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Re: AC C compression #

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Rod (NH)

08-17-2002 19:24:34

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Hi Smitty,

I think the compression ratio for a C is about 6:1. That would equate to an absolute maximum theoretical compression pressure of about 155 psig. It also depends a lot on how you conduct the compression test...the cranking speed and if the throttle is wide open or not. Higher cranking speeds (all plugs removed) and wide open throttle would give the highest test pressures. Plus it obviously depends on the amount of leakage past rings and valve seats...the greater the leakage, the lower the observed compression pressures.

What I am getting at is that there is no single number for any compression test that indicates "good" as opposed to "bad". I think it is more important that you compare the differences between cylinders rather than any absolute value of what the pressure "should" be. For example, a test might show about 100 psig in the cylinders with a variation of say 10 or 15 psig. I certainly would not tear the engine down with such readings. On the other hand, if all cylinders were less than 50 psig I think it would indicate that some major action would be advised, at least for best performance. More commonly, you would find one or more cylinders much less than others. This would indicate that something is amiss that is not attributable to general wear.

My '42 B has a compression ratio of 5:1, according to the I&T manual. The engine has never been apart and I have never checked the compression on it. I have not felt the need to. I occaisionally crank it by hand and it seems to have reasonable compression. Plus it runs well and has decent power. No need to try to put a number to what it "should" be.

Maybe someone with specific experience can give you a "band" of pressures that are considered acceptable for this tractor but I would not be concerned too much if all cylinders are some reasonable number and are within 10 or 15% of each other.


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