straight 8 spark plug questions

[Guest milehighjimmy]

While searching for a set of AC46 spark plugs for a '40 248, I came across sets of R46's, C46's and R46S's. Could someone please explain the differences and if any of these sets would be acceptable to use in a straight 8? Thanks in advance.

[39BuickEight]

I just bought some on eBay, I got a set of both regular AC 46's and 46s's just to see the difference myself. The 46s's are longer and thus likely will not fit under the spark plug cover on my '39, so I will relist them. I got them so cheap I'll come out ahead, just have to watch it. The 46's are shorter and appear to be what I need based on research.

[jenz38]

hope this helps ..

[Mark Shaw]

Or, you could use a cover spacer to accommodate the longer plugs...

[60FlatTop]

A few years ago I did a major tune-up on this old gal for a member:

I decided to take a tip from people who have really fussy cars to tune (like my little brown one in the background). I took an original AC plug out of my pocket at the parts store and said "Give me an NGK that has a tip like that." We found one and I put in a set. I was pretty happy with the outcome.

Bernie

[FLYER15015]

While searching for a set of AC46 spark plugs for a '40 248, I came across sets of R46's, C46's and R46S's. Could someone please explain the differences and if any of these sets would be acceptable to use in a straight 8? Thanks in advance.

Jimmy,

My '40 LTD came with a set of THE AC-46. I tried the AC-48 and the AC-49c (which is still used in a lot of generators).

I went back to the AC46 set and I just keep them clean.

Pay attention to Jenz's posted chart, and beware of letters in the prefix / suffix (front and back) as you will end up buying plugs you cannot use. Many times the insulator is too tall, or the reach into the cylinder is too deep. Both will cause problems..........

You want AC-46 Period..........

If you like your health plan, you can keep it, period.............

Keep searching e-bay and Amazon and you will find them.

Mike in Colorado

Edited January 11, 2014 by FLYER15015 (see edit history)

[Larry Schramm]

The chart that JENZ38 put up is great. The operative question on AC 46 spark plugs or AC R46 plugs is the R46 plugs has a resistor material inside of the spark plug insulator that was used to help reduce ignition noise in radios, etc. Both plugs in my opinion can be used interchangeably. They are both the same size and heat range.

For plugs with an S after the number, the chart says extended tip. This is easy to see because those of us that have been around spark plug manufacturing call it a skirted plug. If you look at the area below the threaded portion, there is a smooth land that extends from the end of the threaded area to the end of the spark plug shell. If the number of the spark plugs are the same, the heat range is the same. Just a little more reach.

On the comment about having AC-46 plugs in their car and they got some AC-48 & AC-49C plugs, these are plugs that run hotter. If you compare the tips by looking from the bottom electrode area, the higher the number the skinner the insulator around the electrode, therefore the hotter plug.

The C after the number indicates that it was a "commercial" sparkplug.

In the older days if you had a car/truck that was burning a fair amount of oil and fouling a sparkplug, you could put a hotter plug in that cylinder or maybe the whole engine to nurse it along until it was either overhauled or disposed of. So if the car had a regular AC-45 plug and over time it started to foul out the plugs, you could put in an AC-46, or AC-47 plug in depending on how much the plugs were oil fouling out.