PTSchram said:
First off, I think Mike might have it backward as there is very little difference between a B suffix 3.9 and a 4.0. Everything should be interchangeable except the main caps. The suffix A engines were more similar to the 3.5 than the 4.0. The suffix B engines were manufactured after June 15, 1995 and incorporate the ten bolt head style, versus the 14 bolt heads. Also, the water pumps are different between the two suffixes (suffices?). Mike, remember our discussions on water pumps? Yours from the 4.2 was completely different from those from my B suffix 3.9.
Supposedly, replacement shortblocks can be had from of all places, the dealer for $1200.
Paul
Waterpumps are not the differences between the blocks. Waterpump differences are due to the configuration of the timing covers.
The 3.9's used in the serpentine-belt Disco's, for example, used a completely different waterpump than in the v-belt RRC 3.9's. In fact, the front cover on the RRC V-belt 3.9's are a throwback from the grey-market 3.5's from the late-70's, and possibly earlier.
I'd assume the suffix B you refer to are the later Disco 3.9's which had the later front cover using the serpentine belt system which is why the pumps were different.
The 3.9 and 4.2 short-blocks are basically identicle save for the displacement. Main journal diameters are the same as is the length of the crankshaft nose and camshaft nose. The 4.0's used a larger main journal diameter (same as the Buick 300 FYI), cross-bolted mains...but the differences are directly outside of the block: longer crankshaft nose from what I remember, and most importantly, a shorter camshaft nose.
Because the 4.0 was introduced with the distributerless GEMS engine management system, there was no need to drive a distributor off of the camshaft. The oil-pump is also powered differently on the 4.0 than on the 3.9/4.2.
BTW, the 4.0 and Disco 3.9's (serpentine belt) use the same waterpump, but the timing covers are different due to the longer crankshaft nose, and the lack of a distributor + provisions on the crankshaft to drive such.
This is not a huge problem. The 3.5/3.9 timing cover will fit the 4.0, you'll have to use some spacers on the pully bolt and washer to affix the crankshaft pullies, and you will have to replace the 4.0 camshaft with one from a 3.9 (or any Buick 215 aftermarket cam) so you can get the distributer (and oil-pump) drive back.