A Royal Enfield riddle: is this the answer?

Amid all the shouting at the Three Down bar it was easy to miss the fact that Matthew's riddle probably wasn't nonsense. Acrimony quickly swallowed up the answer. Remember the question he posed?

"If a spark plug is removed from the combustion chamber and has been smashed flat, apparently by the piston, even though the entire engine block is 100% stock and has not been touched in any way, what does that indicate, and might one fix it?"

There had been a clue earlier, when Matthew, advised he needed to set the timing on his Bullet, returned saying that he couldn't because "I absolutely can not get the piston past TDC" (Top Dead Center, the top of its travel, nearest the spark plug).

Matthew had told us that he had a Bullet 500 and he certainly believed it, because he said he had ordered a replacement plug for a 500. The 500 uses a long-reach spark plug. However, the Bullet 350 uses a short-reach plug.

Matthew must have put the 500 plug into what was really a 350cc Bullet, and smashed it flat.

He only said later that he tried another spark plug and it was "fine."

Did you guess the answer? Leave a comment here.