Scored cylinder bore?

Tiny
JRUSHER
  • MEMBER
  • 1969 MERCURY COUGAR
  • 5.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 131,000 MILES
I recently rebuilt a 351w - it is completely stock except for a mild cam. While going through the flat-tappet cam break-in, I noticed a miss above 1500 rpm. I inspected the plugs and #4 was fouled (wet), all others were fine. I swapped plugs and then wires and there was no change. I did a compression test, and all cylinders were 150+/_5. I ran a borescope into the bore, and noticed both radial and vertical scoring on #4, all other cylinders were okay. It looks like possible oil starvation. What do you think?
Monday, November 18th, 2024 AT 11:17 AM

1 Reply

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,602 POSTS
Yep, the walls do not look good on #4, like you had contamination when the engine has assembled, or something is wrong with the piston rings. I would remove that piston to see what's going on. There is a problem with the oil control on that piston as well. Please upload pictures or videos in your response of any problems so we can see what to help you with.
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Monday, November 18th, 2024 AT 12:28 PM

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