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In this question comments raised the question of whether or not it could harm an engine if it was forced to turn in the "wrong" direction – for example if a car is parked on a hill pointing uphill and left in first gear, could damage be done if it rolled down the hill?

I would imagine that this is specific to the engine and transmission as some can do this with no obvious harm – for example old SAAB two-stroke engines would run in either direction depending on the ignition timing (and it seemed the phase of the moon) and direct reversing marine engines which "shift" into reverse by stopping, moving the camshaft, and restarting.

So, if there are potential problems, what components are involved and how would someone know if there is a risk of damage?

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under specific circumstances yes this can cause damage but it would need to spin fast in some cases:

  • obviously it can wear out bearings and oil pumps due to no oil since it will not get pumped backward
  • it can pressurize the intake and the vacuum system which it is not designed to do. this may rupture diaphragms or damage sensors that are being pushed in the wrong direction
  • you could blow a return line on a power steering system by spinning it backwards. The return lines may not be built to withstand the same pressure the feed lines are
  • you can cause electrical issues from spinning the alternator or generator backwards.
  • there are specific engines like RDS_JAF mentioned that may get damaged because of the way they were built
  • you can cause a valve strike by tightening the slack on the wrong side of the timing belt and overpowering the tensioner.
  • it could start and run in reverse if it's direct injected
  • there are also special engines designed to run in reverse which some of the above issues will not be an issue.

none of these are likely to happen but if it's a steep enough hill and a low enough compression engine set in first...

The best thing to do is stick it in the gear it should be in: pointing uphill should be reverse, pointing downhill should be first. And always set the hand brake.

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The engines I could think of that would be susceptible to damage in this scenario would be the desmodromic cam engines used by Ducati. They have a 'positive-follower' cam setup that could in theory cause lash issues if forced to turn backwards. There are probably some other engines that would have problems too because of pumps turning backwards.

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