Reducing Bushes & Plugs

A selection of tapered- and parallel-thread fittings for cast iron radiators. We stock sizes to fit antique brands such as Ideal Standard, American Radiator Company and Crane.

Showing all 5 results

Showing all 5 results

Using Thread Types To Date Sectional Radiators (Part 1)

Tapered Bushes On Pre 1970 Radiators

Cast iron radiators made before the mid 1970s had tapered right hand threads on the four openings. The photos adjacent show tapered-thread bushes, all of which were made before 1970.

Use our tapered-thread Black Iron Reducing Bushes to replace these.

Using Thread Types To Date Sectional Radiators (Part 2)

Flanged Bushes On Sectional Radiators Made Post 1970s

Sectional radiators have used flanged, straight-thread bushes since the mid 1970s. This type of bush forms a seal between the joining face of the outermost radiator section and the flange of the bush by compressing a gasket in between the two. They are left hand thread on one side of the radiator and right hand thread on the other.

Our new cast iron radiators use this kind of bush, as do those made by our contemporaries from the last fifty years.  The adjacent image shows parallel-thread bushes on radiators including our own and a model made in Italy until the early 2000s.

If you have this kind of radiator, use our Forged Steel Reducing Bushes. You’ll also need the appropriate size gasket.

BSP: British Standard Pipe

British Standard Pipe is the standard thread used on plumbing fittings across most of the world (but not in North America). Below, you’ll find tables showing BSP sizes in millimetres, an explanation of straight and tapered thread types and a handy thread size ruler and template to print out and use at home to check thread sizes.

Our Printable Thread Size Template allows you to check the size of pipes, bushes or openings before ordering your components.

Download Our Printable Thread Size Template

Nominal BSP Sizes In Millimetres

pipe size (BSP)
Threads per inch Average thread
diameter, mm
⅛” 29 9.1
¼” 19 12.3
⅜” 19 15.8
½” 14 19.8
¾” 14 25.3
1″ 11 31.8
1-¼” 11 40.4
1-½ 11 52.3
2″ 11 58.1
2-½” 11 73.7

Tapered And Parallel Threads On Radiators

G & R Thread Types

G is the international standard shorthand for BSPP  – BSP Parallel threads.

R is the standard for BSPT – BSP Tapered.

Parallel (G) threads are used where the seal is made on a mating face. Most cast iron radiators have parallel threads and a gasket between each section.

Tapered (R) fittings achieve a tight thread-to-thread seal with the help of a little sealant such as PTFE tape.