Honda-SOHC

having had some helicoils replaced i tried to assemble rocker box apart from some helicoils coming loose (i'll deal with that guy later!).
however one of the rocker box bolts would not tighten. taking the rocker box off (again) i found this (see pics). i think the phrase according to James May is "oh cock" although i used stronger! and before anyone says it, yes i did have the torque at the right setting!
i guess find someone who does alloy welding and have it rebuilt or any other ideas?

I'm not that far off putting the engine back in the frame, at the moment the oil pan is not on, neither is the clutch assembly, starter motor, oil pump ands right/left hand crank case covers (at the polishers) or carbs. Obviously the engine is pigging heavy and to keep the  weight down I thought I'd fit the aforementioned bits once in situ. Seems logical and looks like nothing will foul the frame if I do but I've a sneaky feeling I'm setting myself up for some problems. Any thoughts, anyone done it like this?

Yes it's 6mm. One of middle four on the head that has 2 either side of the cam chain. It's the rear right , from front of engine. I've only got a bog standard hand drill, will I get hole straight enough with that? I suppose original hole will act as guide. 

Thanks all. There are 2 dowels to the rear and they're new and holes are clean. I'll look out for taco drive meshing fully too. It's great to feel able to ask apparently dumb questions and get good advice thanks guys. 

Thanks for advice - prized chain off and after removing head again found chain not on crank fully. After that plain sailing.

Wish it was Peter , it's just a naff angle and my poor photographic skills. unfortunately it is round 
John  - i thought i might try that but there doesn't even seem to be enough slack to persuade it off the sprocket - unless you have a nifty trick. i have tried it. my thoughts would be take off the chain, remove head and have a look inside - if i could just get that b****y chain of.!

Yep , that was the video clip i watched earlier in the week which filled me with such hope! i notice he mentions jiggery pokery with the cam chain - sounds like the bottom end of the chain may not be on correctly. So all i need to do is get out the half wedged in camshaft and try again.!!!!! 
 any other ideas that come to mind would be great. i'm trying to persuade the chain off the sprocket but it doesn't have enough slack to even do that. the cam shaft is in to the point that all i need is the flange, that the sprocket bolts into, to get past the last bearing journal and it's home - although its 2 -3 mm bigger than the gap available.

can anyone explain installing the cam shaft on a 400/4. the engine is out of the frame and being reassembled on the bench. the Haynes manual procedure just will not work as the cam shaft will not go all the way through the sprocket with the chain on. it is a new linkless chain. a you tube clip i saw said to hold up chain, slide camshaft through and then slide on sprocket and put chain over - easy peasy i thought! however there just doesn't seem to be enough slack in the chain to get it over. i now have the camshaft half in and half out and the chain is so tight i'm afraid i'll break or stretch it. i've removed the cam guides front and rear to give a bit more room but to no avail. the flange on the shaft that the sprocket fits on fouls the bearing seat as well. any ideas. the chain was from David Silver so i guess should be the right length. Any ideas before i resort to the club hammer. Totally wrong but just feels satisfying!