Thread Size of Spikes for Revel F206

Was hoping that someone might know the thread size for the floor spikes. Bought a pair of used F206s that came without the floor spikes. I am looking to put on a pair of outriggers and need to find the right size bolt.

I found somewhere on internet that it was 1/4"-20 but this is way too small. A 5/16"-16 is close. I can get a couple of threads on and then it binds up.

Any help would be appreciated !