Cast iron radiator construction
Have you ever wondered how a cast iron radiator is put together?
When Cast Iron Cracks: What a Frozen Radiator Can Teach Us
We recently had a radiator come into the workshop with severe damage caused by a combination of poor-quality cast iron and frozen pipes. The results were dramatic—and if the radiator looked this bad, you can imagine what the rest of the central heating system (and house!) must have gone through.
Let this be a reminder for everyone (including us!):
Double-check pipe insulation
Ensure the frost guard is switched on
Drain the system if the property will be left unoccupied
Also see our related article:
How Reliable Are Reproduction Radiators?
Repair, Don’t Replace
Fortunately, we were able to help. In this case, the damage was isolated to a single cracked section. That meant we could dismantle the radiator, remove the broken piece, and reassemble it one section shorter.
Our customer was delighted—they avoided the cost of buying a whole new radiator, and we had a chance to demonstrate how these beautiful old radiators are constructed and how they can often be repaired rather than replaced.
Understanding How Cast Iron Radiators Are Assembled
Whether you’re dealing with original Victorian models or high-quality modern reproductions, the construction method is largely the same. Most cast iron radiators are built section by section using threaded hollow bolts called nipples.
(Some radiators do use a slightly different construction known as the “push nipple” method, but the principles are similar.)
How It Works: Threaded Nipple Construction
At the top and bottom of each section, you’ll find holes on both sides.
These holes allow hot water to flow from one section to the next along the length of the radiator.
Each hole is tapped with a female thread—one side with a right-hand thread, the other with a left-hand thread.
The nipple is a hollow bolt with matching male threads—right-hand on one side, left-hand on the other—so it can be screwed into both sections at once.
Between each section, a gasket is placed to seal the joint and prevent leaks.
Tools of the Trade: Inside the Nipple
Inside the hollow of the nipple is a shaped profile that allows a tool to be inserted to rotate it during assembly or disassembly.
Just like any threaded part:
Clockwise = tighten
Anti-clockwise = loosen
This method allows each section to be pulled tight against the gasket, creating a sealed, pressure-resistant joint that can last for generations.
Why Original Radiators Take More Effort
Reassembling modern radiators is relatively straightforward—nipples and gaskets are clean, tolerances are consistent, and materials are usually uniform.
However, original radiators, especially those over 100 years old, present a different challenge. Dirt, rust, scale, and age-related distortion make dismantling and reassembling them far more difficult. In many cases, we need to fabricate custom tools, apply controlled heat, and use experience (and a bit of force) to safely get the job done.
Watch the Process in Action
Curious to see how it’s done?
Watch us dismantle an original cast iron radiator in the video below.
It’s a fascinating glimpse into traditional craftsmanship—and why proper restoration takes more than just muscle.