I asked an ex-racer who also welds aluminum to help me with my clutch cover issue. He recommended heli-coiling all the case holes to ensure perfect threads, using allen bolts with washers on the clutch cover, a paper gasket only, with no sealer, loctite and TORQUE all the clutch cover bolts to ensure even clamping to the case, and careful kicking. It's a known issue with Tri-Z's, mine was broken so badly when i bought it that it couldn't be welded, so I bought a NOS cover. I have not had the bike fully together yet, so I can't report on the results.
1985 Tri-Z 250
1985 ATC250R