These Pyrex® reaction flasks have cylindrical side walls with an inner diameter of 4 inches (approx. 100mm) and round bottoms. Upper end is tooled to obtain a thickened flange (135mm O.D.) and flat top. Top is finely ground to provide for a leak-resistant fit with cover Cat.Nos. 1-5390, 1-5392 and 1-5394. For aluminum clamps, see Cat.No. 1-5398-C.