Spark Plug for 346 ci

[vicbrincat]

Hello folks,

What is the available replacement for the original A.C. 104 spark plug used on the 346 ci engine?

I just had my 1948 engine rebuilt. No modifications..as original with  40 thou. oversize piston. What is everyone using for spark plugs for their 346 L-head?

Thanks in advance.

Vic

[dadscad]

Don't know if this will help but here is an Autolite list. Year, plug number and gap. I found it at,
http://www.autolitecatalog.com/RelatedInfo.aspx?b=A&f=Autolite/ASP2009PassCar_Early0427.pdf

CADILLAC (1959-1902)
1959-57..................................85 .035
1956 331cid Engines .............86 .035
1956 365cid Engines .............85 .035
1955-49..................................86 .035
1948-46 10mm Heads ...........3136 .028
1942-39 10mm Heads ...........3136 .028
1940-38 16 Cyl. Models ........308 .025
1938-36 8 Cyl. Models ..........308 .025
1937-36 12 & 16 Cyl.
Models .................................386 .025
1935-30..................................386 .025
1929 w/18mm Threads..........386 .025
1929 w/7/8"-18 Threads ........3076 .025
Thru 1928...............................3076 .025

[Bill Ingler #7799]

A/C M-8, Champion UY-6, Blue Crown T-4X or Auto-Lite P-4. All 10 MMplugs. The A/C M-8 plug is sometimes on Ebay. The UY-6 in my opinion is the better plug, but hard to find.

[Carfreak]

http://www.modifiedcadillac.org/documents/Multiple_Years/1939-73%20AC%20Spark%20Plug%20App/tn/1939-73%20AC%20Spark%20Plug%20App-2.jpg.html

I've found a couple boxes of NOS AC plugs on Fleabay.  One set was encased in cosmoline; PITA to clean but they were in perfect original condition.  That's what's in my car right now. 

[Carfreak]

Here's a set ending in a day and half and quite a bit cheaper than what I paid.   

Then you'd get a chance to clean cosmoline too. 

http://cgi.ebay.com/ebaymotors/NOS-AC-104-Spark-Plugs-fit-mid-30s-40s-GM-vehicles_W0QQcmdZViewItemQQhashZitem2c52d40979QQitemZ190368188793QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

[Brad Ipsen CLC #737]

NOS AC 104 are relatively easy to find.  I had a set in my car and happened to open the hood at night when the car was running.  There were sparks jumping all over the place.  Since then I have run new AC M-8 without any problems.  It is nice to have the originals but check to make sure they are functional.  Maybe the ceramic material fails with time.

[vicbrincat]

Thanks for all the replies.
I managed to find a set of Champion UY-6, I think I will start off with them. I will eventual get on eBay and get a set of AC 104. Let's see how the Champions work out, first.

Thanks again folks.

Vic

[Don Boshara #594]

Got a set of Champion UY6 from NAPA last week, they're not hard to find.  Also, Champion has a new number for them: 842. Tried AC104s years ago, some leaked.

[vicbrincat]

I purchased a set of Champion and installed them on my motor tonight. I used a spark plug
socket and exerted the same amount of force that I always have on AC plugs.

Two of  the Champion plugs broke leaving the threaded part only in the cylinder
head.

For half an hour, the language that came out of my mouth would have made a
sailor blush! I didn't think that such verbs and nouns could be used in the same
sentence like that...anyway...

The good news is that I was able to wedge a flat-blade screw driver in the
remnant and turn the broken pieces out without dropping anything into the
cylinder.
I haven't even fired up this engine since it was rebuilt. I would have really
lost it, if I had to rip the heads off to recover  broken pieces of spark plug!

I compared the Champion to the old AC plug.. There is no comparison. The
Champion is made of cheap soft metal, the AC plug is far more sturdy.

I put the old AC plugs back in and tighten them twice has hard as the
Champions...and they did not break,

No more Champions for me....

I will try the AC M-8s tomorrow.




Vic

[dadscad]

I quit using Champions plugs back in the late 70's, Fortunately I never had a Champion twist off like you did. My reason was from inferior service, they didn't last as long as other brands and they were prone to foul quicker.

David

[vicbrincat]

Just thought I'd post a picture of the culprit:

http://www.eldobrghm.com/Evil_Plug/IMG_2289z.jpg

[Otto Skorzeny]

I've never seen a plug break like that. If they do break, it's usually the insulator.