The factory book on modifications for the Spit engine (same rods as the
TR6 and presumably same rod bolts) says change the stock rod bolts
for grade 8 or better bolts, or use socket head bolts. The TRF bolts are
stroud head bolts (have that extra flare on the head to spear the load)
whereas the local hardware store grade 8 bolts are NOT stroud head bolts.
I don't think that the TRF bolts are grade 8, probably grade 5 or
phosphate hardened or something similar.
APT sells rod bolts better than stock for not much more. At first I was
going to use grade 8 bolts from the local fastener supply store,
now i think i will go to an aircraft supply fastener place.
About stretch, I don't think that stretch to essental at all. Stretch
has something to do with the the torque required to clamp at the correct
force. I *think* if memory serves correctly, that when you torque the bolt
it will stretch (any bolt), if you over torque you overstretch and exceed
the tolerances of the bolt. The higher grade the bolt the less it will
stretch for a given torque. The higher the grade bolt, the less likely
it will stretch beyond its yield point for any given torque.
But UNLESS you plan on widning this engine up to redline on
a regular basis obviously the stock bolts should work fine. About thinkness
differences, don't think it should matter. I don't think that the
bolt is needed to keep the cap from moving around, but i'm not sure.
Cheers Dennis
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