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Compression Test kit


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I just bought a moderately priced kit with a 300psi  gauge fitted with a quick connect,   two hand held plug probes, a hose and a range of spark plug adapters- the common ebay model in a red carry box.

 Compression tests are comparative things and a really good, accurate and expensive  gauge is not needed. That said, I would have preferred  a second hand Snap-On set if I could have found a reasonably priced one in time.

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This compression test gauge is basic, and all you really need- for a Triumph engine. For over-head cam engines you would need a longer reach gauge.

You will get your best reading if the engine has been run until normal operating temperature is reached; but not essential to do that,  just better.

Remove all spark plugs.

While you hold the gauge tightly against the plug opening, have an assistant crank the engine with throttle pedal to the floor.

After the gauge needle stops increasing, write down that cylinders reading.

Move to the next cylinder, repeat.

If you are unsure that the reading you got is accurate, simply repeat for that cylinder.

After this dry test, you could perform a wet test- in which you pour a tablespoon of engine oil into each cylinder, in turn, in hopes of getting a higher reading.

The purpose of the wet test is to check if the engines piston rings are weak- thus if the reading is increased by sealing the rings with oil, you will know the rings are getting blow-by.

Most important is to learn that all readings are relatively close in PSI. Large variations will indicate a problem.

 

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Edited by Sapphire72
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I like the gauge sets with the hose and spark plug adaptors- that way you can leave them screwed in while you operate the engine and operate them long enough for the pressure reading to level out. Also makes it a bit easier to take WOT readings.

I really like Snap-On tools because I like high quality tools- even if I only use them once every few years. But  Snap-On stuff is always hard to fine at reasonable prices.

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Apologies Jon, I don't want to hijack this thread but its related. Does anyone know where to get a schrager valve for a Gunson compression gauge? I bought the gauge at Stoneleigh last year for £20. Used but in its original packaging and was told it worked but it doesn't because the spring on the valve is broken. I'm told that ordinary tyre valves do give a correct reading.

Thanks

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33 minutes ago, SeanF said:

Apologies Jon, I don't want to hijack this thread but its related. Does anyone know where to get a schrager valve for a Gunson compression gauge? I bought the gauge at Stoneleigh last year for £20. Used but in its original packaging and was told it worked but it doesn't because the spring on the valve is broken. I'm told that ordinary tyre valves do give a correct reading.

Thanks

Pop to your local tyre fitter they will probably give you one. 

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1 hour ago, Hamish said:

Pop to your local tyre fitter they will probably give you one. 

Thanks Hamish. I've read that ordinary tyre valves do the job properly though, that they do not give a proper reading. Is this not correct?

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As compression test are more about comparing cylinders to each other (I know overal pressure is good to know)  it’s worth a go surely 

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Like Nigel I've used a Gunson gauge for many years ,it has a reasonably long hose and the screw in adaptor make for easy single handed operation.

As others have said unless your going into the realms of specialist engine builds all you really need to know is a rough guide to overall compression  the comparison between cylinders is more important.

Brian

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