tester , I needed to see if I could find where a leak may be occurring in my Model A. Start with an old spark-plug punch out the white insulator, then drill the opening out to get a clean hole, take a 18mm anti fouling adapter for 18mm spark plugs (NAPA) cheap. braze it to the old plug. install a T fitting one side a tank pressure inlet the other side any kind of pressure gauge. I like the larger dial so It's easy to see! (old eyes). Thinking of starting at 100lbs at the compression stroke , use a wire to get the TDC of each cylinder. Easy to remember firing order 1243 two up two down on the crank shaft. You will know if your wrong if you get no build up of pressure your on the wrong stroke. Mine was made from fittings I had in my junk drawer. The gauge I had for testing other vehicles. the plug was an old one, I'm old that's why my junk drawer has so many fittings in it, I never throw any good stuff away! LOL Test as of yet not done, will report when I think I have a reasonable diagnosis . Frank PKNY

[whirnot]

I have one that I made over forty years ago to do cylinder leakage tests. Just knocked out the insulator and welded an air fitting to the top. Then adjust your air pressure at the compressor. Works great.

[Patrick L.]

If a piston isn't quite at TDC the engine may want to roll over. The piston doesn't really have to be exactly at TDC for the test. Putting the car in gear and chocking the wheels [ or using the hand brake as Tom has mentioned] will keep the engine put.
Also, 100 lbs shouldn't be needed. But, thats your call.

[James Rogers]

Why not just buy one of these?
http://www.ebay.com/itm/ENGINE-CYLIN..._Tools&vxp=mtr

[MikeK]

There is a huge difference between an actual leakdown tester, as linked by James Rogers, and a simple adapter to pressurize a cylinder.

If you want an actual measurement versus a calibrated orifice, and means to quantitatively compare cylinders and engines the true leakdown tester is a great tool.

If you just want to pressurize a cylinder and listen at the carb, oil filler tube, tailpipe and radiator filler for gross problems, then a simple adapter as posted will work.

[Fordors]

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MikeK is correct. Most leakdown testers will have a .040 orifice, but some manufacturers use different sizes. The size of the orifice has an effect on the percentage of leak seen on the gauge, so different leakdown testers may show different results on the same engine.
On a race car I was involved with we used a long handled ratchet to bar over the engine to TDC. Be advised that it's wise to take the tool off upon setting TDC, if you are just a hair off the air pressure could push the piston down with the ratchet striking you or the car.

[John S]

Here is a pretty good explanation of both a compression check and a leakdown test and what each one does:
Every decent maintenance schedule will have you doing a compression check every 15,000 or 30,000 miles. One way to tell engine wear & health. But the old compression tester may be on the way out.


Most better mechanics now have a "leakdown" tester. I once worked with a factory race team whose engine man didn't even own a compression tester. With 4 straight 24-hours of Daytona class wins, and a number of national and world championships or records with his engines, I tend to listen. He uses a leakdown tester exclusively.

The principle is the same. A compression tester relies on the pumping action of the engine to build up pressure in the cylinder. A gauge with a valve measures the cumulative pressure, giving an indication of how good the engine's internal seal is. A low cylinder may have a burned valve, blown gasket or scored cylinder. All low compression probably means worn out rings.

The problem is that it's difficult to get accurate and consistent readings. Engine & oil temperature affect the seal, as does whether the throttle is wide open (preferred method) or not. Cranking speed, how many cycles cranked, and even atmospheric pressure and temperature affect the readings. Getting the same readings a few days apart or with a second gauge is impossible. Consider it a one-time snap-shot.

A leakdown tester basically does the same, except it relies on a continuous supply of compressed air. The air is regulated, typically 50-100 pounds depending on instrument manufacturer, and then goes through a calibrated orifice in the tester, past a gauge, and into the spark plug hole. The higher the compressed air loss, the lower the leakage gauge will read. Better gauge sets will have two gauges -- inlet and outlet; other's (economy models) will rely on one.

With a two-gauge set, 5% leakage of the psi applied is excellent -- there has to be some leakage around the ring gaps. 10% is still pretty good. At about 20% you should start thinking why, where & are repairs due. One-gauge sets, though they may have percentage numbers on them, are more of a Good-Warning-Bad measure (Milton Industries is even colored green-yellow-red) as they can't measure the actually air drop in PSI. The gauge manual will give you reading interpretation.

Unlike a compression gauge, the leakdown test helps diagnose the problem. Using a mechanic's stethoscope (without probe) a leak at the tail pipe indicates exhaust valve leakage. At the FI or carburetor air supply, an intake valve leak. At an adjacent spark-plug hole or to the edge of the head, a blown head gasket. Gurgling back through the radiator can indicate blown head gasket or cylinder wall crack. There will always be some through the oil dip-stick (ring leakage) but excessive hissing there indicates worn rings or cylinder wall leakage.

Performing a leakdown test differs from a compression test in that you test each cylinder at TDC, much in the way you adjust valves. Remove all plugs; turn the engine by hand to #1 TDC; insert plug adapter and apply shop air per gauge manufacturer's instruction. Record reading. Rotate engine to TDC on the next cylinder in the firing order and repeat until all are done. A nicety of the leakdown test is it can be done with the engine out of the vehicle.

My maintenance log has a place for both compression & leakdown readings. [I'm not quite ready to chuck the compression gauge yet! A running record of both can tell you a lot about where you're at in the engine's life cycle, and warn of impending problems. Like a compression check, there should be only minimal difference between cylinders.

[Fordors]

Here is a diagram for one I put together years ago, the .040 orifice was just a piece of aluminum to plug the pipe and then I drilled the hole. The 1/4 NPT hose end was for late model tapered seat plugs, naturally you need what suits your application.
I put mine together because I had the components laying around, and also I enjoy doing "shop made" tools.

[FRANK PKNY]

Changed the configuration and added a .040 orifice, set the compressor regulator at 50lbs and made a comparison of each cyl found the problem one .Replace and repaired all is good n

[Bob C]

And the problem was??????

Bob

Never mind, I found the answer in this thread http://www.fordbarn.com/forum/showthread.php?t=157023
Hard to follow with three threads going at once.

[Chuck Sea/Tac]

What's the orfice for? To control/stabilize the airflow rate?

[MikeK]

Originally Posted by Chuck Sea/Tac

What's the orfice for? To control/stabilize the airflow rate?

The orifice is a flow reference, standardized at 0.040 by the FAA for aircraft engines under 1000 cubic inches. If a cylinder has leakage equal to the orifice, then the gauge, being between the leaking cyl and the orifice will indicate exactly half (50%) the regulated inlet pressure. Actual leakage % readings are really arbitrary numbers with no units. Their function is strictly comparative.

If you make your own and want it to match readings from other FAA approved leakdown gauges for spec book comparisons then you can't drill just any old 0.040 hole. The flow needs to be non-turbulent and laminar. The length of the hole and the entrance/exit chamfers figure into the actual flow dynamics. Just like carb jets, two with the same hole size may not flow the same rate. All orifice geometry must match exactly for a homemade device to have the same differential pressure drop slope as the FAA standard.

[Fordors]

All you want are comparative readings on a simple automotive engine and the l/d tester will do that just fine. At the low prices many testers sell for I doubt the geometry of the orifice has been engineered, much less checked, simply because they need not apply to a rigid standard set by the FAA.
Say most of the cylinders leak at 8-12 psi but one shows you 20, that one is suspect and then you could listen at the intake, exhaust or oil filler for the sound of air flow. Mine always did what we expected of it and did it cheaply to boot, naturally your mileage may vary.

[FRANK PKNY]

My home made tester found the problem cylinder spot on. I used 50lbs at the compressor and found a reading of 20lbs steady, one cyl that had the leak was 10lbs. In this situation you only are making a comparison between cylinders.

[ABento]

For conversation on this subject I'm working on a 312, it ran like a top,started quickly all the time and ran as smooth as could be, but had a lot of oil coming from the blow by tube,about a 3" puddle on the shop floor after running for a few minutes,so I ran a compression test 150 #'s in all holes except 2 and they were 145 so I jury rigged a leak down set up with just a paint gun regulator,I figured I had bad rings but wanted to check,once I had it all plugged in and started to turn the regulator on I had air blowing into the crankcase as soon as it turned on I ran it up to 50 #'s just to see if it would seal up and nothing,I checked 4 out of 8 and all were the same,when it got torn down we found that it looked like the walls were washed clean. My question is how come it will show high compression and then not hold a drop of air on a leak down.

[MikeK]

Originally Posted by ABento

. . . My question is how come it will show high compression and then not hold a drop of air on a leak down.

Compression readings are instantaneous peak readings. Cranking speed, cam timing, and valve lash all proportionally influence compression readings. The leakdown test is not influenced by these factors. A leakdown test shows leakage during fixed, sustained pressure.

An engine that shows equal, fairly high (for the given C.R.) compression readings across all cylinders may fail a leakdown by showing only 65% across all. It is for this reason that a leakdown tester fairly close to the standard should be used. With a homemade tester with an effectively 0.036 orifice the same engine may show 80% (all OK) despite every exhaust valve seat being pitted and ready to fail.

If you want no external comparison to cylinder fitness, use any old homemade, uncalibrated tester. There is a place for homemade tools and a place for calibrated measuring devices. I could easily make my own micrometers, pressure gauges, volt and amp meters, and torque wrenches. I choose not to do so for one simple reason: ease of comparison to industry standardized data.

[ABento]

Thanks Mike,your answer is kind of what I pictured but wanted to throw this out to see what it generated.
Percentage of leak would have been Ok should this engine have held any air in the cylinder,but what went in came right back out.