Anyone know what size/type of bolts to get for the 3 holes in the harmonic balancer to remove or install the new one?

Or is there a easy way to get the crank to stop spinning? I was able to drive the harmonic balancer on with the main bolt and an impact but not being able to have the impact on straight enough caused it to loose some of the drive impact. It drove the balancer on most of the way but not all the way. As soon as I put a breaker bar on to try and tighten the main bolt down the crank rotates.

So I either need a way to keep things from moving so I can use the main bolt to drive the balancer on more and apply the right torque specs for the bolt...or possible the size/type of bolt that fit the 3 holes in the balancer so I can use those to keeps things from rotating.

I tried search..but with the wording I used I didn't come across the bolt sizes.
Thanks!