Potential worries in spinning an engine backwards? (French timing belt content)

So I'm getting ready to do the timing belt on the Renault this weekend. It IS an interference engine, after all. I have all the parts and a copy of the FSM, but there's one part of the job that puzzles me.

They want you to drop the oil pan and lock the crankshaft by slotting a pin into a slot on one of the weights before you try to remove the crank pulley. And dropping the oil pan requires removing a bunch of other BS first. This single recommendation makes what would otherwise be a very simple job far more complicated than it needs to be.

Now, I'm tempted to skip this step and attempt hitting the crank pulley with an air impact gun before I go through that extra 2-3 hours worth of work. Is there any reason I wouldn't want to do this? Potential risk to the engine if it spins backwards and doesn't come loose? Anything like that? If it's risky I'll just have a long sigh and go through the motions, but I've typically been able to just gun them off or use the breaker bar trick in the past, no locked crank necessary.

It should be reasonably safe to go backwards at the low rpms that an impact produces so long as the timing belt is on the car to keep the cams in time.

Timing belts really don't like to go backwards and the risk is higher that you'll have valves playing rugby with pistons.

Unless you can come up with another way to keep the crank from spinning (like fabbing a flysheel lock for example), I'd say you're a bit stuck and will have to follow the FSM.

I would see about getting access to the flywheel some how and jamming it. You don't want to have it spin backwards especially with an impact gun powering it. You will bend valves if you spin it back far enough with a 500ft lb impact. I would be tempted to set things up at TDC on #1 then back the motor up a bit by hand (pulling all he plugs) and put rope in the #1 cylinder to prevent it form being able to spin through TDC. Then take the impact gun (Or a BFR) to the pulley. DONT PUT ROPE IN ANY OTHER CYLINDER. If you put it in to one that the valve is open the force on the rope from the piston will bend the open valve. You could put it in to the other cylinder that is in the exhaust stroke but I would not risk it.

Once it is loose I would then rotate the motor back to #1 TDC and lock the flywheel and finish the removal.

The FSM is unclear (simply shows an illustration of the crankshaft with an arrow pointing to the slot) but a fellow Renault enthusiast is telling me there SHOULD be an access plug on the side of the block you can stick a 6mm punch into to lock the crank. I sure hope he's right...

This must be for an automatic car? I usually just find a way to leave the car in gear on the ground.

I have done in on a BMW 6 cylinder auto by drilling some holes in a long bar that corresponded to threaded holes on the crank pulley, then the bar holds the pulley (or gear) while you loosen.

Finally, the ghetto move would be to clamp the old timing belt to something stationary...

In reply to rcutclif:

No, it's manual, but it's also FWD (transverse, not much room to work) and the timing belt is 40k past the service interval...in addition to being 27 years old. So I'm really trying not to play with fire here! The car is also very, very light, so I'm not sure just leaving it in gear on the ground will work, but I suppose crazier things have worked. With a wheel off it's a straight shot to the crank pulley bolt through the wheel well, so that's the route I was planning on taking. I guess the first step would be to swing by the garage tomorrow and see if the access plug to the crankshaft slot does, in fact, exist...

SlickDizzy wrote: The FSM is unclear (simply shows an illustration of the crankshaft with an arrow pointing to the slot) but a fellow Renault enthusiast is telling me there SHOULD be an access plug on the side of the block you can stick a 6mm punch into to lock the crank. I sure hope he's right...

Hondas are done this way. I don't think even the French are so cheap they wouldn't do that (think of warranty claim numbers!)

QED. With the proper sized socket and a long breaker bar. support the end of the bar on something. Disconnect the ignition, you don't want the engine to start. Turn the switch lightly to briefly engage the starter to loosen the bolt.

Put it in gear, take a stick and put it between the seat and the brake pedal. Move the seat forward. THe engine is locked. Alternately, have a strong leg mash them while you hit it.

If there is a lot of slop in the driveline that might make and impact ineffective... take the starter off and use a crowbar on the gear.

SlickDizzy wrote: The car is also very, very light, so I'm not sure just leaving it in gear on the ground will work, but I suppose crazier things have worked.

Put it in gear and have a helper stand on the brakes while you remove the crank bolt?

In reply to SlickDizzy:

Just put the transmission in it's top gear, and keep the front hubs from turning. Have someone push on the brakes, or drop a screw driver between the vanes in the brake rotor and the caliper. Just make sure the transmission is in top gear, not first or reverse. You don't want torque multiplication to the wheels. You won't hurt anything if the crank moves a couple degrees.

You bitches are too slow!

in the fiat world (and this worked on my old Hyundai when I did the t-belt) we would disconnect the coil to keep the car from starting and put a socket on the pulley not and brace it with a long breaker bar against the subframe and very lightly tap the key to engage the starter.

You do not want to spin the engine, just get it to turn enough to break the nut free

If there is no access to the flywheel, you can drill a hole there, and hold it with a large screwdriver. Otherwise, I agree with the "lean a breaker bar on the frame and hit the key" group.

I did the breaker bar on the frame and use the starter trick on the 318is when I did the timing chain. It's quite exciting. Took 3 hits to get mine off.

gearheadE30 wrote: I did the breaker bar on the frame and use the starter trick on the 318is when I did the timing chain. It's quite exciting. Took 3 hits to get mine off.

It's a test to make sure you know which way your engine turns.

That's an old trick I've used on Spitfires/GT6 for the past 45 years.
Realize that the torque from a stalled starter is not all that great, and may not break loose the nut.
OTOH, if you arrange the breaker bar so that the starter can spin it and turn a few degrees before it
impacts a fixed stop, then all of the kinetic energy is stored in the crank and flywheel is released all at once.
On a Spit/GT6 this is easy to do, since there are 2 sturdy chassis rails on each side of the motor.
With the car raised to clear, I place the breaker bar handle against one rail, such that the starter will
spin it to crash into the other rail.
With all clear, hit the starter, and hold for second or two;
That not only breaks the nut loose, but conveniently unscrews it from the crank snout.

BTW, I've taken a lot of grief for this method "Oh my goodness, that sounds really dangerous"

Oh well. ;)

Hmmmm, might there be another reason for locking the crank @ one point???

Is the crankshaft timing belt pulley keyed? Once you loosen the bolt, if it isn't keyed, it might be near impossible to get it set to TDC, or where ever if you don't follow the procedure.....