Fyi
Golden rule is one and a half times the diameter located in the female thread, this can vary on application and thread type but is a good place to start when thinking is this bolt long enough.
Also if you know the pitch you know how far it enters per rotation.
Hey? So standard 14mm bolt at 1.5 its diameter is 21mm of thread in the disc. Or have I read that wrong?
Standard bolts only go in about 14/15mm
Yeah that's about it, its a rule for determining bolt lengths without working out the loading, materials and application etc.
You should not really intrude lower than the diameter or design your housing in such a way this is possible, and obviously not use say a 14mm bolt into a tapped thickness of 6mm or have bolts hanging through excessively on applications like a hub....
Point is that if you are x 1.5 then you are safe, x 1.0 and you have the same engagement as dia so match the location area to the area under tensile load (it gets really boring and complicated here due to load and material calculations so one rule does not fit all)
With the standard depth being 14/15mm and calculated at that to be safe a 5.0 mm plain spacer reduces this factor by nearly 50%, in the case of conversion spacers your bolt to the hub from the adaptor should have 14mm engagement into the hub with at least the same grade bolts as the original vw part to match the original calculation.
How far this is stressed by the non standard wheel offset and thickness of spacer is open to question and I doubt most take this into consideration when manufacturing or supplying spacers.
The bolt through the wheel to the spacer when fitting 4/5 stud conversion spacers you would think needs to have at least the same engagement in this case 14mm...
If you compare Ali tensile/shearing strength to Cast/Wrought/Rolled steel you can quickly see how much weaker it is and why a x 1.5 rule would stand you in good stead.....
I would have reservations fitting any Ali 4/5 stud conversion/large offset spacers if I could not see that this had been tested and proved in some way.
In short for the plain spacing of 5.0mm you should use longer bolts if it reduces the engagement i.e if all the 14mm is in contact and not clear out the back (and only ever use hub centric spacers as tensile bolts are not designed primarily to take shearing force), 3mm spacers only reduce engagement by 20% at worst so you could argue to yourself they are safe but how the police/insurers take this and any mods like this is open question and circumstance.
I have run 3 mm spacers on standard bolts to clear a calliper upgrade on my old Mk2 no probs and binned it pretty bad without the wheels coming off
Hope all that kind of makes sense, I've glossed over a whole load of stuff I cant fully remember without going over books in the office and drawing you a sketch or two
