Actually they are not the same block in those years. The 8hp and 6hp shared the same twin cylinder 197cc block for years up until the last couple of years where now the 6hp is the same single banger block as the 4hp and the 8hp now shares the 212cc block with the 9.9. The 9.9 has always had that 212cc block since it started being a four stroke i think. It may be possible that they are interchangable against the rest of the outboard, but they are definitely not exact copies of each other if those are the years you are comparing.
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