I'm looking at it right here....M6 and 65mm long....approx 3 inches or so long.

A 1/4 will not thread into the balancer correctly and you run the risk of pulling the threads and crossthreading it.

It could just be my engine, so take that with a grain of salt. if the 1/4 threads in without binding up, go for it....otherwise it could be a different size.

Go with what works.