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401 compression help needed


Todd H
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the compression on all piston except for #5 is 200psi. # 5 piston is 45psi. does any one know if its a bad piston, valves or rings... also a lot of air is being pumped into the crank case from whatever this problem is... if its a bad piston can i replace it with the motor in the car????? any info greatly appreciated... thanks Todd

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Todd,<P>I would try a few squirts of oil in the cylinder first to see if that improves the compression. It is a ring problem if the oil improves the compression. <P>Take the valve cover off if the oil doesn't do the trick. Have someone crank it over slowly while you watch the valves for that cylinder. They should go as high or low as the valves beside them. They should be about level with surrounding valves when closed. The rockers should be tight on the valve stem when the valve is closed. Make sure the rocker tips are in good order. I don't think it would smoke much if it was the valves. <P>That leaves the pistons if it is not the valves or rings. The ring lands can give out in the pistons. This has happened to my car. The blow by was so bad that it would push the dipstick up and off the seat. It smoked pretty well too.<P>You can remove the pistons in the car, but it is not easy. #5 is on the passenger side on the nailhead so there is no interference problem removing the head. The engine has to be lifted an inch or two to remove the pan. In my opinion, it would be easier to remove the engine and work on it off a stand.<p>[ 12-12-2001: Message edited by: Steve B ]

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thanks for the reply.... the guy i bought my 1964 wildcat convertable from 2 years ago said he had the motor rebuilt 20000 miles ago. it seems to run ok...the reason i started checking for trouble was the oil and air pumping out of all the hole in the block.(dip stick, oil filler etc). I tried oil on a compression check and there was no improvment, so i guess its time to pull a head or the whole motro..... thanks again... Todd

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If you've got air & oil coming out, it's caused by 'blowby' which is what happen when the piston rings fail. Instead of the air/fuel staying on top of the piston during the compression stroke, it escapes past the rings into the crankcase. The higher pressure inside the engine is trying to find its way out, via the dipstick tube & valve cover breather.<P>That would also explain the low cyl pressure in #5...the rings are worn or broken...in the extreme, the piston could be damaged too.

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