Description Thread size 1/4 x 32 tpi This valve prevents a vacuum being created in the boiler when it cools down and stops steam oil and residue from the engine and lubricator being sucked into the boiler and dirtying the sight glass. Ideally it needs to replace the top sight glass blanking plug or in the highest bush on the boiler. It is straightforward to fit, using a 2BA spanner and a drop of sealant on the thread.
