I am changing to a GM alternator on the 1974 B20 engine I have in my PV444. Since I was using the OE alternator there have been no bolts in those holes until now. I have not been able to get any of my stash of 5/16 or 8mm bolts to go in cleanly and I do not want to force them.

Ron Kwas tells me that these bolts are normally 5/16 coarse thread but he wonders if there might have beene some changes by 1974. I saw in a 2013 thread on Turbo Bricks that there were a number of changes from Imperial to Metric in 1973 B20 engines but the alternator holes were not among those mentioned as changed.

Before I try running a tap through them to clean them up I want to be sure I use the right tap. Does anyone know if they were still 5/16 coarse thread for the 1974 B20?

Thanks for the help.

Keith;

Normally, those holes are indeed 5/16 NC, and if the engine previously had an OE Alt installed, they would have been using the single big mounting/pivot bolt in-line with the Crankshaft, and not those three mounting holes, which would have been left empty so typically would have developed some rust over time. I recommend chasing these threads with a "bottoming tap" (where the taper at the end of the tap is much less than a normal tap, as it only needs to clean out an existing threaded hole, not cut new threads, and it will do this to a much deeper level because of the limited taper).

I understand the internals of the later engines went to Metric, but I don't believe this included these external features. Maybe highly experienced Eric can chime is on this also to confirm...

Let me know how you make out.

The threads in the block for the generator on the side of the block for all B18, early B20 and late B20(8 bolt crankshaft) are 5/16 (U)NC.

Most of the threads on the outside of the late B20 blocks remain UNC or NPT(pipe thread) so exterior parts could be transferred with the same hardware.

The bellhousing bolts and transmission brace threads do change to metric on the late B20 blocks as well as the crankshaft main cap bolts, the connecting rod nuts and studs, the flywheel bolts on the crankshaft and on the clutch mounting bolts and the front crankshaft pulley bolt.

There are a couple of more holes with metric threads for attaching A/C and P/S brackets on the late B20s.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

Eric;

Thanks for clarification!