As mentioned above all the info is here
https://foc-u.co.uk/index.php?topic=92.0 back in the day what 19 odd yrs ago now, where did all that time go. As stated earlier 638 isn't a thread-lock neither is 620. All Loctite thread-lock compounds start with 2 and their adhesives or retaining compounds start with 6.
Here's what retaining compounds do:Here are link to the different loctite versions:
I remember talking to a chap in Yam tech dept at the time, asking why they'd chosen 620, the reply was to "fill the gaps in the thread" I see, so why not use a thread-locker then? I used to know a fair few people who dealt with and knew personalities in Yams tech Dept back in the day, let's say their comments weren't gleaming and may answer why they picked the wrong product, who knows.
What the bulletin doesn't tell you and Yamaha didn't want to face because of cost. Is the the condition of the threads is vital, as that's where the problem is, machining tolerance, which is why some fail others don't. Meaning the lay shaft on used bikes could well be screwed, beyond this fix. In fact Yamaha did replace some shafts but all on the quiet, they didn't want a recall as it's a big and expensive job and are not alone in that approach.
Many manufactures will only recall when there's no other option. e.g. good old Hotpoint and their dryers springs to mind. Bearing in mind not all bikes were affected, Yamaha wanted to avoid a recall and have a cheap a fix as possible and one that would kick the problem into the long grass, which it did. If you slap on 620 and the threads are sufficiently worn, yours look worn, how much I can't say from a picture. It
will still come loose, quite when is all dependant on how worn the lay shaft threads are, could be days, months even years depending on your mileage.
What I do know is if you use 270 the chances of it undoing are reduced further, bearing in mind you had on idea the sprocket nut had come undone
until you removed the cover and looked. It's also possible your chain has a good few tight spots by now too.
The advice offered is to help you, it's up to you, if you use it. Stick on 243, 638 or 620 even superglue if you wish, it's your bike and pocket. Best of luck with whatever option you choose.
[I have cleaned up the post bit as this is very useful info and will link it in the FZS600 FAQ - bkd.]