How easy are they to fit yourself?
Sorry for the late reply, timezones, workshop, just back to the forum. Already lots of good replies.
As said by those before, an easy job, I usually would not use the drill particularly in the alloy head, because of the shavings, that as said will get all down the pushrod tunnel bores, if the thread is pulled right out of the holes the Helicoil tap will start straight into the hole that's left, put some grease on the tap to hold the shavings, for alloy you should use the
1.5 x diameter length coils, this will give the stud the best hold in the coil and head (8mm dia thread X1.25 pitch X 1.5 long coil gives 12 mm of engagement in the alloy)
By load testing each thread, (I'm assuming maybe only one or two have stripped), you can work out if they all need doing or you will get away with doing just a few.
Any holes that still have the threads in will require use of the drill which can be really "grabby" when drilling a threaded hole in alloy and bring the problem of swarf getting in the engine, it will take a bit care, and also as said , the tap needs to be kept square to the longitudinal surface of the head, all sounds complex but not really so.
