Screws - American Sizes

Bolts with inch threads

On all Harleys from the 1904 single cylinders to the current Milwaukee Eight models, there are bolts with special dimensions that you won't find in your average Central European hardware store. The length, diameter and thread pitch of the bolt are specified in multiples or parts of an inch. 1 inch corresponds to 25.4 mm and the abbreviation for this is ". It is the basis of these special dimensions.

Like metric bolts, inch bolts are available in many finishes and with very different heads. Hexagon and hexagon socket bolt heads are also based on the basic inch dimension. Torx, cross recess and slot are (almost) the same for metric and imperial bolts.

What do the three lines on the bolt head mean? What do the six mean?

Both are markings that indicate the strength. Three star-shaped lines stand for "Grade 5" and six star-shaped lines for "Grade 8".

Bolts marked Grade 5 should have a tensile strength of 105,000 to 120,000 psi, i.e. around 720-820 N/mm2. Bolts marked Grade 8 have a tensile strength of at least 150,000 psi, which corresponds to around 1035 N/mm2. Even though the designations are not 100% comparable, you can use an imperial grade 5 bolt in the same way as a metric grade 8.8 and a grade 8 is roughly equivalent to a grade 10.9 bolt.

I can't see any special markings on the bolt head? Will this bolt be strong enough for my purpose?

Not an easy question to answer. If you go by SAE, bolts without special markings on the head have a maximum tensile strength of 60,000 psi, i.e. just under 410 N/mm2 and would therefore only correspond roughly to the 4.6 bolts that you get in Central European DIY stores. And to be honest, we don't know what bolts and bolts have been used on your Harley over the decades.

But: You have to know that Harley-Davidson has installed a whole lot of bolts over the course of more than 100 years that do not comply with SAE regulations (there are more important things), but where the factory has taken extra care to ensure that quality steels are used and that the factory's own high quality requirements are met - whether the bolts were bought in or even manufactured in-house. And when it comes to high-quality restorations in particular, modern line markings on the bolt heads should not be present. Colony, as an outstanding supplier of reproductions of vintage and classic nuts and bolts, marks the heads as they were marked at the time – that includes bolts which were not marked at all.

To summarize: if you buy a bolt from W&W, it definitely meets the specified strength requirements for the respective component. Everything else can be found in the store. And if in doubt, the number below will help.