What spark plugs for 340

RN12YC are 318 , 360 plugs . They also work in 72 , 73 8.5 compression 340's but they too prefer RN9YC's. The 68,69 and early 70 10.5 compression and the late 70 and 71 10.3 compression take a colder plug RN9YC. A big difference in performance will be seen with the correct plug in the early 68-71 340 . The power will be greatly seen in the mid range and up. Years ago I threw some RN12YC plugs in my 69 swinger. I left them in for one ride and put the old RN9YC's back in.