Compression check procedure?

qzjrd5

qzjrd5

Member
Guys,

Gotta '32 B engine that I would like to perform a compression check on. I have the fitting to fit the spark plug hole and the compression gauge (thanks forever4). Should it be done with the engine warm/cold? What is the "allowable" variation between cylinders in pressure? Can I do it just by hand cranking or should it be done with the starter (does it matter)?

Any details would be helpful.

Thanks,
Mike
 
First : perform the test with FULL trotle
Second : Alway's use the starter.
Third : give each cylinder a few rev's before going to the next.

Temperature ? I always do it ( once a year ) hot and cold.

Diference between cylinders ? prefereble none.
2 to 4 pounds difference is alloweble,

Succes
 
According to my 1942 Motors Manual, the motor should be at normal operating temperature to allow all the parts to expand to normal running friction. Remove all plugs. With the choke open and the throttle open crank as many times as needed untill the gauge no longer rises. Repeat on the rest of the cylinders. They specify a 5 to 10 lb. variation as suspicious. Two adjacent cylinder with low compression could indicate a blown head gasket. Also if the compression is low in one cylinder, squirt some oil into the cylinder and see if the compression improves. If it does it is probably rings. If it doesn't it's probably valves. Hope this helps?
 
Compression Testing

Besides a good high reading, uniformity is very important. I don't like to see them vary on a Model A/B more than a few pounds, maybe 5 at the most. However, on modern engines according to Ford Motor Co. and others, the indicated compression is considered normal if the lowest reading cylinder is within 75% of the highest. This means that a Model A could have 75 pounds in one cylinder and 56 in another and still be considered normal. There's already enough shakin' going on for me!

My method to conduct the test is as follows:

1. Make sure the battery is hot and the oil is of the correct viscosity.

2. Run the engine until it is at full operating temperature. This means about 30 minutes of run time in my opinion at a pretty good RPM, say 1200 or so.

3. Turn the engine off and remove all 4 spark plugs.

4. Set the throttle and choke plate in the wide open position and make sure they stay that way throughout the test.

5. Pick a cylinder and install the compression gauge. I like to start with number 1.

6. Use the starter to crank the engine at least 5 pumping strokes and record the highest reading indicated on the gauge. Note the approximate number of compression strokes required to obtain the highest reading.

7. Repeat the check on each cylinder cranking the engine approximately the SAME NUMBER of compression strokes.

Large variations imply an improperly seated valve or worn or maybe broken rings. If one or more cylinders read real low, squirt approximately a tablespoon or so of engine oil on top of the pistons and repeat the test on these cylinders. Then look for the following:

*If compression improves considerably, the piston rings are at fault.

*If compression does not improve, valves are sticking or leaking.

*If 2 adjacent cyliners indicate low compression pressures and squirting oil on the pistons does not increase the compression, the cause may be the head gasket leaking between the cylinders.

Larry B.
 
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I would like to add something I learned the hard way to all the other good advice, if the plugs have not been disturbed for a long time there can be a carbon buildup around the plug base, when the plug is loosened it falls off and can get between the valve and seat causing a bad reading ---the cure is to loosen the plugs , then start the engine and rev it up a little to clear out any loosened debris before removing the plugs and doing the compression check.
 
kurt , thank you for that info ............. steve

i do mine when warn , not hot , normally about 5 compressions will work . probaly on this small motor , 3-4# different would be alot . mine are within 1 # ........... steve
 
"3-4# different would be alot"

You obviously have not run many tests.

Moreover, operating temperature is where the engine is when you're runnng it, not warm.

Larry B.
 
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