Compression Test Gear?

[Chrisc161076]

What tools are you using to complete the test? Any links to purchase in the UK?

How should l carry out the test?

The test will be on my featherweight. What psi should l expect to see when the piston or rings need replacing?

Thanks

[Horsley-Anarak]

I use one of these

http://www.halfords.com/webapp/wcs/stor ... yId_255216

You can pick them up at boot sales for a couple of quid.

Remove plug, screw in gauge, pull over 2 or 3 times, engine cold in as run condition, not oiled up for the test.

Compression should be over 100 psi on a good 40, I will be corrected if wrong

H-A

[Chrisc161076]

Great, when you say "as run" what do you mean?

[Horsley-Anarak]

When I say "as run" I mean no additional oil in the bore.

It is very easy to get a false high reading if the bore has additional or excess oil in it. Also the engine should be cold.

H-A

[Stelios_Rjk]

I have one very similar to the one on the link but it's Draper. It's essential to have the 18mm fitting. The directions on this tool mention that the engine should have run for about 10min to have reached the working temperature before measuring. Should I follow this or measure when cold?

Is there a chart showing min-max psi for each model?

Nice thread by the way.

[Horsley-Anarak]

Nice thread by the way.

Which one, 14mm or 18mm?

H-A

[Stelios_Rjk]

The topic, I meant the topic!

[phil]

If you test every motor cold and dry, you will have a baseline for comparison. That is if we are talking about vintage or older motors. If you test them after running that is fine especially for newer running motors, but it gives you no baseline or standard for comparison when testing older motors prior to buying.

As you build up a knowledge of typical cold and dry compression figures for various motors you then have the ability to assess non-running motors. After all many motors I buy are not in running condition. The price I will pay depends on how much work the motor needs.

By the way, for many the cold and dry method means pulling the motor over about 5 times or at least until there is no further rise in compression by the gauge.

When you shop for a gauge try to get one that has the compression release valve on or near the gauge. This is especially important for low volume motors such as the 64 cc model 40. Some compression gauges have the release at the end of a hose, in this case the hose becomes part of the combustion chamber which you are measuring. Also avoid those that do not screw in, to awkward to use, making it a two person job.
Those with more experience may be able to estimate acceptable compression just by pulling the motor over and feeling the compression " bounce" and the secondary much weaker crankcase compression.

[Charles uk]

Don't expect an accurate compression pressure reading on the SD, Marston, 170,125, or any of the QB motors as they all use a 3/4" reach spark plug, this type of compression gauge only has a 1/2" reach tip, giving an extra 0.9cc volume in the combustion chamber on a 14mm plug motor & about 1.4cc on an 18mm plug motor.

[Niander101]

This compression tester is ok for a novice but after a few years experience just turning the flywheel
you can feel whats good or not ...and the compression will have to be really bad for them not to run ...in other words they will run with far lower than normal compression

[Stelios_Rjk]

I can also understand whether an engine is worn or not by turning the flywheel. I would like to know in numbers how good a good engine is or how bad a bad engine is.

I will check the difference between cold and hot measurement on the same engine. That seems interesting to me.

[skyetoyman]

Nice thread by the way.

Which one, 14mm or 18mm?

Sometimes H-A , British humour gets lost in translation.

[Horsley-Anarak]

Those with more experience may be able to estimate acceptable compression just by pulling the motor over and feeling the compression " bounce" and the secondary much weaker crankcase compression.

How does that work on a twin, when you are looking for weak cylinder?

H-A

[Niander101]

Those with more experience may be able to estimate acceptable compression just by pulling the motor over and feeling the compression " bounce" and the secondary much weaker crankcase compression.

Actually I know what you mean but i think you'll find that secondary is actually the vacuum as the piston moves down away from cylinder head
doesn't start compressing in the crankcase till piston is lower in cylinder.

[Horsley-Anarak]

I did not mean the "secondary" was the second cylinder, I meant that without a pressure gauge it would be hard using the bounce test to see if both cylinders in a twin were in good condition, or similar state.

H-A