I need some 10x1.25x30 bolts but I see most have a M# in front. What does it stand for? I'm guessing Metric? So would the bolts I need be M10-1.25x30?

 

The first number is the bolt diameter, so 10mm diameter. The second is the thread pitch which is the distance separating each thread in millimeters, and the last number is the bolt length in millimeters. There are usually a standard metric bolt size with very few different thread pitches. Usually 10x1.25 or 10x1.5.

 

Metric

 

it could be co. initial

no?

 

yes Metric

So M10x1.25 (thread and pitch) and 30mm in length.

 

They use the M because there are #4, #12, etc size bolts in SAE terms. There's yet another number, the bolt's grade. Usually 10.8 (SAE 8) for cars, 8.8 (SAE 5) for non-structural parts.

FYI Honda tends to use the finer thread pitch (threads per inch), sometimes they're very hard to find for a decent price.

 

Yeah thats why it has 1.25 pitch... Thats fine pitch for 10mm bolt, having 1.25 threads per mm. You cant really alk about TPI on metric bolts (well you can, but its weird).

as for the grades, 12.9 is the hardest, then 10.9 and 8.8, then it gets to so weak stuff that people dont even bother. 8.8 is fine for automotive use, and you'll see body fasteners are often 8.8. As well as waterpump bolts, etc non stressed parts. Heck intake manifolds and exhaust manifolds can be fastened fine with 8.8, they arent torqued that much.
10.9 for high stress parts, such as heads, connecting rod bearing caps, etc.