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Most folks recommend the diamond shaped gasket. I used it on my 150 cc 157QMJ, GY6 engine, & the engine was backfiring. I returned to the use of the round gasket & the backfiring stopped.
I concluded that if the exhaust (header) pipe flange mates flat with the exhaust port of the cylinder head, the diamond shaped gasket is desirable.
If, however, for some reason, the flange does not mate flat with the cylinder head, it is better to use the round crushable gasket, as it accommodates a mismatch or takes up space between the flange & the cylinder head.
Hope this helps with your decision-making.
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