Sorry to read about your broken bolt. Same thing happened to me on my 300D right after I acquired it and needed to R&R the thermostat.
Hopefully, you can remove the broken bolt with vise grips or some other tool to grip the shank - as others have suggested. The penetrating oil and a bit of heat from a propane torch will help.
However, if you have to use an easy out tool I suggest using the straight sided type (some call them straight fluted easy outs). I have done 100's (if not 1,000's) of broken screw/bolt/fastener extractions when I worked as a professional technician and the straight sided screw extractors are the best because they will really grip the inside of the broken bolt (using the hole you drill through the broken bolt). My extractors are made by Proto and worth their weight in gold for this operation (another company making them is Ace from the Hanson line of drilling tools). The extractors that look like a spiraling reverse thread do not work as well in my opinion because they do not grip the bolt as well - but if they are all you can find . . .
The procedure I use is similar to what some of the others have written and you are already doing - soaking the bolt and then using some heat. However, if you have to drill the broken bolt to use an easy out I suggest this procedure:
1. use a small file, a dremel tool, or other grinding tool to make a flat spot (as large as possible) on top of the broken bolt and as close to perpendicular to the bore as possible
2. use a small very sharp center punch to make a starter indention for the drill bit - this is a crucial step because the drill hole needs to be as close to center as possible so the ensuing drill hole is through the center line of the broken bolt, plus you are dealing with a 6mm shank that is pretty small - so precision is the key word
3. begin drilling with a small diameter drill bit (1/16" or 5/64") keeping the hole as close to perpendicular as possible, use new or very sharp drill bits, add a bit of oil after starting the drill hole - you want to make sure the drill hole is as close to center as possible and if it is not you can re-punch with the center punch to get a better start position, using oil right off makes it harder to see how centered the drill hole is
4. using the first drill bit, drill all the way through the broken bolt, this will relieve the bolt's force on the threads and as you drill with progressively larger drill bits the bolt's force on the threads should lessen
5. use progressively larger drill bits to enlarge the hole and drill entirely through the bolt (being careful not to remove aluminum from the housing, or the least amount) until you reach the drill bit size recommended by the manuf. for the easy out tool you are going to use (for a 6mm bolt usually the easy out tool requires a 1/8" or 3/64" drill bit)
6. take the easy out tool and insert it (if the straight sided type I tap the extractor in until it fully contacts the ID of the bolt), apply a bit of heat to the aluminum around the bolt (not much heat) and back out the bolt - follow the directions if you use the reverse thread type of extractor
7. if this fails, then you may need to remove the extractor (do not break the extractor as one member warned) and drill out the bolt until the threads just show - this is another reason why the beginning drill hole must be centered, take a small chisel and break out the old bolt in pieces, I know this sounds radical, but I have done it lots of times and had to do so on my 300D because the corrosion was so bad the bolt could not be removed with the extractor. If you have to do this procedure and are very careful, then the threaded part of the aluminum housing may be in good enough condition to reuse a 6mm bolt after using a tap to clean up the threads, if not then you are in a good position to tap the hole to use a 7mm or 7/16" bolt (both with fine threads, not course) with a minimum of effort.
8. as was written, use anti-seize compound (or at least a bit of high temp grease) when reinstalling the bolts, use flat washers on the bolts (not lock washers) and then torque the bolts to the proper spec (10Nm or 7.4 ft lbs or 88.8 in. lbs.). Use an inch pound torque wrench to gain better accuracy - 88 in. lbs. is like a strong wrist tight #3 Phillips head screw. In other words, these bolts do NOT need much torque because the o-ring on top of the thermostat is the sealing point and a new o-ring does not need much compression to form a water tight preassure seal to keep the coolant from leaking.
I am sure others will have their own methods and suggestions to help. My procedure has been developed over many years as a professional to remove broken fasteners, but I am always on the look out for other's input that may make the job easier and more successful.
America: Land of the Free!
1977 300D: 300,000+ miles
American Honda: Factory Trained Technician & Honor Grad.
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.
Last edited by tcane; 01-17-2002 at 12:19 PM.