VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Compresion readings for the 207

I am fiddling with my engine prior to starting it up to decide whether I put it in like this or whether it will need help. The first test I did was the compression. It shows 60 dry and about 90 with a bit of oil. My searches of this forum showed 75 as good for a '28 and a bit low for a '49. This motor hasn't run in several years.

First question: What is the difference between dry and with oil which indicates a need for rings?
Second question: How much turning of the engine will the starter tolerate from a 12 volt battery? That's what I was using for my tests.

If you put oil in it and it reads better thats normal. What you should do, is a leak down test. Make sure the gauge is on proper, and see what the reading is after a half hour. If it hasn't moved much, your in the clear. Being that the motor hasn't been active in awhile, I would put oil in all the cylinders, and rotate it abit, then do the test.

Also, you are looking for Uniformity amongst cylinders, as opposed to 6 specific different #'s

As for the starter,


6 volts draws double the amperage as a 12v. The windings in the starter are heavy. As long as you don't crank for an hour strait without cool down, and the cables that go to the battery are up to task, you should be set.

Pat,
You mentioned a 1949 engine? The 1949 engines ran from 110 to 120 pounds.
Your 1934 Master 207 engine should have about 90 pounds at 6 volt cranking speed. At the higher 6 V starter and 12 battery cranking speed it would possibly be higher.
The oil seals the rings and helps to boost the reading. On an engine that has not been run for a period of time you could have a light coat of rust on the cylinder walls or stuck compression rings,, both of which would give lower readings.
With the plugs removed there is not much load on the starter so that should be no problem, with in reason.

What can I do do loosen up the rings short of opening the engine? I will also do the leakdown test suggested above.

Fogging oil has unstuck many seized motors for me. Let it sit for at least a half hour.

You could pour some MMO or ATF on top of each piston and let soak for a day. Also remove valve cover and squirt some om the valve stems.When was this engine last run? Do you have any history on it?

This morning It was going up to 100. So the oil from two days ago and whatever I put down the valve stems probably helped.
This is an engine I got from a guy who hotrodded his car. All I could get out of him is that he drove it about 6 times before he decided he wanted a V8. I couldn't find out how long ago it had been rebuilt. Any way, he stored it right with all the orifices sealed and he kept turning it over. I was right full of oil and very clean.

As far as the leakdown test I left the gauge on one of them for a while and was happy that it wasn't going down when the light came on. It was the checkvalve in the compression gauge that was holding the pressure and not the piston and valves. I suppose the leakdown test suggested by Stylemater would have to be done with compressed air injected into a cylinder at TDC on the cxompression stroke. Is that correct?

I do it with a hand held vacuum pump with a gauge. Make sure both valves are closed.

When a cylinder leakage test is done with a leakage tester compressed air is blown into the cylinder. This will force the piston down and both valve will also be closed. If it leaks air out of the tail pipe the exhaust valves are bad, if out of the carburetor the intakes are bad, if out of the road draft tube the rings are bad.

That's right. I was reading in my "Motor Manual" about the same thing. They suggested breaking the ceramic off an old spark plug and brazing a tire valve on it to inject the air. I guess they had valves stems you could braze back then. Now as far as I know they're all rubber.

KD tools makes a fittting that threads into the spark plug hole and a air hose can be attached. It called an air hold.
You can also braze an air fitting to the spark plug, just using the metal part of the spark plug.

My son-in-law suggested loosening the rocker arm bolts just enough to release pressure on all the valves, then they should all be closed. Would that work easier than trying to figure if each individual cylinder's valves are closed or not?

How would you draw in the air to compress?

Only an option for the leak-down test. If taking compression measurement then you will get zero if the valves are both closed.

When the air pressure pushes the piston down it will get to where the valves are closed and lock-up and the engine will stop turning and stay at that point. Then you can check for leakage. Could be done without using a leak tester to get a rough idea of leakage.

What a wonderful diagnostic tool this air hold is. I has\d a wheeze in the crankcase everywhere indicating an issue with the rings (which confirms why my compression tests were much better after some oil), then on one cylinder I had a definite rush of air out of the Exhaust manifold. I guess the writing is on the wall, I'm going to have to open her up and resolve those issues.

In a long run you will be glad you did. Often these so called rebuilt engines had little more than some gaskets replaced.

The air comes from a remote compressor.
Also if it blows bubbles in the radiator the head gasket has let go or cracked head.
I set the cylinder on TDC compression put trans in high gear with park brake applied to stop the air pressure forcing the piston down. DONT put your hands near ANY moving part while the air is connected.
Tony

Hi Tony,

I'm doing this with the engine on a dolly. Thanks for the warning. Safety is always best.

You'd better have a good engine dolly. They get really tipsy once the motor has got a head on it.

If you are doing it on a dolly once you get each cylinder to ignition point lock the flywheel to the block before applying the air pressure.
Safety is most important. It would be rather difficult to fix your Chev without fingers or more.
Tony

Stylemaster & Tony,

Have a look at the pictures captioned Dolly/Setup. It ia actually quite stabe. You can't even rock it. The only part I touch is the air hose.