Good Afternoon HML74 et al,

       I am sending you some info direct from AC Delco regarding plug identification.

  S = Shielded (5/8-24 thread)better know as "peanut plug"

  X= .060 gap predomiatly used on HEI V6 Buick some Olds etc. too wide of a gap for any small/big block chevy..We tend to push the envelope at a setting at .048,using 50,000 volt coils and low ressistance wires (50-500 ohms per ft.)

 TS =Tapered seat with extended tip..These to get the core closer to the top of the piston i.e. stock plug for 305/350 is AC 45TS.

  I can FAX the original sheet to you with the factoids of plugs etc. if you can send your fax # I would be pleased to do so..

  To my knowledge there is no engine that would come from the factory with two different gap settings for the plugs in one type engine..We in racing have done tricks like that on wet cylinders etc.,however never on OEM engines.

  Also I think you mentioned you had "hump" heads if so are the short hump heads(my favorite 186's with accesory holes)then there are the parents of the 186 which are 461's and 462 heads with a hump that measures 3/4''-1'' at the top of the humps..In the event you are unaware of where these numbers come from,and they are the last three numbers on the heads between the springs/valves.Yes you have to take valve covers sorry..

   The aforementioned heads also have the "big plug"13/16" socket with the crush washer and the later model heads use the "peanut plug"(5/8'' spark plug socket)

   If there is additional questions I might be able to help you with do not hesitate to send a message ..

    Sal        nosal1  80Vette

Pacific Performance Motors

Dunedin,Florida  Formerly from Paso Robles,CA.

thanks for all of the info guys.

i think there might be a slight misunderstanding. 

i do not have  different gaps in the plugs that are in my car.  they are all currently gapped at .035".  i was just curious as to why one type of plug (such as the extended tip) might be gapped at such a large gap of .060 and why an r44t plug might be gapped at .035".   i'm trying to figure out which type of plug/gap would give me the best performance for my type of motor and driving style.  i like to do mostly "in town" cruising with more low-end performance.

i like to leave redlights with an an attitude

i think i'm going to try the r44x plugs with a gap of .045" to get a more powerful spark than the .035".

the car is running ok as it is now, but i like to experiment