I finally cleaned out the crankshaft threads after removing the flywheel. To recap, I couldn't get the tap down the flywheel hole (and nor could some others on the forum who tried) so the flywheel had to come off. It wasn’t that hard tho it did take some practice. I thought for I’d record some notes for the next home spannerer facing this. Best protect the journals, threads and timing bush before starting. Also put mark at a) the top of the journal lobe (use paint), b) the flywheel adjacent to this mark and c) the flywheel 180* from this. Hold it in a tightly in a vice by the crank web. Have heavy gloves – I used wool gloves inside big welding gloves.
1. Use heat before taking the bolts off as they may have red Loctite on them. Use a short wrench (less force) and if its super tight use more heat. You can wreck or even break a loctited bolt with too much force. See this vid:
https://www.youtube.com/watch?v=mUZUyqBhg_02. You can get the flywheel off with only heat – no hammers! The trick is apply lots of heat, then rotate the flywheel 180* (so mark ‘a’ is now aligned with mark ‘c’), add more heat, then slide it off. Some people say one side is better than the other but mine were exactly the same. A mapp-pro torch will do. A second pair of hands will make it easier:
https://www.youtube.com/watch?v=mMHIe8oSG-A. Once it is cool wriggle the flywheel off:
https://www.youtube.com/watch?v=27jGDRv-S343. Cleaning the bolts - 3/8 BSCy 26tpi. The high tensile bolts are tight in their threads. I found my 3 fluted taps did not fit into the holes. But I borrowed a 4 fluted tap and this one did. No idea why the difference. But find a tap that fits and only remove the minimum of metal, if any. Side note - this borrowed tap also did not fit down the flywheel holes.
4. Practice the manoeuvre of refitting the flywheel when both are cold. Then put the crank in the freezer and the flywheel in an oven – I used 230*C. Wait an hour or so. Crank back in the vice, smear oil around the central flange where the flywheel goes, get the hot flywheel. Do the refit manoeuvre with mark ‘c’ starting near mark ‘a’. Slide on then rotate it around 180* so mark ‘b’ is by mark ‘a’. Wriggle the flywheel until you can drop an m8 bolt in the hole to locate it. Quickly, fit the other two old flywheel bolts using a socket (so you don’t get burnt) – not tight. Wriggle the crank as you do this to locate it. Swap the M8 for the last old flywheel bolt and loosely tighten it, wriggling if you can for alignment. If the flywheel tightens on the crank so that you can’t move it, then add some more heat with your torch. And or lightly tap with a copper hammer. If its all stuck then let it cool down and reheat (as per getting it off) and wriggle to fit the bolts. See vid:
https://youtu.be/K-PPLHry6kQ5. Check the bolts move freely and tap or reheat and wriggle as required. Swap out the old bolts for new, having cleaned the threads. When torquing them I found if only one bolt was fitted to the flywheel, then torquing it moved the flywheel a tiny bit slightly misaligning the other bolts/holes. So hand tighten all 3, remove each one it turn to clean, Loctite and torque. BSA says 30 ft-lbs, SRM says 45 ft-lbs for their new bolts. I used 35 ft-lbs and red Loctite.
Always possible I've done some dumb thing so comments welcome to help the next guy.
YMMV
