"EASY-OUT, EASYOUT, EZ OUT, EZY OUT, BOLT AND STUD EXTRACTOR:
(AKA) Bolt Extractor, Screw Extractor, Bolt Remover, Stud Extractor, Stud Remover.
The EASY-OUT or bolt extractor tool by design can be a very useful tool for the removal of broken bolts and studs "IF" conditions for its use are perfect and preparation procedure are followed exact. Without going into when and how the easy-out or extractor will work I"ll give you some circumstances when they generally don't and reasons why you might want to try another approach.
For example a 3/8 inch bolt breaks when you are trying to remove it. The only reason a bolt or stud will break when removing is severe seizure. In this scenario the extractor recommended for removal would be 3/l6 inch to 1/4 inch in diameter. Most easy-outs are designed and manufactured using hardened material making them tough but also very brittle. In a this case the recommended extraction tool is about half the size of the broken bolt and three times as brittle, the easy-out will break practically every time.
In a case where the easy-out might have a chance for success, the steps for preparing the broken bolt are critical. For example, you have a broken exhaust bolt in the cylinder head on the engine in a car. The location of the broken bolt may be difficult to access, making the proper preparation that much more difficult. When drilling a hole for an extractor it is critical the hole is the correct size, perfectly on center and straight. In this situation a mistake in any one of the proper steps could result in the following problems:
1. If the hole is drilled to big, the extractor expands the remaining shell of the bolt defeating its purpose.
2. If the hole is the proper size but off center, the same result will occur.
3. If the hole is the right size and on center but on an angle to the bolt, the chance of drilling that hole into the threaded hole is highly probable."
http://www.brokentap.com/easy-outs.html
Unfortunately the bolt did not snap off cleanly, so it's going to be challenging to ensure that I drill the centre perfectly, and am shirt scared of drilling the sides, as this could be a very expensive mistake indeed. Also the bolt is buried about 8 cm deep, and the buried piece would probably be about 5 cm long. Bolt head takes a size 20 spanner so would probably about 14 along the length? But a good suggestion worth consideration, thanks.
. There goes the Christmas bonus...