Hi all
I was running the stock narrow steel wheels with the "wheel bolt black" (M14X1,25, L:49mm) bolts, also stock.
But I just mounted an aftermarket alloy wheels (17", ET30) using my stock lug bolts. The cone seat fits nicely, though the thread seems slightly short, or is it? How do I determine proper length?

Car is R56 2009.
Thanks.

 

Length is usually measured from
the threaded end to where the
cone just starts. By this measure,
the stock length for a lug bolt for
an alloy wheel is about 27-28 mm.
With a spacer of x mm, the length
should be about 28 + x mm.
The total length of a stock (28mm)
lug bolt is about 48-52 mm,
depending on how tall the head is.

Your bolts look just fine on the
alloys, and a little long on the steel
wheels, which, by the way, usually
But not always take ball seat bolts
instead of cone seat ones.

 

Only taking into account the wheel thickness, they look ok. However, the bolts still need to go through the brake rotor before threading into the hub. They “may” be too short, but hard to tell.

What is the length of the thread sticking out of the back of each wheel? What is the thickness of your brake rotor? Ideally, you want the wheel bolt to thread fully into the hub, with 1-2 threads sticking out the back side for full holding capability.

 

The rotor is pretty thin over the hub.
Usual rule of thumb is 6+ threads into
the hub, so if you can get 6 full turns
of the lug bolt into the hub before it’s
fully tight, you’re doing fine.
This is 7.5 mm plus the distance from
the rotor edge to the first thread on
14x1.25 bolts and hub.
By your pictures you look fine.

If you’re of the opinion that the
rule of thumb was based on 1.5
pitch, then you might want at
least 7.2 turns at 1.25.

 

They are probably ok, but I would want the same thread engagement you had with the stock wheels. Measure the difference between the 2 and order some longer lugs.
When I bought TSW wheels for my VW, they sent lugs that were really short. I emailed them the issue and they sent longer ones. Eventually everyone that ordered those wheels were getting the longer lugs, so I'm glad I caught it.

 

Id screw it into the hub and see how many turns it takes until the lug gets to the back of the hub. Its not very thick. See if you get that many turn while installing the wheel.

 

Thanks all, that put me at ease. Hat tip @@cristo for the measurements.

I got 8 full turns before tightening, so that should be enough. Through the brake rotor and out the other side of the hub was 18mm.
The lug bolt is just about flush with the back of the hub (2nd picture)