thrust bolts.

[JoeMeg]

http://rtjtool.com/images/tapanddrillchart.jpg

[curtisbyrne]

ok. so fine thread? and what size?

[dirtdobber]

so fine thread? and what size?

I see no reason to use fine threads. Coarse thread should be fine just add a drop of loctite for insurance.

there are holes in the axles already but not threaded

You bolt size will be determined by the existing holes. The bolt can't be smaller than the existing holes, obviously. A 3/8 bolt should be plenty big enough. If you're lucky the existing hole was sized for tapping to some size. If you can get an accurate measurement on the existing hole, match it up to the drill size on the chart JoeMeg posted.