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Discussion Starter · #1 ·
I have an A-code 289 and I ran a compression test on it before removing it from my '66 fastback. Cylinders 1-4 were around 110-125 psi whereas cylinders 5-8 were about 10-15 psi higher on average. There was not much variation from cylinder to cylinder so I was wondering whether the difference may be due to improper torque or a worn head gasket on the right head. The engine runs good and I was thinking of just cleaning it up, putting on new gaskets, and sticking it back in... Any suggestions? Thanks!
 

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I think you're close to the stage of needing rebuild; but a better judge would be some the the machinists here. Your low reading is about 25% under your high average if I'm reading your numbers correctly; although it could just be a gasket. When I checked my A code 289 last year all reading were within 5 of 165.
Your 110 vs avg of 135 to 140 might signal being close.
 

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The consistency and pattern of your readings suggests a possible head change at some point. It's normal for readings to vary from cylinder to cylinder but for banks to consistently different, to me, indicates something else. Does it appear that the engine has been rebuilt at some point?

Since the engine runs good, I'm not too concerned. You might post oil consumption and plug readings and, as a further diagnostic, run a leakdown test as well.

Unless someone stacked two head gaskets on the DS bank, I doubt head gaskets or torque would cause your readings...
 

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Which side did you do first? Did you completely warm the engine to operating temp first?

It's possibe that by the time you got to the second bank of cylinders, the motor had cooled some. Thus impacting it's sealing properties.

Just a thought.
 
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Discussion Starter · #5 ·
Thanks for the help. The motor is not original, it is a 68 I think (actually, it has a 4-barrel intake but I can't verify that it is an A-code). And yes, I suspect it has been rebuilt. The gaskets look fairly good and I can see some honing marks on the cylinder walls... What do you mean by "head change"?
 
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Discussion Starter · #6 ·
That's a good thought. In fact, the motor was probably cold when I tested it (oops). I tested the right side (low side) first...
 

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What do you mean by "head change"

It's possible, though not likely, to put heads from different years/engines on the same engine, which might produce a different mean compression ratio, which does affect cranking compression readings.

For example, a head from a '69 351W or certain '68 302's might lower compression by as much as one point, due to a larger combustion chamber.

It just seems odd to me that one bank is substantially lower than the other, in spite of good engine performance.

Wish I had a better answer *G*
 
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