Gun Metal Bushes, Bars and Castings (LG2 / C83600)

Each of the four elements does a job. Copper is the base and gives corrosion resistance in water. Tin is the strengthener and the reason the alloy is a bronze. Zinc helps deoxidise the melt and adds strength cheaply. Lead does not dissolve in copper; it sits as tiny globules that make the metal machine with short, broken chips and that smear into a thin film if a bearing runs short of oil.

The result is a medium-strength alloy that seals well, machines easily and has good wear behaviour at moderate loads. It is always used as a casting or as continuous or centrifugally cast bar, never as rolled sheet or drawn wire.

LG2 is the old British BS 1400 name and is still the word most Indian foundries and buyers use. The current European designation is CuSn5Zn5Pb5-C, numbered CC491K in EN 1982. In ASTM it is UNS C83600, cast to B584 (sand castings), B505 (continuous castings) or B271 (centrifugal castings), and in the US it is often called ounce metal or 85 metal.

In India, IS 318 covers leaded tin bronze ingots and castings in grades LTB 1 to LTB 5. Those grades differ in tin and lead content, so if your drawing calls for an IS 318 grade, quote the LTB number on your enquiry rather than just writing gun metal.

Before steel took over, cannon were cast from copper-tin bronze, and the name stuck to bronzes of that general type. The modern 85-5-5-5 alloy has never seen a battlefield. Do not confuse it with gunmetal grey, which is just a colour, or with gunmetal finishes on consumer products.

Gun metal is chosen for bushes that see moderate loads and speeds with reliable lubrication: pump gland and neck bushes, impeller wear rings, valve stem guides, conveyor and agricultural machine bushes, and the bearing shells many sugar mills still cast for roller journals. Under heavy shock loads or high speed, a high-tin phosphor bronze lasts longer; for light, occasional movement, gun metal is more than enough and cheaper to machine.

For pressure parts the alloy is used for valve bodies and bonnets, pump casings and impellers, water meter bodies and pipe fittings. Its tight, fine-grained castings pass hydrostatic tests that leaded brasses of the same wall thickness may fail.

For bushes, cored or hollow cast bar saves a great deal of metal and lathe time compared with boring a solid round. Give us the finished bush OD, ID and length, and we will suggest a cast bar size with sensible machining allowance; actual available sizes are confirmed at enquiry. For valve and pump castings, send the drawing and the standard.

Every supply comes with a test certificate through a Govt. recognised test house, showing copper, tin, lead and zinc. That is worth having, because remelted scrap gun metal of uncertain tin content is common in the trade, and a low-tin bush wears out fast. If you want a stronger, springier alternative, see phosphor bronze; for melting stock, our ingot range covers tin, lead and zinc.

• Leaded gun metal is one of the easiest copper alloys to turn; chips break short and tool wear is low.
• Use sharp tools with near-zero or slightly positive rake, which avoids the tool digging in.
• Bore bushes slightly after pressing in if the housing interference closes the ID.
• Machine oil grooves in bearing bushes; lead helps in a dry start but is no substitute for lubrication.
• Keep gun metal swarf separate from brass and steel turnings; clean, segregated gun metal scrap fetches a far better price from the scrap dealer.
• For long thin bushes, finish the bore last and support the part well, because the alloy's modest strength lets thin walls deflect under tool pressure.