the 198ci is the original Buick v6, from 1962-63. sorry, we don't have a service manual for that. the general systems and architecture will be pretty much the same in 66 but i can't vouch for any specific measurements.
https://www.teambuick.com/reference/...ssis/index.php
the main line development of the Buick 90* v6 comes from the 1964 and up 225ci, which is the 300ci v8 with two cylinders knocked off.
the 198ci is cast iron, but the small block Buick from 1961-63 was the aluminum 215ci which went on to become the Land Rover 3.5L v8. what design modifications transferred from the 198ci to the 225ci / 300ci, i don't think i've ever met anybody that really knows that.
the 198ci has two major problems, aside from the age. rarity, due to only have two years of production. and the wide availability of FAR larger displacements in the same basic engine family and package size.
you can get a Buick v6 up to 252ci / 4.1L and the 231ci even fire was available from the late 70s to the late 80s. these are all HEI distributor engines, some of them have factory turbo.
and, quite often, you can bump up to the Buick small block with minimal issues, which would give you 300, 340 or 350ci. the 350ci ran from 1968-81. obviously, these are still often available at junk yards with older cars sitting around.
if the 198ci has the 215ci bell housing, the obvious choice would be to get a Rover v8 engine. those kept the same transmission bell from the old Buick 215ci all the way up to 2004. common displacements range up to 4.6L ( 277.8ci ) with some 5.0L available.
Buick went to the standard BOPC ( Buick, Olds, Pontiac, Cadillac ) bell in 1964 for the small block / v6 and 1967 for the big block.
www.TAPerformance.com is the primary Buick vendor, and you're right, they are pretty sporty for the 198ci parts.
i have a hard time believing that someplace like NAPA couldn't cross reference the original bearings though.
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