Monitor Audio Silver RX6 Spike Thread size?

The majority of those threads are 1/4-20 or some metric near equivalent. The suggestion to get different screws and try them in the holes is good. The "dash 20" or dash with another number is how many threads per inch there are. A 1/4-20 screw or threaded insert has 20 threads per inch.

monitor audio originates in the UK, so the thread will be metric.

Thanks to all who responded! Went to the hardware this morning with the footer and found that it is actually a M8. I appreciate the suggestions...