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Grading bolts

I snapped a bolt off recently tightening down a pipe flange desperately needing to get an irrigation pump running again. As much as I wanted to blame the pump’s manufacturer, I discovered the mistake was mine. I had lost several bolts when disassembling the flange and, in a hurry, grabbed some no-name fasteners from the bolt bin.

And so began my fascination with fasteners. I assumed that bolts were all the same, ignoring evidence to the contrary and the marks obviously pressed into the top of a bolt’s head. Those slash marks (officially called radial lines) or numbers reveal a great deal about a bolt’s capacity.

Go with the original as it may need to fail

This marking system pro- vides proof that a bolt meets the strength standards set forth by the Society of Automotive Engineers (SAE) – in the case of conventional-size fasteners – or the European Community  – in the case of metric fasteners. The markings on a bolt’s head offer a handy guide when replacing fasteners. In other words, replace a bolt with three slashes in its head with another bolt with three slashes.

You may think that trading up to a bolt with six slash marks to replace a three-slash bolt is a smart move. The general rule is to always find an exact replacement for missing or broken bolts, advises Jordan Cazeault of the Bolt Depot ( “For example, replacing a bolt with a stronger one doesn’t always work,” he says.

Stronger bolts are often harder and so tend to be more brittle. Rather than stretch as required for situations the equipment’s manufacturer had selected the original bolt to meet, the higher grade fastener snaps off. Then, too, some equipment calls for the use of a specific bolt that fails (often shearing) on purpose to protect more expensive or critical components.

It’s all about tensile strength for most bolts


But most of the time for bolts, it’s all about tensile strength as illustrated in the bolt descriptions on the next page. Tensile strength indicates the maximum load in tension (pulling apart) that a fastener can withstand before breaking or fracturing. But don’t confuse tensile strength with shear strength. A bolt’s resistance (in psi) to shearing apart is about 60% of a bolt’s tensile strength as a rule of thumb.

The vast majority of fasteners in use on agricultural equipment are foraged from carbon steel that is quenched and tempered (hardened). The exception to this is stainless steel bolts, which are created from a combination of chromium and nickle.

Stainless bolts are not stronger but do resist rust

One common misconception about stainless steel bolts is that they are stronger than bolts made of carbon steel. Due to a low carbon content, stainless steel cannot be hardened, which makes these bolts significantly weaker than a hardened steel bolt. Stainless steel bolts excel in their resistance to corrosion, however.


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