3/8"-16x1-1/4", make sure you reuse the dished washers under the screw heads!
I prefer using Si-brz or Al-brz (silicon bronze or aluminum bronze) for a number of reasons. If I can't get those, I'lll use SAE Grade 5 carbon steel (they retain more stregnth longer than SAE Grade 8.)
The OEM studs (one at either end, possibly one in the top centre) are threaded 3/8"-16 on the cylinder head end and 3/8"-24 on the outer end. There are valid reasons for doing something like that - but they don't apply in this case (the joint doesn't need that much strength.) You can cut new studs out of 3/8"-16 threaded rod, I typically use brass. You can also get carbon steel hardware store pieces - either way, get them about 2" long. Make sure you get new nuts (brass or bronze) if you get new studs - the OEM nuts are threaded 3/8"-24.
I am building a chart of known fastener sizes on my site (link in sig) in the Tech section. You may want to make a note of it...
