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I'm looking to buy a compression tester kit and am a little puzzled by the purpose of the two rubber-ended attachments shown below.

Am I right in assuming that these two attachments can be jammed into the spark plug hole without threading to perform a compression test? If so, wouldn't it blow out due to pressure build-up inside the cylinder?

Compression Tester Kit

The product description says:

This item is perfect for checking compression on petrol engine quickly and accurately. It diagnoses problems quickly with 2 rubber cones ends which do not need to thread into head. Our item is suitable for petrol engines on lawn mowers, motorbikes, cars, trucks and so on.

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You just hold the rubber ended bit against the plug hole whilst cranking the engine. No, it does not blow out of the hole due to pressure build up. This is how such a unit is designed to be used.

The different designs of tube allow use on different designs of engines, choose the one which feels most appropriate to the vehicle you are testing.

  • +1 for HOLD against the plug hole rather than jam it in the hole. – Mauro Dec 9 '15 at 14:56
  • Yea, you'd be surprised at how much pressure can be contained by just pushing a rubber stopper against an opening. – JPhi1618 Dec 9 '15 at 16:04
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    The compression on most non-competition engines rarely exceeds 12:1. 12 * 14 PSI / 2 (about half a square inch of area) works out to significantly less that 100 pounds, which most adults can generate by simply leaning on the stopper. – dotancohen Dec 9 '15 at 17:36

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