Compression Test As promised. I forgot to take pictures yesterday when I did this, plus I did it a slightly different way than the way I am going to explain. My gauge I have had for years, I drilled the check valve out WAY bigger than when it came from snap on. This way, I can do a check without the rockers on...... Anyway. You will need to get your hands on. 1. A compression test gauge suitable for a diesel engine's high compression. I have a snap-