On the subject of TR6 rods I would be very careful of any rod which
uses 1/4" bolts. The only TR6 race engines I know of that had a rod
failure were the aforementioned 1/4" bolt variety. As Kas said, the rods
are not bad. We never broke a rod in 17 seasons of racing. I think we
used 5 different sets of stock prepped rods (we had two engines most of
the time) and the only time they needed replaced was when they were
damaged from a broken crank blowup.
We broke 4 cranks in rapid succession after we figured out how to
make the flywheel stay on. We were shifting at 7800. The first one was
the first time we went to the chassis dyno with a new head and header in
'85. (BTW that head and header are the still the best combo we have. I
think that head must have 100-120 hours on it - use your air filters).
We figured it was because of the sustained loading. The next one was
when #6 rod ventilated the side of the block on the starter side when
(as we found out later) the crank broke. The people who witnessed the
blowup at full throttle and 7600 rpm in the esses at Riverside said it
was a spectacular fire when all the oil hit the headers. (The corner
workers delight in bringing back pieces of block, rod, crank, etc. I
can't blame them because they have a tedious and mostly thankless job).
We figured a rod bolt had broken. The next 2 times Rod felt the crank go
before it did any damage to anything else. All four of those cranks
broke in exactly the same place between #6 journal and #4 main. They
looked like matched ends - you could fit one end from one crank to one
of the others and they would match. We looked at lap charts and found
that non of these engines had gone over 30 minutes. Finally we got the
hint and decided to change the cam and work on making good power between
5500 and 7000. A billet crank was not legal until the early '90s but
after we talked with one of Paul Newman's old mechanics we decided it
wasn't worth it anyway. It seems they had gotten tired of breaking
cranks and tried a couple of billets in '76. The billets lengthened the
time before failure by maybe 30 minutes. This would mean roughly an hour
of run time. They had also decided it wasn't worth it.
The stock TR6 crank is a non twisted forging which is very good -
better than a plain billet. The TR6 crank problem is not the material
but the small overlap due to the extremely long stroke. So for about $4k
you can have a crank that will give you maybe 30 minutes more run time -
if the nose of the cam doesn't break off, the oil pump drive doesn't
shear, or the distributor drive dogs don't break. There is just too much
shakin' goin'on.
Aftermarket rods for the TR6 are a solution for a problem that
isn't there (except for weight). A billet crank won't fix the harmonic.
This engine is less than thrilled to spend any time over 7000 so
optimize it's strong points like torque and dependability. Spend the
money on better shocks, better transmission ratios, track time, dyno
time, etc. Prep the rods and crank according to Kas' instructions and
you can't go wrong. SCCA fixed the rpm problem anyway by restricting us
to 28mm chokes. It needs 36mm.
All inline sixes have this harmonic but at different revs. BMW M1s I
believe were at about 9000, Datsun 240s about 8500, Jag 4.2 s were 7500
I think. Crank rigidity has a huge effect on the rpm where this occurs.
Balance can help.
Greg Lund
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