Had a go at the splash guards at the weekend and what a b!tch.Ended up cutting them off and leaving the old bolts still in as the are completly corroded.Any sugestions how to remove them,will I just have to drill them out?
Is it the front or rear? (I noticed you posted in the other thread as well)
Here is how I would tackle it.
1) Clean up bolt head and surroundings as best as possible
2) Soak in penetrating oil / release oil / WD-40
3) tap arround the bolt head were it contacts the hub with a cold chisel to break any seal the corrosion may have formed
4) Soak in penetrating oil and leave for a cup of tea
5) Using a spanner, socket (or grips if rounded) try and remove the bolt
6) Try an impact driver if the heads are not rounded by now
7) Try heating with a propane / butane flame paint stripper (with care to surrounding components). You usually have to heat them hard to shift them. When hot try steps 5) and 6) again

if no joy then you have a choice of soaking again overnight with oil or getting more serious
you will need good quality tools for the next steps....
9) drill into the bolts (or through it if you know whats behind with a small sharp drill to make a pilot hole that is perpendicualr to the bolt
10) use larger drills then have a go with an "easy out"
11) If still no joy use succesively larger drill until you are close to the threads. Use a chisel (or drill the bolts head off - you can do this earlier if you like) to smash the bolt head off and with a bit of luck the remaining bolt in the threads will collapes with persuasion.
12) If not you could send a threading tool into the hole to chase out the remaining bolt.
13) failing this you will have to drill it out and use a helicoil
When putting new bolts in use some copper grease to help stop this in the future
Hope this helps
