Aluminum threads and steel bolts or sparkplugs are a recipe for disaster. I've dealt with many machines with aluminum blocks and stainless steel bolts. Usually they came with Heli-coiled threaded holes, but if they didn't, they got galled up in a short time. I'm guessing Timeserts are similar to Heli coils. You might as well do all four holes, because you will eventually need to do them anyway. The ford triton engines would blow the spark plugs right out of the head. The fix was to Heli coil them.