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kdjose
05-07-2007, 10:44 AM
Hello Oh Revered Mechanical gurus. (I am not). I have been looking seriously at a boat that has rebuilt heads due to an overheat when the hose came off the water intake. apparently runs great and had made a deal, and was on my way to pick up while the owner had a compression test done at my request. All cylinders were good (110-130 psf) except for #3 which was 70. The mechanic who rebuilt the heads says that sometimes you need to change out a push rod for a shorter one since material is being removed during the rebuild. Both the Seller and this mechanic are clearly trustworthy, however I asked a friend who is a very astute mechanic locally whether this sounded right and he said absolutely not -- best case when there is low compression is bad ring, worst is crack in the block.

Thought I'd submit it to group for consideration -- is there something I'm missing here? Thanks.

kj:confused:

wesgardner
05-07-2007, 11:12 AM
Head gasket could be faulty - this is another "best case" scenario...

Seems if you have roller rockers and/or a "race oriented" engine build, push rod length comes into play...so both answers MAY be correct, if your engine is a fairly standard stamped rocker design, then push rod length may not be an issue at all..

I'll let the engine gurus take it from there...

TRBenj
05-07-2007, 02:37 PM
The mechanic who rebuilt the heads says that sometimes you need to change out a push rod for a shorter one since material is being removed during the rebuild.

This is true. When having the heads rebuilt, it is common to resurface them. This changes the geometry of the valvetrain and will usually require the use of shims under the rocker arms (assuming stock Ford non-adjustable pedestal type rockers) or different length pushrods.

However, this has nothing to do with the fact that one cylinder has low compression. That could be explained by worn piston ring, bad head gasket, cracked head, or hung valve. Not good news in any case.

Have you had them perform a leak down test?

kdjose
05-07-2007, 02:49 PM
I have not yet but understand this to be the next level of diagnosis. The seller is having this done today. The mechanics theory (redimentarily summarized due to my lame level of knowledge), is that the low compression could be due to the cylinders not closing fully because of a push rod that is too long (I think)????? Thanks for the sage advice! The head gasket was replaced as was the manifold, but the compression was never tested after this repair.

kj

TRBenj
05-07-2007, 03:33 PM
I have not yet but understand this to be the next level of diagnosis. The seller is having this done today. The mechanics theory (redimentarily summarized due to my lame level of knowledge), is that the low compression could be due to the cylinders not closing fully because of a push rod that is too long (I think)????? Thanks for the sage advice! The head gasket was replaced as was the manifold, but the compression was never tested after this repair.

kj

You probably mean that he things a valve is not fully closing due to a long pushrod. While that theory is somewhat valid, it is unlikely to be true. Heads are usually shaved 0.03" when theyre rebuilt- this may cause excessive lifter preload when reusing a stock pushrod, but it shouldnt cause a valve to remain open. Theres absolutely no reason why 1 valve would be affected and not the others- Im betting that the valve job was botched.

kdjose
05-07-2007, 03:36 PM
Good tip! I appreciate the help.

kj

EricB
05-08-2007, 05:16 PM
The next step is to do a Leak Down test on the weak cylinder. This involves applying compressed air into the cylinder to see where the leakage is occuring.
If it leaks out the intake or exhaust, it is most likely a valve issue. If it leaks out the crankcase Oil fill, then it can be rings. If it leaks out the cooling system, well then (no matter what) it's not good.