this is referencing 61x model engines.

the force from a tensioned chain will push the tensioner into its bore (only on non-ratcheting models) - as everyone said, this will cause slack and the IP side will get bungled up and skip or get caught.

There is nothing dangerous about doing this if your rocker arms are removed and you have a ratcheting tensioner. Never do this unless you are certain you have a ratcheting tensioner! When you spin backwards, you allow your chain stretch to accumulate on the opposite side - I would be concerned that valves might hit if it is excessive, but I have no proof.

Judging from diagrams of the IP, spinning backwards does not appear to be an issue.

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