Having cut off some nuts and bolts on the rear suspension of my suzuki, I need to buy some replacements. 8 bolts, 12 nuts and some washers add up to £65 in genuine parts so I'll be buying generic items. Most of the bolts are M12x1.25 pitch. Why do they use fine pitch for these? Do I need to replace with the same? Also, they use nuts with a flange, like a built in washer. What is the reason for these? Will a normal Nyloc and a washer do the same job?

A fine pitch bolt will have a higher clamping load at the same tightening torque. That's why they are used and should be replaced by either the same or a higher pitch bolt with increased tightening torque (if feasible...)

Bolts with integral washers are often used to reduce assembly time. No problem to replace with a nyloc + washer combo.

HTH
Sebastiaan

Fine pitch bolts have a higher ultimate tensile strength over their coarse pitch brothers and allow greater precision over clamping force per turn and more clamping load at the same torque. Take it to an extreme and you see that a 5mm pitch bolt will undo and back right off to slack with the slightest turn. I personally would replace like for like, they don't often spec odd sizes for the fun of it. Nuts with a flange spread the load area and massively increase friction with the contact surface, locking the nut to the outer. Try to replicate the situation and you should be fine, but it's usually best to get the right parts if you can.

[Edited on 22/3/11 by coyoteboy]

Fine pitch threads also give better fatigue life.

Agreed with above.

Replace like for like, but don't waste money with Suzuki for these.


Maybe these guys, if you haven't got a local stockist.
http://www.namrick.co.uk/acatalog/Home_Metric_Fine_HT_Head_Bolts_29.html

Right Ho! M12x1.25 it is. I suspected there was a reason but coudln't think what.

Course threads are usually for parts that are removed often and on weaker materials.