It's one of those things you really should have, those RC valve cable bracket bolts on the top cylinder are a pain without one too.
There's no real excuse as cheap ones are super cheap (seach ebay for "wobble extension"), a 3/8's one is also pretty handy for getting to the top spark plug.

Though I will confess to repeatedly using a 3/8 socket bar on my bottom engine mount;
I thought it "fitted" until Monkey's didn't, either his 3/8's extension is slightly more rounded on it's corners than mine or his mount was tighter, however a proper look at it revealed what was really needed, i.e. some kind of 12mm hex drive, clearly a good bolt with a deep head is fine though a 12mm Allen key (or even better Allen socket) is the correct tool.
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