Be very careful even when the glow plug igniter is not connected. I had an engine start while priming it, no glow igniter connected. The engine was cold and was being started for the first time of the day. I nearly cut up my fingers.

It has only happened once in 15 years of flying. But cutting the fingers badly once will stay with you for years. I did cut some fingers down to the bone back in 1996. It took about a year to heal and I am still missing sensation in the tip of one finger.