best way to check cylinder compression

[marc silva]

What is the best way to do a compression check? After the engine has been warmed up and the plugs out or cold with the plugs out? Thanks

[SeaSlugs]

i think cold with all the plugs out. if any seem odd dump about 1/4 cup of oil down each cyl and run the tests again.

[Jim Johnston]

I would suggest a thoroughly warm engine. Make sure the throttle is wide open during the test to insure complete cylinder filling. If you get a low cylinder you can do the oil in that cylinder as suggested by SeaSlugs. If the reading improves with the addition of oil in the cylinder it indicates the rings may be worn or not seated.

[BRENT in 10-uh-C]

Originally Posted by marc silva

What is the best way to do a compression check? After the engine has been warmed up and the plugs out or cold with the plugs out? Thanks

What are you trying to achieve? Is this to diagnose an ailment, or just for something to do? Most good mechanics will tell you that compression gauges today are really cheap (i.e.: inaccurate) so they really are lacking in accurate diagnosis.

[Mitch//pa]

sometimes i have to check compression on a car that wont start so then its cold. a leak down is a much better test on things IMO. opening the throttle never made a noticeable difference when doing a comp test even with a good gauge.

[TDO]

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I do a compression check cold and warmed up for comparison. I do not open up the throttle. I also only take out one plug at a time.

[Patrick L.]

Remove all the sparklers and test cold. Its important to check the very first needle stroke and then the last. Warm it up and do it again. I don't worry about the throttle plate position on something like an 'A'. Compare readings between cylinders, should be fairly close to each other. You could then do the oil thing if needed. I don't worry much about how high the readings are as all gauges are not created equal, there will be differences between gauges.
If you have an air chuck adapter type fitting, then you could perform a leakage test also or if there is a problem.

[Bill Goddard]

Exactly how do you do a leak down test on a Model A. I am thinking of a differential compression test done on an aircraft piston engine where you move the propeller back and forth during the test. How do you do this on a Model A? You can't move the crankshaft back and forth. Bill G

[Patrick L.]

Originally Posted by Bill Goddard

Exactly how do you do a leak down test on a Model A. I am thinking of a differential compression test done on an aircraft piston engine where you move the propeller back and forth during the test. How do you do this on a Model A? You can't move the crankshaft back and forth. Bill G

Easy. You'll need a way to get air into the cylinder. All I do is set the piston near TDC on compression, stick it in high gear and set the parking brake. Adjust the air about 35-40# and put in the cylinder. Then as in any similar test, just listen for the air escaping. Easier than a plane.

[marc silva]

Thanks I just want to see what the motor has for compression as it is tired. My compression guage is a snap on so I don't know if it is any better than any other quality wise. It definitely costs more , I hope that means it is better than a Harbor Freight or some cheap auto parts store junk.

[Patrick L.]

Originally Posted by marc silva

Thanks I just want to see what the motor has for compression as it is tired. My compression guage is a snap on so I don't know if it is any better than any other quality wise. It definitely costs more , I hope that means it is better than a Harbor Freight or some cheap auto parts store junk.

Well, a Snap-On is an expensive gauge and should be a good one. What do mean by tired ? Is it leaking or smoking ?
Mine shows a grand total compression of 45# on a 50+ year old engine rebuild. So its really getting tired, but, It doesn't leak or burn anything and still has enough power for what I do.

[Bill Goddard]

Originally Posted by Patrick L.

Easy. You'll need a way to get air into the cylinder. All I do is set the piston near TDC on compression, stick it in high gear and set the parking brake. Adjust the air about 35-40# and put in the cylinder. Then as in any similar test, just listen for the air escaping. Easier than a plane.

Do you put the nozzle into the sparkpug hole or into a screw in fitting in the sparkplug hole? Do you need a stethascope to hear the leakage?
Sorry to be so dumb about this. Bill G

[Patrick L.]

Originally Posted by Bill Goddard

Do you put the nozzle into the sparkpug hole or into a screw in fitting in the sparkplug hole? Do you need a stethascope to hear the leakage?
Sorry to be so dumb about this. Bill G

Most compression gauges have the spark plug hole fitting with an air chuck adapter. But, just as long as you have something that screws in the plug hole an air line can be rigged to fit it. I never needed a stethoscope, your ear will work fine. If there is a problem you'll notice it at the oil filler, exhaust pipe, the intake or possibly the radiator.

[jkeesey]

Most modern comp gauges do not fit the Model A spark plug hole. You can buy the adapter used for installing the 14mm plugs into the A and thread your gauge into that. I do not trust the nipple ended gauges, only the thread in type.

[BILL WILLIAMSON]

Well, Marc, to answer your original question, here's the way I have ALWAYS done it:
1 Engine FULLY warm.
2 Throttle WIDE open.
3 Battery FULLY charged.
4 Crank it at least 10 strokes, or until guage no longer rises.
5 If one is low, retest it several times, as carbon may be stuck under a valve from taking out the plugs.
I've used the same old inexpensive hand held guage for 35 years & checking it with a pressure regulator, it's STILL RIGHT ON. I've had to replace the valve core a time or two. And it's rubber dooley is BIG enough fur a Model A.
You only need a leakdown tester when your basic test shows a puny cylinder, that will tell you WHERE the problem RESIDES. Keep it simple, it AIN'T ROCKET SCIENCE or MEDICAL RESEARCH or ATOM SPLITTING! Bill W.

[Patrick L.]

Originally Posted by BILL WILLIAMSON

Well, Marc, to answer your original question, here's the way I have ALWAYS done it:
1 Engine FULLY warm.
2 Throttle WIDE open.
3 Battery FULLY charged.
4 Crank it at least 10 strokes, or until guage no longer rises.
5 If one is low, retest it several times, as carbon may be stuck under a valve from taking out the plugs.
I've used the same old inexpensive hand held guage for 35 years & checking it with a pressure regulator, it's STILL RIGHT ON. I've had to replace the valve core a time or two. And it's rubber dooley is BIG enough fur a Model A.
You only need a leakdown tester when your basic test shows a puny cylinder, that will tell you WHERE the problem RESIDES. Keep it simple, it AIN'T ROCKET SCIENCE or MEDICAL RESEARCH or ATOM SPLITTING! Bill W.

Bill, your compression gauge is only 35 year old !? Thats still just a 'pup'!

[marc silva]

Originally Posted by BILL WILLIAMSON

Well, Marc, to answer your original question, here's the way I have ALWAYS done it:
1 Engine FULLY warm.
2 Throttle WIDE open.
3 Battery FULLY charged.
4 Crank it at least 10 strokes, or until guage no longer rises.
5 If one is low, retest it several times, as carbon may be stuck under a valve from taking out the plugs.
I've used the same old inexpensive hand held guage for 35 years & checking it with a pressure regulator, it's STILL RIGHT ON. I've had to replace the valve core a time or two. And it's rubber dooley is BIG enough fur a Model A.
You only need a leakdown tester when your basic test shows a puny cylinder, that will tell you WHERE the problem RESIDES. Keep it simple, it AIN'T ROCKET SCIENCE or MEDICAL RESEARCH or ATOM SPLITTING! Bill W.

Thanks Bill. I agree it isn't rocket science. I have built a few engines over the past 35 years, Harley Davidsons, modern Fords, small block Chevys not to mention a couple V.W.'s, lawnmowers and even a few golf carts. The model A thing is new to me and because of the little details that are different than a modern , I like to ask questions. The good part of that is , it always turns out that I am second guessing myself for nothing. Marc