is it un-advisable to spin an engine backwards?

[healtoeit]

can anyone clarify or dispel the advice i received? thanks.

Spin it one way... everything is designed to go that way

[GoldenEagleFan]

I would also agree. As an engine designer for 30 years, slack side and tension side of belts and chains for timing are typically different lengths with the slack side radii on guides designed for one rotation. Reversing the rotation can cause a slack side chain to "catch". If you do it slow however and stop at the slightest odd resistance, you should be OK. Belts are not as big of a deal.

To me the bigger issue would be for valve timing.

For as close to exact timing as possible, the engine should be rotated the "normal" direction to take up all slack and then timing marks lined up. If you miss a mark, go around and try again. Backing up uses up all the inherent tolerances in a drive and will not be as accurate as rotating the 720 degrees and trying again.

[GoldenEagleFan]

If you want to be paranoid at a micron level....

It's normal in an engine to have small particles of contaminants become embedded in the crank, con rod, and cam bearings. As you can imagine, these particles can "catch" on the micro barbs that are formed on the corresponding steel running surfaces and be retracted if the engine is rotated backwards. when the engine starts rotating correctly these particles can cause more damage to the bearing before becoming embedded again or getting flushed through the oil system.

Probably over thinking but generally another reason to not turn our engines the "wrong" way.

[Mordan]

Here you go.
http://www.m5board.com/vbulletin/e34...-disaster.html

this makes me mad.. I had a BMW mother****er who bent my M20 valves... I was a noob then. i hadn't a clue why my car was running badly.

I learned to do the cylinder head job and did it. I will never go back to any mechanics except my friend who helped me at times.

80% of mechanics are crooks or incompetent. they say here