Just want to clarify, M12 does not mean it's a 12 mm hex head. M12 means the bolt has a 12 mm diameter, usually a 17 mm hex head. The engine/transmission are held together with ~5 M12 bolts and maybe a few M10 on the bottom. 1.25 thread pitch sounds right. The length of the bolt entirely depends on the engine stand being used, specifically how long the circled part is in the below pic. You really don't need grade 10.9 bolts for this. The ones I use I picked up at Home Depot, Lowe's, or Rural King for cheap (maybe grade 5).

Does your stand look similar to the one below? You just adjust the arms to line up with the threaded holes in the block then bolt it on, no adapter plate needed.

 

I am aware that M12 doesn't mean 12mm hex. I never said that it did. In the pics I referenced it shows most of the bolts are 10x90 and a couple are 10x100.
I was advised that using a lower bolt rating would be unsafe because they had to be equal or greater to the automotive rating listed by Honda. I read that in a few threads.

Transmission Case

Ironton 47033

 

it seems you can mount any Kseries transmission to any Kseries block.

Which would suggest the bolts that mount to the kseries block would be all the same.

Thus, as Bro said, they are all m12x1.25, except the two bolts on the cover plate at the bottom. Length will differ.

 

I’ve always used the bell housing bolts that hold the engine and transmission togethe.

 

They were too short for my engine stand but that's definitely a good idea. The bolts I use are grade 8.8 by the way.

 

Be careful using the trans bolts. I used them on a Harbor Freight engine stand, and there wasn't enough thread engagement. Got caught by surprise and dropped a block on the stand (thankfully not my foot!) when the threads stripped. For anyone else using the Harbor Freight engine stand, I believe I ended up with an m12x1.25 80mm bolt. Make sure you get lots of thread engagement!

I've had good luck finding metric bolts at Ace Hardware by me also. 👍

 

I ordered them from Bolt Depot.