Spark Plugs for a 1948 Chrysler 251cu. in. L Head

[Tom Skinner]

Gents,

I purchased some AC Delco R45S for my 251 six I have been using Champion 592 Plugs.

I noticed the AC R45S are 1/16" Longer into the block than the Champions.

Can I use them or will I bend a Valve Stem? Do they cross reference?

Tom

[keithb7]

I can’t say for sure on your questions. I can offer that my 265 engine, (same bore and head, just a ¼” longer stroke) uses Autolite 295 plugs. 

[Sniper]

I maybe that the long plugs are the right length and the short plugs too short.  Pull a plug out of the head, measure the threaded part of the hole and compare.  That will answer that.

[Tom Skinner]

Sniper,

I did. The R45S are 1/16" Longer than the Champion RJ12C or 592.

I think I better return them for Champions. I looked up a Cross reference Chart.

It seems the R45S might be for a C-39 or 323cu.in. and my Parts man rushed looking at it on his computer

I'm afraid they would damage the 251 Valves if seated longer in the head by even 1/16"

Tom

[Sniper]

Measure the threaded portion of the hole in the head not the plugs, that will tell you what's what.  If the plug's threaded part is longer than the head's threaded hole then pass.   Even if it didn't hit the valves, not sure if it would or wouldn't, the thread sitting in the combustion chamber can act like a glow plug and cause preignition.  

[martybose]

Worse comes to worse, you can always add sparkplug indexing washers (available at any hot rod online store) to back out the plug a little.  Had to do that for my Edmunds head, which would close the gaps on my Autolites without them.

 

Marty

[desoto1939]

Also did you get a cold or hot plug range. This also might increase the amount that the bottom of the plug extends into the cylinder chamber.

 

Rich Hartung

[belvedere]

The correct AC plug looks to be a R45. Not sure how that differs from your R45S.

[Sniper]

S means extended tip

[Tom Skinner]

Sniper,

Yes. The tip is longer by 1/16".

I am swapping them out at NAPA tomorrow for Champion 592.

They R45S) are for an eight cylinder not a six.

Thank you for your help. I wasn't about to risk monking up my Valves.

When I bought the car in 2007 it had R45 Plugs in it not R45S they (R45) measured the same as Champion 592

7/16" into the block from the top of the threads to the bottom of the tip of the plug. The R45S measured 1/2"

That was what bothered me as I have heard of guys bending a valve by using the wrong plugs.

Tom

[Dodgeb4ya]

All 1946-1950 MoPar flatheads ...Plymouth, Dodge, DeSoto and Chrysler.... both six and eight cylinder engines all use the same reach spark plug.

The correct plug number back then for all 1946-1950 Chrysler's was an AR8.

I use Autolite's on all my MoPar flatheads..

295's and 306's or Champion J11C's..all 1/2"....to tip of electrode

But of course others use their own preferred plugs.

[desoto1939]

On my 1939 Desoto I use the old style Autolite A9 but the original was an Autolite A7.  Was recommended this 15 -20 years ago and still use the A9 today. They are getting harder to find but I have a couple of boxes.  The a9 is a hotter plug than the A7.  If you find the AL series this is a longer reach plug and the A9 is the regular 14mm plug  The Al series plugs would have been used with the aluminum heads and the a plugs would have been used with the cast iron heads.

 

Rich Hartung

Desoto1939@aol.com

[Tom Skinner]

Got my Champions (6 Spark Plugs) from NAPA swapped out (R45S) for 6 RJ12C (592) for a trade out of $2.49 each.

Life is Good! I have used Champion Spark Plugs for 47 Years without any issues. Never had one fail. Although they be made in Mexico

I still find they are as reliable as ever! God Bless the Mexican People! And All Hard working People everywhere!

Tom

[greg g]

Been running the r 45 in my 56 230 fr 15 years.  Yeah the same ones. Replaced champs that started hard and missfired under load.  Watching the champs after dark was quite a light show! Lots of lightning going along the porcelain and down into the block, not much left to go into the combustion chamber.