Radiator Thread Sizes

Removing Worn Bleed Screws

It is a shame to have to throw away a perfectly good radiator just because the bleed screw doesn’t work anymore.

There are three main problems you will come up against:

  1. A screw where the square section has become rounded
  2. A screw that is seized up in the hole due to corrosion
  3. A thread that has become damaged or corroded

Or you might have all of them

Here are some tips to help

Before You Start

It is a good idea to spray the screw and the hole with easing oil such as WD40 each day for a few days before starting. Scrape any corrosion or paint from the screw and clear the hole before starting.

Bleed Key

Standard Bleed key measures 5mm across the flats but there are a number of radiators that use different size keys.

4mm / 4.25mm / 4.5mm / 4.75mm / 5mm / 5.25mm / 5.5mm / 5.75mm / 6mm

Make sure you select the best fit

Standard Bleed Key

Notice the hole has tapered edges on an original bleed key. This is a bad design as when inserting the key over the bleed screw it only grabs hold of 75-80% of the bleed screw.

Filled Standard Bleed Key

Here I have filled down the tapered edges using a standard metal file. The key will now cover more of the bleed screw and should avoid the key slipping and rounding the screw edges.

Sometimes the hole itself is blocked at the bottom. You know this because no water or gas escapes when the screw is out. Use a wooden cocktail stick to poke away any blockage. Take care to catch spills. If you have the radiator off the wall you can then flush it through using a hose until you see water squirting out of the bleed hole. Turn the radiator upside down before moving it. Try to avoid getting the black liquid on your clothes and carpet.  Wash the muck immediately if you do spill it or it might stain.

Using the new screw

Take care the first time you screw it in. If it is stiff you might need to clean the thread in the hole more. It might not seal well the first time you use it as the soft brass has to bed into the bottom of the hole.  If it leaks turn off the valves at both ends of the radiator, remove the screw and screw it in again. It might take several goes. Don’t force it or you will back to needing a new screw again.  When bedded in you should only tighten it till it just bites. Then turn on one valve. If it doesn’t leak turn on the other.

Option 1

Use a tapered socket (4.5mm / 4.8mm) Place over the bleed screw and gently hammer for a tight fit. Undo slowly to prevent slipping. A small amount of heat can be used to break any seal (do not over heat as brass expands faster than steel)

Option 2

Using a Dremil tool with a thin cutting disc attached, cut a small slit into the worn bleedscrew. With a suitable flat blade screwdriver, carefully undo a small amount of heat can be used to break any seal (do not overheat as brass expands faster than steel).

screw removal 2
option 3 removing bleed screws

Option 3

Bleed Valve Repair Kit for Cast Iron Radiator ⅛” BSP

Ideal for reclaimed cast iron radiators with broken bleed valves. Many bleed valves on reclaimed radiators are set in the bleed bush.

Here’s how to overcome the problem with our Bleed Valve Replacement Kits:

  • Drill out a hole using the HSS drill bit provided
  • Create a new thread in the freshly drilled hole using the 1/8th BSP bleed valve sealing the threads with PTFE tape or LSX sealant

Repair complete, about 20 minutes!