Bit more on this.
@RW...Before TDC (AIUI) refers to valve or ignition timing.... ie how far before TDC does the spark fire.
Which is all dynamic and driven from the ECU these days but in the days of dizzies and rotor arms, critical to know.
Nice bit was you could see.... so you turned the crank towards the TDC mark and often had a little scale showing degrees BTDC so would stop say on 10 degrees, then drop in the dizzy and rotate so the points just opening on relevant pot... usually No1 but could check against valves/cam position, lock it then plug in a strobe light for fine tuning.
Right, some basics for Bradders.
Suck squeeze bang blow... or if starting from Bang...blow, suck, squeeze.
That is what each cylinder does as the crank rotates, twice for the full sequence.
Compare that to the valves/cams for any pot.
Bang... at the start and as the piston goes down, being pushed by the fuel burning/expansion, both inlet and exhaust valves closed and stay that way, nominally for the first half rotation.
Blow... piston starting to come up, exhaust valves open, inlet valves still closed. exhaust gasses are pushed out.
Suck... piston has now come back to TDC, exhaust valve closed, inlet valve opens, as piston goes down, fuel/air mix is sucked in.
Squeeze.... piston coming back up, exhaust and inlet valves both closed, fuel mix compressed....
Back to TDC and repeat from bang.
Opening and closing isn't at TDC, exactly where depends on engine, design, valve timing but it gives an idea.
Next is the firing order.... 1342
So 1 is at TDC just about to go bang... valves are all shut.
3 is next to go bang so is at bottom of stroke and just about to squeeze, inlet valve just closed, exhaust valves closed.
4 is at TDC at end of blow, exhaust valve just closed, inlet opening.
2 has just gone bang so at bottom of stroke about to blow... inlet closed, exhaust just opening.
You can see all that happening if you rotate the engine when belt all on and can use it to set from scratch.
At least nominally. For exact set up you really need to be sure of dwell angles and when opening/closing happens plus measuring kit.....
BUT.....
Makers help us here.
By putting on marks or similar for timing.
Renault AFAIK on all their 16v petrol burners have a slot at the gearbox end of the camshaft, this is what the cam locking tool goes into. Off centre and IIRC, when cams are set correctly for crank at TDC, horizontal and below centre.
From that you really don't need to worry on first principles, just set the slots and all should be well.
On my KIA was dimples on the (keyed) sprockets that faced each other, one at 9 O Clock one at 3 when set correctly.
Renault Diesels (the 1.5dci at least) has location holes in the sprockets and back plate for a location pin
Just need to know what the specific engine has.
You can take off the covers and have a look while you are practicing locking the crank.
Lock them as well and paint on some marks on cam sprockets, belt and back plate for easy reference when changing.