There are two things with the crossmember bolts, as I recall:
1. They're not UNF, they're British fine thread and the cut of the
threads is slightly different, so that using UNF bolts will cause the
fit to be a little too tight (or too loose... can't remember). Of
course, re-threading the frame sleeves using a UNF tap is a one-way
operation...
2. The length of the thread required on the bolts is longer than is
provided on UNF standard bolts. Using a thread die to put more threads
on the bolt is a Bad Idea because it causes a major stress riser right
where the stress on the bolt is the highest. Threads must be rolled onto
the fastener so that they're in effect forged into the bolt.
Tom Hall has a tech tip on repairing the inevitable stress cracks in the
frame, around the crossmember bolt sleeves. If your Tiger shows
distortion around the bolt sleeves (visible from the top of the frame
rail) then it's only a matter of time before the crossmember will get
ripped out of the car... better reinforce it while you're in there.
Theo
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