Maybe a little additional information about the changes would help. In
1971 along with the body changes to the MK4 here in the states, the
Block was changed to the big bearing 1296 motor, I assume to get ready
for the 1500 increase. In 1972 the block was changed to the recessed
head gasket variety. Then in 1974 the stroke was increased to provide
the 1493 increase. There may have been a few late 71's with the
recessed heads. The rods from 71-on are the same (and by the way are
the same rods as used on the later GT6 and TR6), and the rods for the
MK3 and earlier are the same (for the most part). The crankshaft is
unique to the 71-72 model years(only to incorporate the bearing change,
not the stroke change). This much I know, the rest is speculation. Now
something has to be different from 71-72, and from 73 on, because you
can't just put a 73 crank in using the same rods and pistons. It stands
to reason that if you increase the stroke by changing the throw on the
crank to a longer one, AND NOTHING ELSE, (which is what appears to have
happened, same pistons, and rods) the pistons would hit the head,
because they are going farther up and down. You also have to change the
crank center to head deck height to accommodate the additional travel.
The blocks from 71-72 are also unique to those models, and herein lies
the catch. These blocks have a crank center to head deck surface for
the 1296 engines, and the 1500 blocks have the additional increase from
the crank centerline to head deck height for the stroke change. So it
appears that one cannot simply use a crankshaft from a 1500 in a big
bearing 1296 to get a stroke increase. I may be all wet here, because I
haven’t actually measured these block’s and someone will correct me if
I’m wrong I’m sure.-
Barry Schwartz (San Diego) bschwart@pacbell.net
|