Kevin: You will need a reducer from 4-2 1/2". The problem you'll run into is that your shop vac is not powerful enough and have enough suction to keep up with the dust and chips from the t.s. You really need a dust collector. But if you don't have one, then you have to make due with the vac. The shop vac sucks and the d.c. blows air around before pulling it into the d.c. Depending on the s.v. you have, you may only have about 150-200 cfms.