Maybe I've posted this before:https://www.box.com/s/qyubvu9wmljjlqz6zxox
The bolts are 3/8" diameter, but a 10 mm bolt will also fit (they just happen to be the same).
When I had a machinist do the points on a lathe (could have just cut them off with a hackwaw and ground them), he shortened them much to my chagrin.
I was going to put a large washer with a nut on either side to keep the ends of the legs from going too far into the dirt (it doesn't freeze or snow much here but I'm originally from snow country and a large washer/plate might work in snow).
I looked at the Gitzo spikes but they were far too short to function in soft turf or dirt. Ideally I'd like to have 125 mm/5" to push into the ground - it rains here all winter so the turf is soft.
I also use the spikes at low tide in the rocks - placing the points into a depression or between two rocks - no slippage.
I use these extensively for outdoor flower macro - when the points are pushed right in, the tripod won't tip over even with the arm fully extended horizontally (Gitzo Explorer + 5DII + EX2 + one tube).