So i pulled the plugs from my truck to swap out. went to orielly's and they tell me the computer has listed a 3/4 reach plug as the OEM but both 3/8 and 3/4 are listed. I've been running a 3/8 reach plug since getting the truck several years ago. so I've always felt like i haven't been able to get the motor to run as it should. granted I havent done much beyond changing plugs/points/dist. cap/etc.. and rebuilding the leaky POS stromberg a couple times. is it possibly that that 1/2 inch reach difference is keeping it from running correctly? does anyone know the correct plug for the 259 poly motor? I'd be so bummed to put the 3/4 in and have it run worse or even cause something way worse to happen. (piston or valve banging the plug and big damage) it'd be sweet if the extra 1/2 inch made her purr better... (pun intended) thanks for your time guys Stephen
Autolite AR-82 for 270 poly 1955 and AR-52 for a 1956 270 poly.....should be a cross reference somewhere's....
If the plug hole is situated where you can get a sight line on it (possibly requiring the old mirror and flashlight trick), you should be able to see the thread depth in the head. Should be evident which reach plug the engine was set up for. CAUTION: If some previous owner has been running a plug that's too short, the unused lower threads will be full of carbon. Good chance of goobering up the threads if installing the correct length plug. Best to first chase the threads with a tap, liberally coated with grease to catch as much debris as possible.
Additional thoughts: I'm certainly no expert, particularly on that engine, but a 3/8" reach plug sounds awfully short. The last time I used a plug that short was in 2-stroke Suzuki motorcycles back in the '60s. (Factory had to change to 1/2" in self defense. 3/8" threads + aluminum heads + ham-handed bubbas = lots of ruined heads!) Anyhoo, if you've been running a plug that's too short it will be firing way up in the hole. Could certainly have an effect on tune & performance. Good luck!
Stillrunners, by the motor numbers my best guess is its a 241 or 259 but i wouldn't think thats gonna make a difference when it comes to 3/8 versus 3/4... thanks for the tip Retromotors, i was thinking 3/8 seems short. I'll have to put on the headlamp, get in there and see if i can make anything out....
For what its worth, I just pulled a plug from a scrap 270 Dodge poly I've got and its plug is 3/8 " reach and perfect in the chamber
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