Standard metric M8 bolt has a pitch of 1.25mm under DIN 975 standard.
The bolt used for the diff support: M8 X 20,5 - 8.8ZNS3 where 8.8 is the quality, 20.5 mm long, ZNS3 zinc plated (very common).
The standard for hex bolts is DIN 933 (whole length thread) or DIN 931 (partial thread).
The standard lengths in mm are: 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, ...
Most threads on the E46 are standard coarse pitch, except for suspension components, which use a fine pitch.

1. Bolts and nuts used to fasten the heat shields: Insulation
ST - SelfTapping for metal, also a standard for those, "knipping" bolts in German

2. small plastic brackets on top of RTAs use a universal plastic bracket. They also fit from other brands of cars. Or order a universal multi-piece pack for cheap.

 

Scheg - Thanks for your reply. Are you saying that the diff support bolt should have the same pitch as metric coarse pitch bolts from the hardware store? As I mentioned, they fit in the thread matching template at the hardware store, but the template only had a few turns. When I start threading the 8 mm coarse metric bolt from the store into the car it starts to bind.

Regarding the heat shield screws, I spent considerable time on that page before posting here. I just looked again. Part 13 is the one that holds the supports and not self-tapping. One of the numbers for part 10 is 07119901299. Do you think that is the one for the tunnel heat shield?

 

The bolts from the car fit the template. The bolts from the store fit the template. Compare the bolts side by side. If there is a difference you can visually see that. Clean the thread with a M8 tap drill or use some grease. If you dont mind, why do you need to change them? If they snapped, then Im sure the thread in the car is rusted and causes the new bolt to bind aswell.

 

If you dont mind, why do you need to change them? If they snapped, then Im sure the thread in the car is rusted and causes the new bolt to bind aswell.

I mucked up the threads on one. The other 3 are fine, but I added a reinforcement plate so it would be good to replace all with slightly longer ones. I tested the store bolt in a hole on the car with good threads. I just examined the bolts using burnoutrubber's grandpa's method and they look identical, except the BMW bolt seems to be manufactured to a higher standard. The thread ridges come to a thin edge, while the bolt from the store is not as finely wrought. My current plan is to buy a factory replacement bolt and hope the receiving threads aren't toast. It would be nice to find 4 BMW bolts that are 5 mm longer, but not critical.

 

Can I ask why you are buying the bolts at a hardware store? Hardware stores around me don’t carry bolts that carry 8.8 rating. They are ordinarily grade 5, which is not equivalent. And there is also the zinc coating.

You can get 8.8 zinc bolts online.

Are you buying at a Home Depot-type store or specialty?

Also, my grandpa taught me to hold two bolts against each other pointing in opposite directions thread to thread to check thread pitch. If they don’t match exactly, one is wrong.

 

hold two bolts against each other pointing in opposite directions thread to thread to check thread pitch

^ This is the quick and easy way to verify matching thread pitch.
You can also use a thread gauge, or Vernier caliper and count them.
Suggest you search, read up, and watch some youtube videos on the subject.

 

They are ordinarily grade 4.8 or 4.4, which is not equivalent.

metrified that for you. Hardware store bolts also suffer from an unknown supply chain.

I use them regularly- for things like heat shields and such that don't matter.
But not things like suspension component attachment...

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I have a complete set of genuine subframe connecting hardware. Ping me