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Compression #'s


Jedi Hopeful
I did a compression check on my tr3a:

#1-120lbs., #2-120lbs., #3-58lbs., #4-90lbs.

What can I deduce from these numbers.
The car runs great by the way.
Last edited:


Jedi Warrior
#1 and #2 are healthy... #3 is sick and #4 is not so great.... Depending on how you performed the test.

Geo Hahn

Country flag
A leak down test will get more specific about where you are losing compression (e.g. valves, rings, head gasket).

You can do another compression test with a spoonful of oil dumped in the cylinder --if that markedly improves things then suspect the rings.

Have you adjusted the valves lately?


Senior Member
Country flag
Did you use a "screw it into the plug hole" gauge and hold the throttles full open during the test. The rubber bushed "hold it in the hole" gauges are not very good. As far out as 3 is, if your test is accurate, it should be running like crap. As George says, a shot of oil will seal the rings for the comp. test.


Jedi Hopeful
Thanks all.
The compression tester is
the screw in variety. The engine
wasn't completely warmed up and I did put some oil
in the low compression cylinder without
much improvement.
I didn't have the engine running during the test.
And like I said, it runs spectacularly. I decided to see where a 54 year
old engine stood after years of abuse.
I've not adjusted the valves but maybe I'll look into it.
Again thanks all.


Darth Vader
Country flag
You would want to turn the engine over the same number of times for each cylinder, 5 or so should do it.

Absolutely adjust the valves.
Last edited:


Country flag
I am not surprised that it still runs good, the TR 4 cylinder is a good motor, I had times when mine lost spark in one cylinder and still pulled strong, ran pretty rough, but not so bad as you might expect. Ditto on the adjust the valves, if that doesn't fix things you are probably due for a rebuild at some point, but they can often run on and on even when severely worn, my old 4A was burning about a quart for every tank of gas before I rebuilt it, and still ran pretty well (but ran even better after the rebuild).


Jedi Warrior
All plugs out or leaving the remaining three in during test? There seems to be a lot of differing opinions on this. I've always taken all the plugs out. Can't see how leaving the other three in can influence compression. It's also a lot easier to spin the motor with all out.
FWIW All my cyclinders are 150 -155.


Great Pumpkin - R.I.P
Leaving the other plugs in will slow down the starter motor; it turns much faster with only one cylinder coming up on compression than with all 4 doing so. And when you read low compression, you are basically comparing how fast the piston is compressing the air to how fast the air leaks back out of the cylinder. Any change in cranking speed will make a difference when you are looking at a bad cylinder.

That said, as long as the method you use is consistent from cylinder to cylinder, and all you are doing is comparing the relative readings (which is all you should look at), then the exact method, gauge, etc. is not so important. Personally, I always take all the plugs out; but I have never seen any difference at all with the throttle(s) closed or open, so I just leave them alone. I also don't worry about trying to count revolutions, just keep spinning it until the reading doesn't change.

The squirt of oil is a valid test, but note that the pressure will always come up some. What you are looking for is a big change in reading, which implies the air was going past the rings. The oil will temporarily plug up the leak in the rings and hence you'll get closer to a normal compression reading.

With my compression gauge (which is the screw-in type), it is easy to unscrew the hose from the gauge and put an air fitting on the hose. I also have to remove the Schrader valve core from the piece that screws into the engine. Then I can use air pressure to pressurize the cylinder and listen for where the air is escaping. If the hissing is loudest in the exhaust, that means a bad exhaust valve. Loudest in the rocker cover indicates bad rings (or bad cylinder wall, broken rings, broken piston, etc). And you can listen in the adjacent cylinder (#4 in this case) to see if you are actually looking at a blown head gasket (which would be my first guess from those numbers). This is essentially the same test as with a leak-down tester (but without having to buy the tester).


Jedi Warrior
Here is a good explanation of how to adjust the valves: https://www.macysgarage.com/myweb6/valve_adjustment.htm
It is a pretty simple to do and won't take long once you understand it.

Good article. The cam dwell on the "closed valve (rule of nine)" is well ove 180 degrees. No need to be too precise on locating exact cam position. As long as the compliment of 9 is"visibly" open , you can be sure the other valve is closed. I don't turn the engine over by rotating the fan blades. Just bump the starter solenoid a couple of times from under the hood... bonnet.

Geo Hahn

Country flag
...With my compression gauge (which is the screw-in type), it is easy to unscrew the hose from the gauge and put an air fitting on the hose. I also have to remove the Schrader valve core from the piece that screws into the engine. Then I can use air pressure to pressurize the cylinder and listen for where the air is escaping...

It case it isn't obvious, you probably want to do this with the subject cylinder at TDC on the compreesion stroke so both valves are closed. Also have the gearbox in gear and the handbrake set.

I once did this to confirm a blown headgasket. Set up as described and streched a latex product from Walgreens over the neck of the radiator (a rubber glove would work too). When I applied the air pessure to the cylinder the 'balloon' inflated. Quite a sight.
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