Best way to do a Compression Test ?

[DB1380]

Morning all, can anyone tell me the best way to do a Compression Test ? (I have a Com Tester)

Read a few different ways of doing it....just want to make sure. Searched on here but nothing really comes up.

Cheers. Dicky.

[Thorp]

Easiest method is to take the plugs out, screw in the compression tester and spin it on the starter - assuming the engine is in situ. Record the numbers and move the tester across the cylinders and repeat.

If the engine has been sitting for ages, this may not be wholly accurate as the bores may be dry etc.

I'm sure others will share their views.

[JohnA]

Do it with the throttle open

[Exminiman]

If the engine has been sitting for ages, this may not be wholly accurate as the bores may be dry etc.

Ditto above , rings may be stuck etc... be wary of a low reading on an engine that has been sitting for long time....it might improve after its started or even spun round on starter motor...

differing opinions on whether to put oil down bores...

[AndyPen]

I usually put oil down the day before to let it soak. I have an old plant sprayer that makes sure it gets thinly spread everywhere

On the day I do it twice, one with and one without oil - as can show up big differences

[bwaminispeed]

Absolute 1st step is to set the valves.....

Open throttle (you can't compress air if it can't get in), I also block the dash pots open...

Pull all the plugs...

I usually do a dry, then a wet...If the engine will run, I also do cold and hot test....

Also good idea to disconnect the coil....

[DB1380]

Cheers everyone, fantastic.

I had it running yesterday, engine was not too bad, so just checking a few bits n pieces.

Dick.

[Spider]

Ideally, the engine and oil needs to be at operating temperature.

Then as the other guys have said, do a dry test, jot down the numbers, then do a wet (oiled) test jot them down and compare. Any big differences here may indicate a ring and / or bore issue.

All the same, this is not a test I place much faith in as it does not replicate running conditions. Likewise, the Leakdown Test is even more useless as it's a static test, at least with a compression test the engine is being turned over.