Take this for what it's worth.
Executive Summary: Replace the large undertray and rear diffuser bolts as part of a regular maintenance program.
I had the supreme misfortune of shearing one of the two bolts that hold the undertray in place. I'm speaking of the large center bolts, measuring M8 X 1.25 X 25 , with the hex head. There is a matching pair holding the rear diffuser in place.
If you suspect that someone may have ever used a big-torque pneumatic tool to tighten these bolts before, then you want to replace them. PROACTIVELY.
I don't know for certain why I was so fortunate as to face this challenge, but I will say that it only took about approx 20nm of torque to take the bolt's head right off. The bolt broke inside the threaded receiver on the rear "lower wishbone to chassis bracket" requiring a very careful extraction. A replacement bracket costs $78.63, but it is not a simple swap. There are a number of bolts holding it in place and this bracket serves three purposes: securing the undertray, one of the hard points for the lower wishbone and also a stay for the parking brake cable.
I suspect two causes: the first is the aforementioned weakening of the bolt due to overtorquing and the second (to a lesser extent) is the fact that the threaded receiver is open on the top end, thereby allowing grit and dirt to accumulate in the threads. If a thread locker has ever been used in there, then the threads are going to get really nasty when you repeatedly R&R these bolts. If you feel an unusual amount of resistance when installing the bolt, STOP and clean the threads.
I ended up using a bolt extraction kit from Sears. It appeared to be a new, well thought out design. Pricey at $39.99, but the alternative was to have a shop attempt an extraction. The three with whom I spoke, were not very anxious to put a Lotus on lift and sweat this little job. Two actually refused and one just didn't appear very confident.
The kit was similar to this:
I used a very low RPM, very high torque 1/2" right angle drill from Milwaukee. The Sears kit warns against using this size drill, but I wasn't interest in fiddling with one of my 3/8" variable-speed drils.
The extraction tool uses a left-twist drill bit with a tapping collar on it. You drill the center of the bolt and once you have a decent hole, you bring the tapping collar down and manually run it into the bolt. Then you use the power drill to extract the bolt. Of course my drill promptly broke the extraction tool off in the chuck, but wasn't not a problem, as the collar is designed to operated by using a regular box wrench on it.
Bolt came out with no problems, and the stress on the bracket should have been minimal.
Lowe's usually has a pretty decent bolt selection, and they came through yet again. I bought every bag that they had. M8 X 1.25 X 25 . Two per bag at $1.17. Bolts were graded properly at 8.8.
For comparison at lotusgaragedotcom, the bolts are currently $1.05 per.
In searching this website, I have been unable to locate a proper torque value for these bolts. I think my strategy is to tighten the bolt until it feels "right" and then use a torque wrench to measure that value.
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