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To do the job correctly, both cam and crank have to be locked AND the cam wheel nut has to be loosened. This allows the belt to tension correctly. The cam wheel is then back-held with a special tool while the nut is torqued up.
Here’s how I see it
The VVT camshaft pull is set precisely with the correct setting tool
There are three tool needed
1 camshaft lock
2 crankshaft lock
3 camshaft pulley lock
According to Elearn, conti etc it’s not timed via the belt. There is several degrees of movement with the belt in place. There is only one correct timing position per tooth which is set via the use of the proper tool.
The only reason for loosening the pulley is speed. The belt will not quite fit correctly with both the cam and crank both still locked in place. But the pulley still has to be set afterwards with the correct tool. This is not in the same position as it wants to sit naturally. The tight side should be very tight.
The belt can be changed with or without loosening the pulley. As long as all three setting tools are used the results are identical.
Changing the belt using only the belt as the third timing tool will give variable results and IMO is incorrect. Don’t believe me fit the cam locking tool first as soon as it can drop in, just see how much further the crank shaft can rotate
With keyed pulleys and shafts it was never a problem they can only be fitted correctly but to save the machining and assembly time of both parts we have to use a setting tool instead
These are the bits required to be removed to replace the belt.
I first check the timing of the pulley with the locking set. If it’s correct I leave it alone and remove the cam locking tool to fit the belt
I do also change the “O” seals in the cam cover at the same time. 90% of the time they will have gone hard
Just to confuse the issue some other Fiat engines are timed by the belt, and there are some online guides have copied and pasted them and changed the title to 500 or panda