Recently Changed my pulleys on my 2006 r53 and fitted an jcw belt to it, all was fine for 2 days until I started her up and heard s loud squeaking noise!! The noise persists till the temperature need hits 1/4. It dosent seem to be slipping as the power is always there! Do I need a shorter belt?? Any help would be massively appreciated!

I'd have thought so, the JCW pulley is an 11% reduction I believe so that belt is the right length for that.

Your total reduction is 18% so that's a big difference.

It could be an alignment or clamping issue with the pulley though, hard to say without hearing the sound.

I installed the exact same parts in my car with the exact same result.

On cold start it would squeal for about 5 minutes, but when the belt warmed up & loosened the squealing would stop.

I was using the car for about a week or so, as I found spraying WD40 on the belt made the noise go away (btw NEVER use WD40 on a car belt).

Part of the belt shredded & the tensioner needed to be replaced as there was alot of stress being put on it by the belt.

The overall problem is the belt, the JCW belt is too short & tight for the M7 2% Crank & 16% Supercharger Pulley, despite being advertised on their website as suitable. Point being, you need more slack in the belt, so it's not putting extra stress on parts & so you can let the tensioner do it's job.

I'm using the standard belt with the car & I've got another standard size belt in case it ever breaks.

I've had no problems what so ever & definitely no squeak or squeal.

On a side note, if you ever replace the belt remember to make sure you have the pin removed from the tensioner spring when you're finished, otherwise you will have more than just a squeak & please invest in a tensioner stop ASAP.

I hope you've solved your problem, as it was extremely frustrating for me.