Engine stand: Yes, grade 8 bolts are more brittle than grade 5, but they
have a higher yield strength. What this means is that the grade 5 bolts
will bend before they break after you drop the engine and stand off a 5 story
building, the grade 8 bolts will not bend first, but simply break when you
drop the engine and stand off an eight story building. What you need to do
to the motor should be well within the working limits of the bolts you used.
Head: Yes, a later Mark 2 or 3 will have the TR5 type casting, and not the
early Vitesse/2000 straight port head. Years ago I started working on the
following chart, compiled from various books and measurements:
Application & CR & Thickness & CC depth & CC volume
------------------------------------------------------------
GT6 Mk I & 9.50 & & &
GT6+ & 9.25 & 3.450 & 0.425 & 34
GT6 Mk III & 9.25 & 3.32 & .47 & 37
(early) & & & &
GT6 Mk III & 8.0 & 3.40 & .55 & 44
(late) & & & &
TR250, TR6 & 8.5 & 3.55 & .60 & 48.5
TR6 middle & 7.75 & 3.54 & .68 & 55
TR6 late & 7.5 & 3.55 & .70 & 57
TR6 race 1 & 10.25 & 3.375 & .52 & 41.6
TR6 race 2 & 12.3 & 3.295 & .44 & 33
GT6 race 1 & 10.75 & 3.31 & .385 & 30
GT6 race 2 & 12.75 & 3.22 & .300 & 24.4
In the same booklet which I will never finish, I also say:
The observant enthusiast may notice some of the above info points to a
quick way to increase the compression ratio on the 2.5 liter TR6 motor:
drop on a stock GT6 head.
So yes, you could get a TR6 core, they can be had for something in the
range of $100, complete, and put the GT6 head and such on it. You may
want to use the TR6 block, as the GT6 block you have may not have the
same casting, with increased clearence for the longer throw crank.
mjb.
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