My tires are starting to show cracks, 8 700 km
Should I be concerned?
About two years old

A little crazing and cracking is normal, particularly riding in hot conditions. Generally it is acceptable on the tread blocks and round the edges of the upper surface, but not if it's round the bottom.

Just out of interest, what's the date moulding on them say? They may very well be a lot older than 2 years.

Cracks like some of yours around the bottom of the blocks along the centre line are often caused by either extended hot running or pressures being too high. Low pressures and hot running tend to damage the outer most tread blocks. It looks like you've had a reasonable innings out of them, probably time for replacement? They are only going likely to get worse and eventually look like this, especially if ridden hot:

__________________

Look at that Michelin T63 http://www.xt660.com/showpost.php?p=211820&postcount=29
I lost some knobbies due to my own stupidity, but it was still possible to drive at normal speed. So don't worry.

Ciao
Solitario

I wouldn't put a pair of tourances on mine if they've gave them to me for free. I have never in 30 years plus riding, experienced a tire that put the ****s into me like they did, especially in damp or wet conditions.

..totally bum clenching

And then some