INTRODUCTION This Item presents static-pressure drop data for flow through helical coils and gives some guidance for non-helical coils. The types of coil considered in this Item are defined in Section 1.2. The data for helical coils are given in the form of non-dimensional friction factors@ f@ and the effects of the following parameters are considered: (i) Reynolds number@ Re@ (ii) non-dimensional surface roughness of the coil tube@ ??d @ (iii) diameter ratio of the coil@ D/d or Dc/d@ (iv) pitch ratio of the coil@ h/D. In addition known temperature variations through the coil can be taken into account. A complementary Data Item (Reference 3) gives the corresponding heat transfer data. The principal restrictions on the application of the data are shown on the Figures to which they apply. Further information on the applicability of the data is given in Section 3 whilst Section 8 gives details on the uncertainty of the data.