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Dear BrickDad,
Hope you're well. It is sound to give the specific model (940/960), engine type and alternator type (Bosch or Nippon Denso). Sometimes these specifics matter.
In Volvo VADIS - the now-superseded dealer parts/service database - the diagram for the '92 940FT shows the top bolt, but does not list the part number. Using the diagram for the '94 940 with turbo, the parts diagram shows the top alternator mounting bolt as #980245. This number has been updated to #985405. It is a flange screw (M8 x 140 mm; pitch not specified). It should cost $4-5, at a Volvo dealer. The nut is part #948645, updated to #985868, which should cost $2.50.
The failure of the nut on the top bolt - which seems still to be in place - should not lead to any loosening of the alternator. That would result only from the failure of the tensioner (Part #1378789, which should include the bracket, the large bolt that goes through the bracket's slot, and the smaller, tensioning bolt, that goes through the opening, in the head of the bolt, that goes through the bracket's slot; it is not clear from the VADIS diagram, if the tensioning bolt's nut is included with Part #1378789).
If by "tightening bolt" you refer to the smaller, tensioning bolt, that is turned to tension the belt, that bolt is an M6 x 90mm x 1.0 (pitch). Once the desired tension has been reached (when index finger pressure can depress the belt about 3/16" Βmm] in the middle of the belt), the large tensioner nut should be tightened fully. Then, the tensioning bolt's head should be turned (counter-clockwise, as you face it), so that there's a gap of, say, 1/8" (3mm) between the underside of the tensioning bolt's head, and the tensioning bracket. If this is done, there is no strain at all on the tensioning bolt. The tensioning bolt is too small to take the strain imposed by the drive belt. That is why the tensioning bolt should be "eased", after the large tensioner's nut has been fully tightened.
Hope this helps.
Yours faithfully,
Spook
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