John,
mjb summed it up pretty well. The 1296 has two journal diameters, depending
on which motor you have. 67 1/2 -> 70 had the small journals, and 71,72 had
the larger ones. If you can find a short block from a 71-72, it's worth
installing. The motor will rev a little slower, (not much), but the lower
end is a little more substantial, and the bearings and thrust washers
interchange with the 1500, GT-6 and TR-6 engines. About the only advantage
to using the small main motor that came with your 69 is that the cam actually
rides in bearings as opposed to the later engines which ride in the block.
Externally they are the same, except for the size of the crank pulley and
the timing cover. I have run 1147, small and large main 1296 and 1500 engines
in cars that I have owned and that I have built for customers, and the large
main 1296 is a good choice. As with any of these Herald-based motors, make
sure the thrust washers are always in good condition and don't overstress them
by starting the car with the clutch depressed.
I hope this helps.
Greg Meboe MEBOE@wsuvm1.csc.wsu.edu
Dept. of Mechanical and Materials Engineering
Washington State University
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