I finally determined the mysterious chick-chick-chick noise when the truck was in gear was not a lifter as initially suspected, nor a leaking exhaust gasket, but was a loose spark plug on on the third cylinder back on the drivers side. (I forget what number that would be.) The plug was so loose that it could be jiggled with my fingertips.

I removed the plug. I inspected it, and saw what may have been minor damage to the first few turns of the threads, but I dismissed it and decided I wouldn't worry about it if it would thread easily into the hole. I cleaned and regapped it and reinstalled it without any unusual trouble other than the contortions needed to access this plug. The engine ran smoothly and the noise was gone. However, ten miles down the highway the noise was back. I returned home, and although the plug was not as loose as it was before, it backed out with hardly any effort with the spark plug socket and rachet. I cleaned and reinstalled it, this time tightening it down much more than the first time. (I have no torque wrench) I am always paranoid about stripping those threads, having done that when I was a kid to a lawn mower engine. I started the engine and the noise has disappeared again.

My questions: before driving it again, should I go ahead and replace the plug even though it seems to thread into the hole okay? If the plug loosens again does this mean the threads in the block are damaged? The reason I didn't buy a new plug is that I am ten miles out in the country and had to reinstall the original in order to move the Bronco outta the driveway.

What has to be done if the threads in the block are damaged or is it more likely the damge will occur to the spark plug threads before the engine block threads are damaged.

1980 bronoc Custom 351M with 87,000 original miles.

 

the third cly on the drivers side down is #6 I had this happen to me on a 300 6 on the number 1 cly the plug was away loose I finally ran a tap in the cly and cleaned up the threads and put new plugs in her and never had a problem again

 

Thanks for the tip. Where can I find out the tap size?

 

it's either 5/8 or 9/16 just runner throught afew times and see whats happens it's most likely 9/16 because the plug it self is instelled with a 5/8 socket. let me no how it worked for you.