Quote:
Originally Posted by Laird Scooby
Are you sure the inhibitor switch thread is M16 x 1.5mm? Many threads on Volvos back in the 60s and 70s were Imperial and if memory serves, it's a Laycock-DeNormanville OD unit which was a British thing so the case for an Imperial thread is stronger - 5/8"UNF-16 would be likely, if not something very similar. Worth double-checking before you tap that thread! 
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Definitely worth double checking, but I was the one that incorrectly tapped the hole to 5/8 UNC based on that very fact. 5/8 UNC outer diameter is something very close to 16mm IIRC, so I just went with it only to find I couldn't screw the switch in.
A helicoil is the professional way to do it, but I've also had some success with two part epoxy based titanium sticks where I've needed to bodge
(
https://jenolite.com/product/titaniu...UaAvCYEALw_wcB)
You may find you don't need to drop the gearbox out to get the top plate off. Unscrew and remove the selector from inside the car, then from underneath there may be space to get a stubby screwdriver in and remove the six(?) hold down screws.
It's a bit shameful that I never did this really, as the destruction would be massive should the OD engage in reverse. I used to just pull the wire from the relay (behind the dash) when I sent the car off for an MOT to ensure nobody did the "I wonder what this switch does" trick. With the back axle ratio being standard (non-OD) then the risk of reversing with the OD engaged is actually very low - I only tended to use it on motorways and the like, and if you'd forgotten to disengage it you'd realise before you came to reverse. Excuses...