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I've read several times that spinning your crankshaft backwards is a big no-no. For example if someone is doing a timing belt job and they are trying to align the crankshaft marks and accidentally turn the crank counter-clockwise for a few revolutions instead of clockwise. Is this really bad? What symptoms might come of this, or how would it effect the engine? To me personally, it doesn't seem that it would be a horrible thing, but I don't know. Any input is appreciated.

I've always wondered this as well. See when I have done timing belt jobs I've often accidently moved it a bit past the mark and then just moved it counterclockwise a little bit to get it lined up instead of moving it two more rotations clockwise.

Exactly why is it bad to turn it backwards?

It's supposedly bad to turn the crank backwards because it put's the slack in the timing belt opposite the tensioner, increasing the chance of a skipped tooth.

So if one were to make sure and rotate the crank clockwise a couple times before putting the belt on, this would remedy the reverse tension, right? The problem only lies in when you put the belt on just after having rotated the crankshaft counter clockwise?

It only applies after the belt is on. If you make sure the belt doesn't skip, then there is no harm in rotating the crank backwards.

The cam lobes provide quite a bit of resistance when you turn them. This is what will place a lot of tension on the belt. Because of the way the belt works the tensioner is almost always placed right before the belt 'returns' to the crank sprocket. This is the point that slack naturally accumulates, so is the best place for the take-up to occur.

If you rotate the engine backwards, the 'natural slack' now developes on the other side of the crank sprocket. In fact, the tension caused by pulling the cams will want to compress the tensioner more then normal causing even more slack to develop on the 'wrong' side. If enough slack occurs, the belt can jump.

AFAIK, there's no reason at all you can't turn the crank counter-clockwise. The rings and bearings don't care, especially if you're turning the engine by hand at 10 RPMs or so. I guess there may be a chance the t-belt could skip a tooth if you went too fast, but even that wouldn't hurt anything...you just gotta re-set the belt.

OK yeah, I thaught so. It only applies if the belt is ON the vehicle. If the belt is off the vehicle you can rotate whatever you want in whatever direction you want without causing any harm, I believe (as long as everything is lined back up properly when installing the belt).

codfizzle wrote:OK yeah, I thaught so. It only applies if the belt is ON the vehicle. If the belt is off the vehicle you can rotate whatever you want in whatever direction you want without causing any harm, I believe.

If its all put back into time and he right place.

codfizzle wrote:OK yeah, I thaught so. It only applies if the belt is ON the vehicle. If the belt is off the vehicle you can rotate whatever you want in whatever direction you want without causing any harm, I believe.

This is assuming you are working on a non intereference motor, correct? I have an EJ20H DOHC motor and I thought that if you turned the crank independently from the cams it will bend your valves.

Depends on where the cams are rotated. If they have nothing holding them in place (like the belt), then the valves will be closed, and the crank can be rotated without interference.

But in general you are right, don't rotate the cams independent of the crank to avoid interference.

Yeah, sorry I forgot to mention that what I said applies to only SOHC engines. I've never worked on a DOHC, but they are interference engines and need to be tended to much more carefully.

And the newer '97+ EJ22/'99+ EJ25 SOHC are interference, too.

(Yes, I know this is an old-school forum.)