These innocuous looking bolts have a big job to do!
It is recommended that the lower bolt is always replaced after use. It must be torqued to the correct figure making sure the threads are cleaned before fitting the bolt using the correct tap so the bolt can be screwed in by hand.
There are two types, the standard MG one or a higher tensile cap head bolt.
Because these have a habit of snapping  I recommend they are changed for the socket cap head ones.
The main issues with these bolts snapping is, the threads are not cleaned (use a M12 tap and wash out any debris) so the bolts can be screwed in by hand (NOT a hammer/impact wrench) then correctly torqued to 110N/m. The alignment of the bolt/thread /bearing rebate in the arm are out of alignment.These must be checked before hand for alignment. DO NOT use a hammer to assemble as I have seen reported by one Guy on a MG forum.That is one way of modifying your MG but not the correct way!!
Check the old bolts when removed, if they have witness marks on them and bent this could be a sign that you need to investigate further as the bolt could be binding on the inner bearing surface and the alignment of the mating part is out. This will require another arm fitting.( I have seen the rebate in the arm out of concentricity with the threaded section by 1mm so the bolt will bind and snap eventually!)The bolt can 'relax' as the correct torque is not reached causing the characteristic failure.
In my opinion the failure of the stock bolt is not because there is a large tolerance fit between the bolt and bore of the bearing but because of the parts not being in alignment/the bolt stretches beyond design limits causing it to relax. Using a tighter fitting bolt is not a way I would recommend as it reduces the tolerances between the bearing/arm rebate and the threaded part of the arm and the internal finish of the bottom bearing is not a true machined fit,fitting a tighter stronger bolt is a recipe for future problems! Imagine removing a high strength bolt that had to be forced into place and is now immovable ..yes it's going to snap off in the arm!!
The standard factory one is no longer  stocked.
NB:  Price is for one bolt.