I do clusters with up to 4 motors all the time. The modern "starters" are no problem. Just make sure they are not damaged and use a 12v source instead of these little 6v or 9v Estes and Quest systems. If you are still paranoid about them, you can dip/paint them in nitrocellulose lacquer (nail polish). A visual inspection and individual continuity check will go a long way in successful clustering with modern "starters".

Cheap lawnmower batteries, gel cells, etc are good, even when they won't kick over an engine anymore.

BTW, I've got a big stash of A8-3's and C6-7's from Walmart closeouts over the years. I've done a bunch of C6-7 clusters because of that. Three C6-7's give you that D20 (actually a baby ~E15 with a peak thrust of about 45N) that a lot of people want Estes to produce.

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