The Stovebolt Site Forums The Shop Area Engine Shop spark plugs

Anyone know the proper spark plug that goes in a 1955 engine (235.) I use points. Just a regular engine but I want the plug that was in it to start with. As always thank you, Steven

I do not have the clear answer to your question, however it stands to reason they would have been AC Delco because that was a GM partner. AC44-5 comes to mind as to what would be original in 1955, however, the new version of that is R44. I am not sure if you can still purchase the originals.

The R is for Resistor. The 44 is the heat of the plug. The 44's were the most popular (sold the most) in the late model 235's and early V8s.

http://www.secondchancegarage.com/public/1955-chevrolet-235-136hp-6-engine-specs.cfm

AC 44-5 on Thriftmaster 235; and, AC 43-5 on Loadmaster 235 & Jobmaster 261

R43s are now on-sale on Rockauto.com for $0.84

I just bought 6 of those last week there, for that price plus shipping.

43 is pretty cold. I wish I knew what the difference really is between colder plugs and hotter plugs. Another research project? LOL!

I would like to see some stats for our 216/235/261 application that showed engine performance over time between the 43 and 45. As I said in my first post, the 44's have the biggest sales. But that is not to say it works the best for our application. SO, I wish I knew what the difference is between the colder and hotter plugs. Your sales brochure isn't giving anyone confidence that the 43 is better than the 44 or 45 for these particular engines.

My 1973 AMC Gremlin with a 232 6 cyl used R45XLS. Just not sure how you become SURE the plug heat you are using is the optimal is all. Am I missing something? Wouldn't be the first time.

All I know about the plugs is that an AC 43 was recommended for the big-truck 235 and the big-truck 261 (and the AC 44 on the small truck Thriftmaster 235).

I'll check tomorrow what plugs I have on the 1960 261 (out of a 2-ton school bus) that I have in my Suburban.

Thanks Tim. Anecdotal is better than nothing. I have 44s in my 261 but that was more because the vendor sent me what I ordered, which was plugs for a 261. How does todays fuels play into this, or things that GM didnt consider when writing the manual? What did they learn from it for later years? I havent researched what the 56, 57, 58, 59, 60, 61, 62 truck manuals suggest. I am just too busy these days to do alot of research. I am not being argumentative. Just curious. Thanks!

AC 44 on truck 235 in 1959 and 1960, CR 8.25:1
AC C-42-1 on truck 261 in 1959 and 1960, CR 8.0:1

AC 45 on truck 235 in 1961 and 1962, CR 8.25:1
AC C-42-1 on truck 261 in 1961 and 1962, CR 8.0:1

Only reason I bought R-43's is that's what Rock Auto part's catalog called for, for my truck.(1955-3100-235)

Oreilly's and AutoZone list the same part number.

AC 43s were the designated spark plug on early 261 engines (54-56).

Wow, I've got some numbers to take to the parts store now. Thanks so much everyone. Believe it or not I couldn't find the plug numbers in none of my book! Again, thanks everyone. Steven