Need Help - Rebuilt Engine w/low compression

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Eric S
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Need Help - Rebuilt Engine w/low compression

#1

Post by Eric S » Fri Jul 03, 2009 9:29 pm

I recently purchased a '59 FLH with a rebuilt engine that has not been started. Other work has to be finished before it can be started with fuel. I decided to take a compression test on both cylinders, plugs out, throttle open, and it appears to only have 40 PSI on both holes. Looking through the spark plug hole the engine appears to be 60 over with flat top pistons. Is this normal for an unstarted panhead after engine rebuild? The individual who rebuilt the engine is not available to answer any questions.



RUBONE
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Re: Need Help - Rebuilt Engine w/low compression

#2

Post by RUBONE » Fri Jul 03, 2009 10:07 pm

The numbers you list are way to low. Try squirting some oil in the plug hole and see if the numbers change significantly. How long ago was the rebuild? Perhaps the rings are stuck in the pistons or the valves are not adjusted properly. You mention flat-top pistons. How flat? Panhead pistons all have a domed crown of varying height depending on the compression ratio. Never FLAT. Without being able to talk to the builder you are left in the gray-zone of multiple and questionable scenarios. If it were mine I would dig deeper to find out what was really done. Some peoples idea of a rebuild is a far cry from a correct job.
Good luck
Robbie

oldbob
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Re: Need Help - Rebuilt Engine w/low compression

#3

Post by oldbob » Fri Jul 03, 2009 11:25 pm

Sounds like some one put ULH pistons in it. They would lower compression also.

Robert

Eric S
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Re: Need Help - Rebuilt Engine w/low compression

#4

Post by Eric S » Sat Jul 04, 2009 12:23 am

Thanks for the replys. I shot oil in both holes and now it cranks to 85psi on both. I may be wrong about the pistons they appear to be flat-tops?? looking in the plug holes and you can clearly see the .060 overbore stamp but the could have a dome? Any help is appreciated.

steinauge
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Re: Need Help - Rebuilt Engine w/low compression

#5

Post by steinauge » Mon Jul 06, 2009 3:10 pm

I would do a leak down on it and if it checks OK start the engine and see how it acts.If it has evo pistons in it or some such(I have seen this twice!!)it will run,it just wont pull a greasy string out of a cats ass.Wont harm anything.(JMO-I hate doing any unneeded work).

Ohio-Rider
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Re: Need Help - Rebuilt Engine w/low compression

#6

Post by Ohio-Rider » Mon Jul 06, 2009 3:46 pm

A low compression test on a rebuilt, never started engine isn't anything I would worry about. After your break in period on the new engine i believe you will see a huge gain in compression. Things just need to wear in a bit. -Steve

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