[Bill UK]

Have you tried screwing in an old head bolt to see if you have the same problem, how much longer are the new studs? I would have thought Toyota would have machined the threads deeper than required in the block to allow for the head and block to be skimmed. Sounds like you may have some rubbish in the threads of the hole. I think I would remove the head and use a long thin round brush to clean out the holes (the type spray shops use to clean out their paint spray guns or you may find them in a gun shop) screw in the new studs to see how far they go, you may need to result to cleaning the threads with a machine tap. You should be able to borrow one from the company that skimmed the head. They come in three types, bottoming, plug and taper, I would recommend you use the taper first as its easer to locate into threads without cross threading and shouldn’t cause any block material to be removed providing the hole is deep enough. As this is the most critical part of the engine I wouldn’t take any chances.