7mm or 10mm spacers and standard length bolts?

[MtGlorious]

I have some 15 X 7&8 Fuchs on the way and at the rear I'd like to edge out the rims to what a 9 inch rim would achieve, which I calculate to imply 7mm or 10mm spacers. I'm wondering if my standard length wheel bolts will be ok with these sized spacers or whether I'd need to get longer ones? My car is a '83SC.
There are a few threads on this topic but I couldn't find a definitive answer. Thanks.

[JJ 911SC]

How much is sticking out right now?

[Oh Haha]

They will work fine. I had 10mm spacers on my SC with factory studs and had plenty of lug nut engagement.

[moneymanager]

Just measure the things!

[safe]

7 is OK-ish, 10mm you should use steel nuts and I would still call it borderline.

[Walt Fricke]

The standard rule is that you should have nut thread engagement at least as long (as many threads) as the stud diameter. Which means at least 14mm of thread. Some nuts do not start engaging until the width of a thread or two (i.e., they have a sort of relief at the end where you start the nut on the stud), so when measuring you want to account for that.

Racing organizations require open end nuts, which means if the inspector doesn't see the end of the stud at least flush with the end of the nut you are apt to fail tech. But with stock closed end nuts you have to measure.

An engineer buddy told me this rule of thumb is quite conservative, as 90% of full strength is achieved within the first 2 or three threads. But try explaining that to an inspector.