> The Dowell bolts are just 3/8 UNF bolts
If they're ordinary bolts, why do they have 6 digit part numbers rather than
standard hardware numbers ? There are even two other ordinary 3/8 UNF bolts
used, which do have the "standard hardware" numbers (HB912) and are probably the
same length.
> they often have to be driven in with a hammer
"Ordinary" bolts have plain shanks considerably smaller than the nominal
diameter, frequently even smaller than the actual major diameter of the threaded
portion (which is smaller than the theoretical major diameter). I'm fairly
certain this should not be true of the "dowel bolts" (the name given in the SPC
for the 6 digit part number).
Furthermore, the dowels (either pin or bolt) are what set the alignment for the
input shaft in the pilot bearing. Any significant error here will cock the
input shaft and cause eventual problems inside the gearbox. In addition, the
input shaft may drag in the pilot bearing, causing difficult shifting. I can't
prove it (never having owned a TR6), but I'll bet this is at least part of the
clutch problems that TR6s are so notorious for.
I rummaged through my 3/8" bolt drawer measuring shanks ... the smallest was
.365" and the largest was .370". That means even choosing the largest one would
allow a misalignment of .010", surely too much.
Thanks for the reply, Graham, but I don't believe this is the right answer.
Note that the flywheel housing is held to the engine block with precision ground
dowels, and the earlier TRs use them as well. I am reasonably certain that the
150743 bolts have precision ground .375" shanks rather than being ordinary
rolled bolts. Same thing goes for the 132872 dowel bolts specified for the TR6
(even though some vendors have said the same thing, that these can be replaced
with ordinary bolts). Size does matter, here.
Happy New Year to all
Randall Y
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