One of the 4 small bolts for securing the diff on my 85' 300TD is buggered. Seems like the nut inside the body is cross threaded. I ran a die over the bolt a few times to help clean it up but it's still not right, too tight as it snuggs up.

I'm guessing that the nut is impossible to get to? I was going to add a lockwasher and use antisieze to reduce the chance of snapping the bolt (M8? what were they thinking?).
Any suggestions?

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1985 300TD 1981 Scirocco 1.6D conv 1986 Golf 1.6D 2003 Golf TDI

The four bolts that hold the rear transmission mount attached to the dif back cover? If so there is a small plate drilled and tapped with threading that those bolt attach to. The plate is loose inside the frame channel. Take the mount off the diff cover, two large 22 mm I think, that will allow you to use a couple of the M8 bolts to secure the plate into place so that you can run a tap up through the threading in the plate were the problem with that on bolt lies. You might need some kind of spacer rather than having to crank the bolts; you'll use to secure the plate, all the way down/in. So use a couple bolts to secure the plate so you can clean up the threading in the plate that is causing you the problem.

Awesome! Thanks for the quick response. I ran a tap up into it and cleaned it out. The bolt still looks good, though I'm sure it's not as good as it should be. Edit: actually I found a new bolt in the size (although with an allen head) so I use that instead, I think the old bolt is stretched.

I am wondering if blue locktite or antisieze is the way to go. I worry about corrosion in fasteners (especially these), but it did have locktite before.

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1985 300TD 1981 Scirocco 1.6D conv 1986 Golf 1.6D 2003 Golf TDI

Blue loctite always with a structural bolt.

Anti seize will cause the bolt to loosen as it is slick.

The prior owner of my car put anti seize on my brake caliper bolts, they worked loose and I started getting these weird clicking sounds when I braked. Turned out the caliper was about to fall off the car.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thanks!
Yea, after reading a bit on it, manual says those (coated) bolts are not to be reused.
I replaced all (4) M8 bolts with my new ones with hex heads. Cleaned out all the threads with a tap and used blue locktite.

The new bolts that I had on hand look good for the application, same length threads.
McMaster-Carr

I need new pads on my front calipers so I'll make sure those are clean with locktite as well; I think they are.
And I'll be sure to do the rear calipers correctly as I still have to put them back on.

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1985 300TD 1981 Scirocco 1.6D conv 1986 Golf 1.6D 2003 Golf TDI

The caution in the manual about not reusing bolts is CYA to make sure they are properly thread locked. They don't want to take a chance on someone reinserting the bolt and not applying more thread locker.

For non stretch to yield applications, in my opinion, thoroughly cleaning off the old thread locker, chasing the receiving threads with a tap, and applying fresh blue Loctite is adequate.

Now if you have a stretch to yield bolt (like a head bolt) that is a different animal. Brake caliper bolts are definitely not stretch to yield.

All that said, I would (and have) replace differential mount bolts on principle. Just because of what it's holding up and the abuse it takes.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech