Spark plug replacement on a 390
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Senior User
Joined: Jul 2017
Posts: 205
Likes: 1
Spark plug replacement on a 390
1974 f100 390, a/c
I am looking to replace the spark plugs on my 390 and I'm concerned about the passenger side dealing with the a/c - heater box causing problems!
Anything I should know to help me out? Special tools needed?
I am looking to replace the spark plugs on my 390 and I'm concerned about the passenger side dealing with the a/c - heater box causing problems!
Anything I should know to help me out? Special tools needed?
Mountain Pass
Joined: Oct 2017
Posts: 213
Likes: 6
From: Maysville, Ky
I would have to say that a swivel and a few different lengths of extensions for a 1/2" ratchet would do the job. That being said, it is still a pain no matter what. I think the best tool for that job is a healthy dose of patience. My old 73 had the factory air and I used to dread pulling the rear plugs. My 75 only has a heater. It had dealer added air when I bought it years ago that I removed. Working on the right side of the motor on the 75 is stress free without the air box in the way.
Mark.
Mark.
Mountain Pass
Joined: Dec 2017
Posts: 126
Likes: 10
From: Nevada
I just changed mine this weekend on a 351M using a 3/8” drive plug socket, a 3/8 x 1/2 adapter and a 1/2” flex head ratchet on the two plugs by the heater core. That combo worked perfect but I don’t know how much space there is to work on a 390 vs 351. Good luck.
Laughing Gas
Joined: Oct 2015
Posts: 991
Likes: 111
From: Dallas, OR
I changed mine with no special tools, just a fair degree of cussing. You might want a spark lug wire boot puller to prevent damage to the wire. That was the hardest part I believe. I was replacing my wires too, so I could yank to my heart's content. I have a 360 w/ factory air box still in place. As mentioned previously, stock up on patience before you start, but very doable within an hour or two.
Senior User
Joined: Feb 2017
Posts: 136
Likes: 0
From: Owasso, OK
I remember doing mine... I have a 360, so basically the same thing. It was a pain in the butt!! But, mine had been sitting for about 14 years untouched.
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Senior User
Joined: Dec 2009
Posts: 176
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From: Spring Hill
This is why i wish my truck didnt have factory a/c. Plus its horrid to look at, one of these days its getting cut out.
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Tuned
Joined: Oct 2010
Posts: 357
Likes: 11
From: Roanoke, VA
Couple extensions and universal joints, some patience, and you should be good to go. The worst one I've done was my old 69 Ford Fairlane w/ a 390... first time I changed plugs (when I was16), took me over 2 1/2 hrs... Dad used to have a 69 Fairlaine w/ factory 428 CJ, and he said once took it to Sears Auto for tuneup and oil change, after 3hrs they told him to never bring it back. This was in the early/mid 70s... haha
Cross-Country
Joined: Jan 2014
Posts: 52
Likes: 0
From: Caldwell, NJ
Agree with what everyone has said and will add one thing that helps me a lot when I'm undertaking this project: an angled ratchet. I believe I got this little goodie at Sears a couple decades ago but basically instead of having just a straight ratchet, the handle angles up slightly. It has worked wonders for me when getting to those rear spark plugs. It's still not gonna be fun, though.
Laughing Gas
Joined: Nov 2013
Posts: 1,207
Likes: 60
From: S.E. Louisiana;so far USA
You have a lot of good suggestions. My '76 460 & factory A/C also produces a lot of grief. Just go slowly & thoughtfully. Try not to injure yourself. If you have an air compressor, blow off any crud or sand lying on the plugs before loosening them. That way nothing will fall into the cylinder, or the threads, or the seats. When replacing them, put anti-seize or a bit of oil on the threads and seats. This will make it easier next time and discourage rust.
More Turbo
Joined: Feb 2018
Posts: 710
Likes: 3
From: Oregon
You have a lot of good suggestions. My '76 460 & factory A/C also produces a lot of grief. Just go slowly & thoughtfully. Try not to injure yourself. If you have an air compressor, blow off any crud or sand lying on the plugs before loosening them. That way nothing will fall into the cylinder, or the threads, or the seats. When replacing them, put anti-seize or a bit of oil on the threads and seats. This will make it easier next time and discourage rust.
Laughing Gas
Joined: Nov 2013
Posts: 1,207
Likes: 60
From: S.E. Louisiana;so far USA
You're actually not supposed to put anti seize on plugs - I researched this a bit before changing mine. So for example NGK plugs they recommend NOT to put anti seize since it can mess with the temperature dissipation, and it can alter the torque rating making it too easy to over-tighten, info straight from the manufacturer...NGK Spark Plugs