A couple of mm is fine.
You want the benefit of the stop if the engine does start to move during a race.
It is unlikely to crack with a few minutes of being butted up to the mount but extended contact will strain the weld area.
Get into the habit of checking in between heats.
It's alweays worth making yourself a little list so that you don't end up on the dummy grid having forgotten something basic.
Tyre pressures fuel Blowing up fuel after carb change Chain lube Chain tension Engine stop, forgotten after tightening clamps Connect battery Gearing Tighten steering wheel bolt Throttle cable gives full throttle Exhaust tightness Power valve spring Bumper retention Floor pan bolts etc
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