Tow bar bolt strength

I am going to buy new bolts for attaching the tow bar to the chassis.

Is the bolt strength class 8.8 good enough or should I select 10.9?

I am having problems finding 10.9, while 8.8 is easily available.

8.8 is the standard strength for tow bracket mounting.

Roger

Although there are bolts marked as strength 7, 8.8 is the minimum that may be regarded as high strength. Companies like Westfield Fasteners who make Unbrako bolts go up to 12.9 and these will be used on cylinder head, c/shaft main and big end, flywheel and crown wheels. Many of these fittings are subject to shock loadings but survive and would also survive on a tow bar.

The choice is yours, 8.8 is the standard that your tow bar manufacturer would have used in their calculations and as such will be perfectly suitable, but the next rating up, although more expensive will not fail you. Also higher rated bolts do not suffer corrosion to the same level as 8.8.

Roger

Thank you all for the suggestions.

I went with 8.8 and coated all the bolts with a ceramic anti-seize. I did not know how much to tighten the bolts, so I took a wild guess and torqued all the bolts to around 50 Nm when tightening. Four bolts are 12 mm dia and one is 10 mm dia. Even that appeared quite tight, I thought that I am going to strip the threads. Any thoughts?

HI,
My tow bar bolts are 8.8 as supplied by the manufacturer, so they will be fine. I just did mine up by hand, as tight as I could, and its been fine for the last 2 years of use. If I don't have any manufacturer figures I prefer this method as you can "feel" if you are going over the limit.  Google suggests you may have over tightened them
M12 8.8 = 40nm
m10 8.8 = 28 nm
These are typical figures from an engineering supplier.
 

Engineering toolbox has a good table and chart for maximum torque settings and different property classes, see below

www.engineeringtoolbox.com/metric-bolts-...m-torque-d_2054.html

M10 8.8 max is 57.3nm, so 50nm should be fine.

I can't remember what torque I used on mine, I suspect that there wasn't space to get my torque wrench in to every bolt, so probably gave it a good pull on a 1/2 socket wrench.

Robin

No eBay APP ID and/or Cert ID defined in Kunena configurationI sell them! Of course!

Thank you all for the info. I added that engineering web article into the tow bar wiki article.