I measured the idler pulley my three blade deck. It only uses one idler pulley which, is used to obtain a belt wrap on all the blade pulleys and to tension the deck belt. The pulley measures 4.0" diameter on the flat surface the back of the belt runs against. Again, I can't say the pulley is OEM as I rebuilt the deck about 10 years back. As long as you can get the belt to tension I think you will be okay regarding pulley size.