Wheel thread engagement

[joshd1994]

Hi guys

I have standard oem 18' Mercedes alloys and I have just fitted 15mm spacers at the rear and I need a bit of a confidence boost on the length of the bolts I have used.

The originals have 27 mm of thread and engage with 6.5 turns. I have bought new 40mm bolts which engage very slightly under 6 turns (say 5.8 turns).

Do youse think this will be ok or not or am I being a bit paranoid?

Cheers

 

[snowdragon58]

Hello Josh, I have just joined the forum. I work in engineering, although I am not a mechanical engineering practitioner as such. There are engineering standards for just about everything mechanical however. This includes thread engagement required to achieve full tensile load rating for bolts. Afraid I don't know the relevant standards to point you to though. Bolt torque is critical too. Hopefully a proper Mech Eng will read this and advise further.

 

[merc85]

Hi guys

I have standard oem 18' Mercedes alloys and I have just fitted 15mm spacers at the rear and I need a bit of a confidence boost on the length of the bolts I have used.

The originals have 27 mm of thread and engage with 6.5 turns. I have bought new 40mm bolts which engage very slightly under 6 turns (say 5.8 turns).

Do youse think this will be ok or not or am I being a bit paranoid?

Cheers

If the original bolt thread length was 27mm, surly you would need a bolt with 42mm long thread? after fitting a 15m spacer to keep it within Oe spec?

 

[KennyN]

On the CLS the standard threaded part of the bolt protrudes by 20mm front and rear, this should equate to just over 13 turns until the tapers touch.

BUT - there is the thickness of the disc before you get to the threads on the hub , possibly 5mm , so in reality there will be around 9 - 10 turns of actual thread that hold the wheel, so i would be aiming for that figure.

Kenny

 

[hotrodder]

The bolt pitch is 1.5mm in this case i.e. 1 full turn moves the fastener 1.5mm

Thread engagement length depends on loads of stuff but very generally is typically an absolute min of 0.8d (0.8 x major diameter). That's in the case of a matching nut* and bolt i.e. same strength class and with the caveat that the bolt protrudes through the nut by at least a few mm because the first thread or two are typically only partially formed (thread chamfer) for example



Thickness of the disc bell and the fact it's effectively a blind hole (can't tell whether the wheel bolts protrude through the hub flange without more measuring/taking stuff apart) means that measuring the amount of bolt sticking out of the back of a wheel and/or counting turns can't be relyed on as a means of accurately working out thread engagement length...

6.5 turns of a 1.5mm pitch bolt = 9.75mm, 5.8 turns = 8.7mm. There should be a 2mm difference if the new bolts are 13mm longer than the old ones and used with 15mm thick spacers...
If the wheel bolts are M12 then 0.8d = 9.6mm, for m14 it's 11.2mm

Personally i'd take a wheel off and use a paperclip or whatever to work out how thick the hub flange is/how much thread there is. Properly bad things are unlikely to happen if they torqued up without stripping and the spacers are hubcentric but personally i'd be happier having to shorten 45 or 50mm long bolts (because 42mm long don't exist) than using shorter bolts if thread engagement is less than 0.8d. Or less than original if the stock set up breaks that rule


* there are exceptions like 1/2 nuts which are (unsurprisingly) thinner but only intended for lowish torque/stress applications or stuff like jam nuts where two of 'em are locked against each other