Pick your sander, router kit or track saw to see its dust port as the maker states it, whether that figure is an inside or outside diameter, the maker's own route to a vacuum and the adapters it names.
Pick your vacuum or collector to see its hose and inlet size, the fitting its hose ends use, and the hoses, adapters and separators whose makers list it.
"4 in" and "2-1/2 in" are family names, not measurements. POWERTEC lists three different "4-inch" ends: 3-61/64 in OD on the 70136, a 3.51 to 3.73 in taper on the 70291 and 3.965 in OD on the 70147. Measure the two surfaces that meet and compare them with the maker's figures.
Forgetting which end goes inside
Two male ends never join, and some reducers are made for hose only. POWERTEC says its 70136 cone reducer joins two hoses and is not for machine ports. Write down the inside and outside diameter of both ends before you order.
Treating all 4 in PVC as one pipe
Sewer-and-drain pipe (ASTM D2729) and Schedule 40 (ASTM D1785) are different standards, and each needs its own adapter. Rockler's 52680 joins a 4 in dust fitting to Schedule 40; Woodcraft's WoodRiver adapter fits inside 4 in S&D. Read the print on the pipe first.
Expecting a threaded cuff to fit the stock vac hose
Cen-Tec says its Quick Click threaded fittings fit 1-1/4 in threaded crushproof hose and are not compatible with most blow-molded hoses supplied with shop vacuums. If your hose came with the vac, plan on a new hose as well as the fittings.
We start from the two ends that have to meet: your tool's dust port and your hose, vacuum or duct. Port sizes, hose diameters, cuff ends and fitting standards come from the tool, vacuum and fitting makers' own pages and manuals, and every value names its source and says whether it is an inside or outside diameter. Where a maker's figures conflict, we say so and publish no number; inch-to-millimeter conversions are arithmetic, never evidence of fit. We list a part only when its maker states the sizes of its ends. A store's name alone does not show that the brand authorized it. Nothing here has been tested by us, and we give fit information, not safety verdicts: for combustible dust and static, follow OSHA, NFPA and your equipment maker.
