Size bolt on idler pulley on 2001 oldsmobile silhouette

$4.87. No one give the size. It is the size of the socket that fits it. Bolts are measured by thread section diameter not head diameter. You don't say which? 13/14mm bolt head? It looks like the top is stamped 9.8mm which is shaft diameter.

https://www.tascaparts.com/oem-parts/mopar-idler-pulley-bolt-6508247aa

All questions are better served with 'year make model', rather than 'year, cc and engine friendly name'.

https://www.google.com/search?q=2008+5.7+hemi+idler+pulley+bolt

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the idler pulley used a bolt to hold in place, you may have received the wrong part, compare to the on you removed. You should have replaced the tensioner with idler attached, again compare the old and new. yje new tensioner usually comes with new bolt.

If you are needing it for the serpentine belt, it should be either a 13mm or a 15mm.

Go down to a local salvage yard and get a bolt.

I have enclosed a picture diagram to help you locate the crankshaft sensor, some have two sensors so you will need to look. the first location is behind the crankshaft harmonic pulley. and the other location is on the right side of the engine. hope this is helpful.

One of the pulleys or idlers is bad now that you have a new belt. Take off belt and check them .

You need a battery group size 75 side terminal with at least 525 cold cranking amps capacity, the Sears Item # 02830375000 ' Model# 303

There may be a label with a diagram showing the routing of the belt on the fan shroud area but basically the grooved side goes against the grooved pulleys and the smooth side goes against the smooth pulleys

relieve the tension by moving the idler pulley to make the belt slack. Some have a hole in it that accepts a 1/2 in. drive ratchet and you pull it to move the pulley. Others have a bolt head you put a wrench on to move the pulley.

is it your belt or is the "IDLER' pulley going bad?

Or a junk yard.