Compression Testing

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Ken I

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Oct 31, 2010
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Cape Town, South Africa
In response to a thread on troubleshooting compression problems, I thought I would share this tip which I use on full size engines (I haven't tried it on a model yet) - but the principal is the same.

Very useful where access is tight / restricted / small.

Make an airline fitting to fit into the sparkplug hole, set the engine on TDC (firing) and then apply compressed air pressure via the sparkplug hole from your compressor. (If the engine is exactly on TDC it won't move - regardless of the pressure.)

By listening at the inlet, exhaust and crankcase breather you can hear how much the valves or piston sealing is the problem.

It doesn't help you fix the problem but it helps to steer you in the right direction.

Over the years I have built up a collection including long extension type for those engines with deeply buried plugs.

By measuring the decay time for the pressure in the air receiver you can even calculate the leak rate or use it comparatively against another engine or cylinder or to determine if your repairs are better / worse than before.

If you blank off the inlet and exhausts you can determine the piston bypass and by a process of removing the cover plates, determine the leak rates for each.

Regards - Ken
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Just be extremely careful doing this as knocking the crank off TDC could result in rapid rotation of the crank and anything attached to it.