I just looked that up for mine. I found this chart: http://www.psep.biz/store/ryobi_trimmer_spark_plugs.htm it depends on which trimmer you have
spark plug is good but when its plug in to coil on spark
.025 inch per the manual.
Champion CJ8 spark plug is for the crsftsman 3.0 edger/trimmer
Champion RDZ19H
.023"
This part number is for a Champion RDJ7Y spark plug. Set gap at 0.025" I bought a replacement plug, DJ7Y, at Home Depot on 10/6/13 for $2.38 + tax. The DJ7Y replaces the RDJ7Y. This plug is used for other tools such as my Ryobi trimmer and McCulloch chain saw.
EZ start 5847 or DJ8J
0.020"
anywhere between 16 and 18 on a spark plug gauge should work fine.
I think it is 0.030".
Mine is 3/4
Rdz19h