Cambelt tension

[RustyUK]

I know this has been debated on and off in the past, but I still have questions

While I managed to rebuild the top end of my engine and replace various other gubbins with a high level of confidence, getting the tension on the timing belt right fills me with dread and uncertainty. I simply have no experience to fall back on - and no electronic tool to measure it!

My first attempt, following some advice I read, was to get set the tension such that the tensioner could just about be turned by hand against the belt. The longest section (between cam and fuel pump sprockets, 2.2HDi C5) felt, to my inexperienced hands, 'about right'. I couldn't see it slipping off or snapping. It idled just fine.

However under revs, the longest section of belt started twisting and flapping everywhere. Sometimes it looked like it was twisting itself at least 45deg when revved. Uhoh!

So, I adjusted just a very few mm tighter on the tensioner, and let it idle to even out the tension. Now it's much firmer. Deflection under 'moderate' (yeah, I know, that's fairly meaningless) finger-pressure downwards is just a few mm, and it feels subjectively 'tight'. Much more than it was originally. It doesn't flap around now at speed.

But curiously it has pulled itself a couple of mm or so towards the outer edge of the cam sprocket, whereas before it was more or less central. Is that normal?!

The other worry-point is that the cam sprocket appears to ever so slightly 'wobble', but very very slightly, as if perhaps the sprocket wasn't 100% flush on the end of the cam, but I'm not sure why that would be. The central bolt and 3 bolts fastening it onto the hub behind are tight. Is this, to a degree, normal too?

[Mandrake]

Most people adjust the tension until you can only twist the belt 90 degrees at the middle of its longest run with your finger tips, (cold engine) although some people say 45 degrees.

IMHO its also important to make sure the belt isn't over tight when the engine is hot, as the belt tension will increase when the engine warms up.

If the belt is too tight when the engine is warmed up the camshaft bearings on some engines will make a weird whirring noise, if you hear that its too tight and may damage the bearings long term. As you've observed if its too loose the belt will flap about!

[RustyUK]

Yeah, I'd read both the 90deg / 45deg twist thing. You could almost twist it 90deg to begin with, but that turned out to be too loose judging by the flapping. It really won't twist more than about 45 deg now, perhaps less (but that was hot).

Camshaft bearings bother me. In general. Since learning the "bearing" is just a thin film of oil between camshaft and the aluminium of the head, it feels sort of... inadequate. One of many things I'll get paranoid about and loose sleep over! Mine at least looked quite shiny after 155k miles, with the odd very light score from I suppose swarf in the oil. I gave the head a thorough de-grease and jetwash and made sure the oilways were clear (and replaced the oil check valve, as it looked to be cracking). The camshafts cleaned up and looked quite unblemished. That was until I let one roll against the other by accident, causing the timing sprocket of one to put a tiny dent in the bearing of the other

As for the tension, my gut tells me to back it off just a fraction. It's difficult to differentiate all the various noises in there, especially when the fan comes on. That's another niggle I'd forgotten about until I got it all back together, the rad fan turns on a few seconds after the engine starts up, even though it's cold and aircon off. Not been able to determine why that is yet.

[Lighty]

The deflection in the belt is reasonably normal when revving the engine, it is caused by the different speeds of each pulley fighting against each other at certain revs.
Just about all cars now have spring tensioned adjuster pulleys, where you line up a marker, but the pulley still "self adjusts " continually. This stops the flapping about you have experienced.
Sounds like you may need to back off a touch, as a few mm deflection is not a lot between pulleys.