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How to measure CR on deck-protruding pistons ?

Practical Engine Building

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i'm building a Nissan RB30 with 25det head. Its an original Nissan A8 NA Block with stock crank , rods and pistons. These engines have the pistons slightly protruding trough the deck of the block. How do I go measure the comp ratio ?

Do i lower the pistons in the engine , measure CC and then subtract the cilindrical capacity of the length i lowered them with ? Possibly coming up with a negative value which I have to subtract of the Headgasket + cilinder head cc's ?

Is this info in the engine building lessons or webinars ? I couldn't find it

Thanks all


I'm not sure I follow? You can measure the deck clearance with the pistons protruding from the block using a deck bridge. There's a webinar in our archive that covers this - https://www.hpacademy.com/previous-webinars/154-measuring-deck-clearance/ This can then be used in the CR calculation provided you know the dome CC (which should be provided by the piston manufacturer0.

If you're trying to measure the dome CC then you can move the piston a measured amount down the bore and calculate based on this distance and the bore diameter what the CC is. Then seal around the edge of the piston with vaseline or grease and use a burette to measure the actual CC. The dome CC is the difference between the calculated value and your measured one.

thanks for clearing that up :)

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