I would recommend the TimeSerts rather than Helicoil on parts that will be removed again. Helicoil is fine one-shot, but I've heard of too many problems with them unscrewing with the bolt when something had to come off again.
The Timesert is swaged in place, plus it's a solid cylinder of steel, not must a "fat thread". The only drawback is that you must have enough material to drill out, cut the shoulder, and insert the TimeSert. If you don't have the space, use the Helicoil.
The jig for drilling and tapping the aluminum block is REQUIRED -- else the head bolts won't go in. Big trouble.... The repair, however, if done correctly is permanent, and the later blocks come with TimeSert or similar inserts, I think, not bare aluminum. Sooner or later a bare steel bolt in an aluminum block is going to remove the thread from the block...
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!