Hello
The size is 1/4-20 tpi. I have some here, but easier to get them from the source in the US;-
http://meniscusaudio.com/std-spikes-p-204.htmlIt is worth gently removing the threaded inserts, refitting carefully with a little mdf glue (a dot or two on the threads about half way up, screw in touch tight immediately). Clean off excess glue!
I lightly oil the threads and add a stainless or brass washer, to prevent binding. Don't go more than light finger touch tight with the lock nuts!
Take care - Graz