Another case of me talking (typing) before I get off my butt and just look at my own engine. It's been a while since I changed an Opel water pump, but my engine is on the floor and just about to receive a new pump. And suddenly the wierdness of the water pump and timing chain bolts came back to me. Here's what I found.
Bob is right (as usual). The Opel water pump bolts are M7x1.00, which exists in my tap and die kit, but I have yet to see one in a bolt store. So even the bolts are probably special order, let alone the Heli-Coil kits. The 7 mm bolt is a bit bigger than a 1/4" bolt (0.278" versus 0.250") but that may be your only choice, unless you move up to an M8 (0.315") bolt. The closer SAE equivalent is the 1/4" x 24 NF, which is even a bit finer thread than the M7x1.00. So your choices, a I see them, are:
1) Find a M7x1.00 Heli-Coil (not likely) and install it in the block. But you will have to remove the chain case. BIG OUCH!
2) Use a SAE bolt of an equivalent length, and convert the hole (JUST in the block!) with an repair insert, either the NF (20 TPI) or NC (24 TPI). You still have to pull the chain case.
3) Drill out the water pump hole to allow you to use an M8x1.25 or a 5/16" NF or NC SAE bolt and insert. The timing chain case hole seems to be big enough to allow a 8 mm or 5/16" bolt through, but the water pump holes are not.
If it is one of the long bolts, they actually thread into the cast iron block, NOT the chain case. Only the short water pump bolts thread into the aluminum timing chain housing. So ignore the comments about not drilling the hole out, no matter which size you chose to use. And unless you have a VERY long tap for the new insert, you have to remove the entire timing chain case. Those long water pump bolts also secure the chain case to the block, so you are inviting a leak if you don't get a good thread for the bolt to grab.
I like oldopelguys suggestion. This is a PAIN to do with the engine in place. If you could find a longer M7 bolt or stud, you might be in business. Otherwise, do a makeshift repair now using a 5/16 NF inch tap, and just get enough thread into the block to get a grab with a SAE bolt. Then do it right (by removing the chain case) someday when you or the next owner next have the engine out.