Passenger side motor mount threads slightly off center....need assistance please!
 

Just give the engine a shove in the right direction. Show it who the boss is.

Next time, just disconnect one mount at a time.

Do you mean the top two bolts (6 mm allen) or the single bottom bolt (8 mm allen)? Either way, insure you aren't cross-threading the bolt. Leave the 2 top bolts loose until you get the bottom bolt started. You will likely need to pry the engine around w/ a crow-bar and perhaps loosen the 2 tranny mount bolts. I just did this 1.5 weeks ago, and it is never easy.

Buy the correct tap if you need to clean the threads. I bought a cheap HF set a while back that is poor quality but works most of the time. I fill in with what isn't included in the set.

Some place like Ace Hardware may have a washer. Lowes etal may have one in the specialty section. Lots of places online will have the washer.

Yes, buy the correct thread chaser and use heavy grease on it.... go in a little at a time.. back out and clean and regrease the threads... until you have a good set of threads remaining...

Taps are designed to cut new threads. Repairing existing threads is best accomplished with an appropriate thread repair tool, especially when dealing with an aluminum alloy like an engine mount arm.

Scroll down this chart.....
 

12.9 is the highest strength bolt you're going to find. The A2 is a specialty bolt. Look at the strength data in the chart you posted. It is NOT a strong bolt, even weaker than the 8.8 grade.

Aircraft fasteners are almost universally SAE.

Unless it's a drag car with a superpump a grade M8.9 (aka SAE grade 5) will be just fine.

I'm currently dealing with the same exact problem on a 1983 240D. I disconnected both mounts for replacment. The driver's side bolt goes in easily, but the passenger side bolt goes in about halfway and hangs up. I didn't have any trouble removing the old bolt, so I don't think there's a problem with the threads in the engine support arm.

I'm going to keep playing around with it without forcing the bolt in. That just feels like a bad idea to me.

I may loosen the smaller mount-to-body bolts to see if the mount itself has enough wiggle room to compensate for this. Does anyone know if I can tighten the smaller bolts with the engine bolted in place?

If you think the threads are toasted, make a thread chaser from an old bolt and clean them.

As for alignment, get creative with ratchet straps and wood blocks. You can line it up, it just takes patience.

There are professionally designed and executed Thread Chasers...
HOME MADE ones are likely to make the situation worse...

AND as to pulling and forcing the engine into a POSITION which allows the bolts to be screwed into the WRONG engine mount..

I suggest that may invalidate the proper function of our engine mounts....