Nick L recons :
buy either grade 8 or allen bolts (grade 12 or 14) easily
grade 12 cap screws are excellent material .........very high tensile , but still machine able / tap able , (just) best to power tap the sods though , probably 4140 grade .....i often buy a few realy long cap screws with a short thread as they are a very cost effective way of getting just a few inches of good grade material without having to go to a steel merchant who will want to sell you a 3.6 meter bar minimum!
if you get them with a UNF
thread, or even metric fine series and tap them to what ever size you need you have made a top of the line thread insert , retap the offending hole using a 2nd cut or taper tap and wind then in as hard as you can the hack them off with a hacksaw , file them flush and you have a forever repair ! done that countless times
oh and by the way .......when tapping high tensiles always go .0,1-.0.2 mm above the drilling size (much more on large diameters ) , depending on pitch and diameter , 80% thread is plenty on that stuff and you wont get so many busted taps