One of the reasons I always use the Wadding ball/tape method prepping cluster motors MMX & UP. Takes all the fight out of wondering if you have the igniter wire in contact with the propellant.
Drat! page-2 of the 4motor cluster igniter installation is to larger. I've substituted simple wadding-ball tape single motor installation...works exactly the same with the motors already inserted in the model. Not all clusters need to have the motors installed first, Only those with long swept back fins the get in the way when taping. Hope this helps.