A question that’s been asked and searched for a fair bit recently, so I thought I’d post the answer

The pistons in all cylinder kits, not just Malossi 70cc big bore kits, have a little arrow on the top of the piston.

This arrow faces the exhaust port, so it is generally facing down on horiztonal engines and towards the front of the bike on vertical engines.

Fitting the piston the right way round on big bore cylinder kits is extremely important, since the big bore piston rings arn’t complete (they need to split to squeeze over the piston), they have a point in the piston there the two ends meet.

These two ends generally line up between the middle and outer transfer (intake) ports on the rear of the cylinder providing you have fitted the piston in the right way round.  Fitting the piston backwards on a big bore cylinder kit will result in these two ends opening up slightly in the exhaust port and possibly catching.  As well as this a lot of pistons have a cut away piece for helping intake induction on a certain side, and sometimes are cast / machined to take more pressue on the exhaust / thrust side.

Another top tip for 70cc big bore kit fitting is not to forget to re-fill the coolant before the test ride on liquid cooled bikes….doh!

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