from 53 to 62, all nailheads were a road draft crank case ventilation system. the way a road draft system works, is at medium to high cruise speeds, the road draft tube acts as a vacuum as wind blows past the engine, under the car, drawing out harmful, poisonous crank case fumes, out onto the road. under these conditions, the engines breather caps (filtered via brillow which typically has a film of oil on it to catch dirt and debri) is the inlet for fresh air entering the crank case. at very low cruise speeds, the breathers change flow direction of pressure and fumes and are an exit outlet for these fumes. 53 to mid 55 engines ran a divider baffle in the middle of the lifter gallery above the cam shaft (53 to 56 used a windage tray baffle bolted under the block.) 57 and 58 blocks have mounting bosses for a windage tray, but they never came with one. the idea was the baffles would help guide the harmful crank case fumes out more efficiently through the engine, as 53 to mid 55 had no breathers on the valve covers. only a breather and fill tube in the front of the valley pan, and the road draft in the rear of the pan. the road draft baffle under the pan was packed full of the same brillow up thru 62, to help catch road dirt/dust at low speed cruise that could potentially enter the crank case at idle and low cruise speeds, via coming up through the road draft tube. when doing an engine restoration, we never re-use breathers, (we sell new reproductions with foam filters that look pretty much identical) and if original valley pans are re-used, the spot welds for the baffle MUST be drilled out and brillo/grit contamination must be removed after blasting, replacing the brillo in valley pan baffle isnt needed, when running a pcv. having a baffle under pcvs and road draft is very important so you dont' suck oil. grit and old deteriorated brillo can otherwise potentially create contamination damage to the engine. starting 63, buick started using PCV valves as emission standards grew. the PCV system (positive crank case ventilation) works essentially at the opposite times as the road draft tube, would typically draw out and relieve pressure and fumes. under high manifold vacuum (idle and low speed/ low load conditions, the pcv valve is open, recycling harmful crank case fumes back through the intake to be burned again, and the breather(s) on the engine turn into inlets for fresh air.... and the engine is tuned to compensate for the vacuum leak. under low manifold vacuum conditions, the valve would shut, and the breathers become an outlet for fumes and pressure. overall the PCV system is much better and keeps your engine cleaner, and is more efficient at relieving crank case pressure, as 50+ years have passed since the PCV introduction to automobiles and we really havnt changed the design much. we sell easy install PCV kits for all road draft application nailheads. not having enough crank case ventilation can and will encourage oil leaks. when picking out aftermarket valve covers and valley cover, always be thinking about crank case ventilation. at bare minimum, you MUST have at least a breather on each valve cover, its better if you have a pcv, on one side and breather on the other... having no breathers on valve covers, and pcv in valley pan with fill tube/breather is also acceptable. you MUST have at least one breather on an engine for the pcv valve or a road draft tube to work. (preferably on the complete opposite end of the engine.) imagine trying to pull air out of a coke bottle, with your lips sealed around the rim. you cant. now imagine a hole at the end of the bottle, allowing air to enter. aftermarket valley pans with fill tube do not work will all intake manifolds due to clearance, and theres often clearance issues with aftermarket valve covers regarding accy brackets and kickdown brackets. call for advice and more information regarding application.