Slant six spark plugs and numbers

-
O

Onus Gumboot

Active Member
Joined
Oct 27, 2021
Messages
25
Reaction score
1
Location
Warrenton Oregon
Hi. I'm new here so bear with me if I get something wrong. I have a '73 swinger I just started working on. I was looking for spark plugs, and all the plugs for this cars 225 slant 6 at the store have a 13/16 hex size. They won't fit, as there is not enough room for the wrench in the cavity where the plug sits. The ones on it are 5/8, but I don't know if they are correct. They are short compared to what the stores say should fit. I can't find numbers on the engine to see if it has been swapped. Is it a thing with the slant six to need a 5/8 hex plug? I have always had V8s before. Is it possible the engine is from something else, and what would require a 5/8 hex plug? No numbers in the places folks say to look that I can see. Any help appreciated.
 
There are two standard sizes for spark plugs and their threads, 5/8" is the socket size for the smaller plugs. Pull out one of your old plugs, there will be a brand name and plug number on it. You can use that info to get new plugs. The plug size isn't unusual, the ones on my slant truck are 5/8" size.
 
Take a few pics, try to slide the phone down to show the spark plug areas if you can.
Not really sure what you have there.
Aren't the plugs recessed the aluminum sleeves?
Either one of the 2 spark plug sockets (5/8 or 13/16ths) with a short extension should work.
A swivel 3/8ths drive ratchet may make getting to those plugs a bit easier.
Screenshot_20211028-080256_Chrome.jpg
 
Last edited:
If it doesn't have the drool tubes, it would be the later head that uses the "peanut plug" and those don't have/use washers.
 
The plugs I pulled out are autolite 26. There is nothing in the NAPA stores, or any other parts store around here, computer that lists these as the correct plug. All the recommended plugs have a longer thread, and a 13/16 hex, with a gasket. There is no room in the recess that the plugs sits in for that size socket. The 26 plugs are 5/8, with a short thread length, and no gasket, but I am not sure if they are the correct plug, or just what the last person to work on it could find that fit in. I found another brand of plug on line with a longer thread, but I don't want to use them if they are going to be too long, and hit the piston when it comes up. They are Denso 4501. The guy at NAPA seems to know a lot about old Mopars, but he doesn't know what is going on there either. My confusion comes from not being able to find any recommended size in any store that will actually fit in the engine. So I don't know what I have there. I assume its the stock engine, but being this old anything is possible. Does anybody else have trouble finding spark plugs for a slant six? Some people on here talk about NGK 3459 plugs, but I can't find any such thing available.
 
In brief....
There are two basic casting versions of the Slant six head. The most common head is on 1960-1978 (ish) and looks like the one in the bottom right of this photo. There will be an aluminum 'tube' that fits in the large round hole with the spark plug inside the tube. Spark plugs for this head will have the longer thread reach and a small metal gasket (remove for most slant applications).
The other style head was used from 1978(ish) to 1987. This head looks like the one in the upper left corner of the picture. Only the spark plug goes in these, no tubes or such. The spark plug for these heads has a tapered seat (NO gasket) and a short thread reach.
You cannot mix spark plugs between the two styles of heads.
two-heads-tubes.jpg
 
The peanut plug head started in 1975. These are the ones that take a 5/8" socket to change.
But any slant 6 head will bolt on to any slant 6 block. If using an 81+ head you will need an 81+ valve cover though. All '60-'80 directly swap among those years.
Your car is a 1973. It's 2021. Something has probably been swapped over all those years since it was new. No big deal.
I have a 1974 block that I'm putting a 1977 head onto, and then my whole works is headed for an 85 pickup.
 
That helps. It is probably a '75 engine, or at least head. Now I have something else to look at. I'll see what plugs a '75 used and see if that matches.
Thanks.
My brother put a Cadillac engine in his Ford car hauler truck. So anything is possible with the right tools and skills.
Edit- Just looked up the '75 parts. Sure enough that's what I see in the engine. Now I know what I'm dealing with. Do you know where the numbers are on a '75 225 engine? I haven't found those either. That would probably have answered my question right from the start if I had found them.
Thanks again. Solved a lot of head scratching.
 
Last edited:
Your engine obviously has the 1975 model year and newer head on it. It still may be the 1973 block with a swapped head or maybe the whole engine was swapped out. See post number 6 and 10.

The guys on slantsix.org have been doing this a long time and have settled on the NGK #3459 (ZFR5N) plug as being the best for most people. (The extended tip gets the spark deeper into the combustion chamber) I generally followed the recommendations of @slantsixdan for parts and they've worked well for me. I recently bought another set of NGKs from Rockauto but mine are still good after 10,000 miles. Check with Autozone, Advance or O'Reillys but you may have to order them.

The vehicle VIN had to be stamped into the engine block starting in 1968. Look for a machined pad below the #6 spark plug and see if it matches your car.

EDIT: These are the plugs I used in my early drool tube head. Not for use in later head.

20211028_152809~2.jpg
 
Last edited:
I thought that ZFR plug was for the older drool tube head?
I know that with the LA series V8's they sold a plug with the smaller hex (5/8) but Still have the seal washer and right thread length. When my son had his '90, with the 360 I thought they'd sold him the wrong plugs, I wound up calling Champion and was told that the plugs that my son bought were identical to the old standbys with the only difference being the wrench size needed.

But in the case of the slant 6, when they went to the peanut plugs, they went to the tapered seat instead of the flat flange with the washer. Don't use the plug pictured above, in a '75 or newer head.
Plugs that will work for a slant 6 for into 2 groups/ the 1st being for 60-74 engines and the 2nd grouping for 75-87 engines (never mind what year the car body may be)
 
You must log in or register to reply here.
-

Back
Top