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1965 Mustang, Convertible, 289, T5
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I had a post on here a week or so ago where i was talking about rebuilding my engine. a lot of people told me to run a compression test and post the numbers here. so i did and i also took pictures of my plugs. id like to hear what any of you have to say about these numbers. Cylinders 1:110 psi 2:95 3:120 4:125 5:105 6:120 7:110 8:95 can i tell if my engine needs to be bored vs honed when i do my rebuild from these numbers and pics? or do i have to wait to pull the block?

 

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I had a post on here a week or so ago where i was talking about rebuilding my engine. a lot of people told me to run a compression test and post the numbers here. so i did and i also took pictures of my plugs. id like to hear what any of you have to say about these numbers. Cylinders 1:110 psi 2:95 3:120 4:125 5:105 6:120 7:110 8:95 can i tell if my engine needs to be bored vs honed when i do my rebuild from these numbers and pics? or do i have to wait to pull the block?

How long were these plugs installed? How was it driven?

 

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Numbers 3 & 6 look a little suspicious but the rest look like they're fairly new. Numbers 3, 4 & 6 are very good compression but 2 & 8 are low compared to the rest (you shouldn't vary more than about 15 psi. I'd run a wet compression test to see if you have issues with rings or valves. You'll have to measure the bores for taper to decide if you need a re-bore.
 

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Was the engine hot or cold? Was the throttle open during the test? How many strokes did it take to reach the value. And yes, do it dry and wet (squeeze some engine oil into the spark plug bore). this way you can check the rings, since the oil will help selling.
The variations between the cylinders should be less (95 vs 125)
 

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Most of the plugs look okay. Numbers 3 & 6 have buildup of ash. I would suspect either leaky intake valve seals or worn guides in those cylinders.... Another possibility would be if those 2 intake runners were the ones picking up the majority of flow from the PCV valve.....

Compression awfully low and a bit imbalanced at 30psi difference between highest and lowest. Did you repeat the test "wet" (after adding 2-3 oz. of fresh engine oil to the cylinders)?
 
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