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MG MGA - Compression test

Have just finished a compleat rebiuld on a 1600 motor . Head , valves, valves guides rebore new pistons and rings the whole works. when setting up the timing . turning the engine over by hand , i checked the compression it was 40 lbs. .is this correct when you turn it over by hand or should i be pulling the head off? i checked all 4 cylinders and they were all the same. cheers mike
mgczenboy

Mike. This is a late "April Fool" joke, right? Not bad, but a poor spoof because it is unrealistic to suppose that anyone would believe that someone would actually turn an engine over by hand to do a compression check. Everyone knows that you need the engine to be powered by the starter for the compression test to work correctly. You could not turn the engine over rapidly enough by hand to get any form of useful reading. Good joke, but a little late and not as well crafted as I should like.

Les
Les Bengtson

Les This was no joke .I had just finished the rebuild and was about to put the engine back in the car when my friend asked me if i wanted a compression tester as he had just got a new one. So there i am engine in the stand new tool in my hands . Now i ask you what would you do . Got to see if this thing works right! So i try it out and i get 40lbs. Now i start to have second thoughts . Is the test right have i done somthing wrong on the rebuild .I could only think the worst . thanks for putting me straight ;] mike
mgczenboy

Mike. You need a fast rotation of the engine, as when being turned over on the starter, to properly fill the cylinders with air and make the compression check valid. The valves stay open a little after the piston is starting to come up and compress the fuel/air mixture. This is because once started flowing into the cylinder the fuel/air will continue to flow due to various fluid dynamic things that a physicist would understand, but the rest of us have to just accept. The net result is that more fuel air gets into the cylinder than happens when the engine is turned over by and and the air has the chance to be expelled throught those partly open intake valves.

As a result, there is less than a full cylinder full of air to be compressed into the combustion chambers when the engine is turned over by hand and this reflects itself as low compression readings when a compression gauge is used and the engine turned over by hand. There is relatively little fluid volume to compress and the readings are low.

Check you engine when you have it installed and the starter wired up. If you only see 40 psi then, it would be time to worry. But, you should see a much higher pressure when you do your testing.

Les
Les Bengtson

This thread was discussed between 11/07/2006 and 12/07/2006

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