is it un-advisable to spin an engine backwards?

[ryan roopnarine]

I'm certainly not talking about spinning it over backwards with a reversed starter motor or anything like that, more along the lines of lining up torque converter bolts, adjusting valve trains, et cetera. I've heard this advice for the second time last weekend, with the explanation that in timing belt driven cars, one does not want to reverse the tension while under load. can anyone clarify or dispel the advice i received? thanks.

[Ross]

I see no harm. Done it myself coutless times with no ill effect.
EDit; Lots of good reasons given here not to do so. I'll consider myself lucky so far or to have had healthy timing gear.

[whiskychaser]

Dont think it matters. But I wouldnt line up my timing marks turning the engine backwards. Why? Think Bill R put it best when he quoted his dad:
' cos I said so'

[632 Regal]

only thing I would avoid reversing it would be in setting timing.

[Blitzkrieg Bob]

Can be very bad if your tensioner is weak.

the tiiming chain will slacken and skip a tooth or two.

[632 Regal]

now that would suck!

Can be very bad if your tensioner is weak.

the tiiming chain will slacken and skip a tooth or two.

[Dave M]

Turned my M50 backwards plenty. Can't see how one could bugger anything up turning it by hand. However, your question mentioned timing belts, not chains. There might be truth in not wanting to turn a belt backwards, but I dunno................

Dave

[bubba966]

Can be very bad if your tensioner is weak.

the tiiming chain will slacken and skip a tooth or two.

+1, big time.

Doing so on a clutch job has taken out S38's before. Will have to look up the last thread on this happening on the M5board. Poor guy took his E34 M5 in for a clutch job and blew up his engine because the timing chain jumped a tooth or two and the pistons hit the valves. The pics weren't pretty.

[bubba966]

Here you go.
http://www.m5board.com/vbulletin/e34-m5-discussion/104719-disaster.html

[Bill R.]

you don't want to turn a timing belt equipped car backward or for that matter any timing chain equipped car that has a tensioner. They can jump a tooth and in the case of a chain bunch up or break a tensioner or guide rail. Sometimes it unavoidable but you want to avoid it whenever possible.



I'm certainly not talking about spinning it over backwards with a reversed starter motor or anything like that, more along the lines of lining up torque converter bolts, adjusting valve trains, et cetera. I've heard this advice for the second time last weekend, with the explanation that in timing belt driven cars, one does not want to reverse the tension while under load. can anyone clarify or dispel the advice i received? thanks.

[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-m5-discussion/104719-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