ModestW12

From searching the internet and this forum it appears stock D3 4.2/W12 wheel lug bolt size is M14x1.5 with 28mm shank/thread length, 17mm hex head and R13 ball seat, torqued to 90 ft-lbs (please correct me if I'm wrong).

With that said, I have a few questions:

It seems that to determine lug bolt thread length to use with spacers, you add the spacer width to the OE thread length.
1) So assuming 12 mm hubcentric H&R DR spacers up front and 20 mm in back, would the ideal lug bolt thread length be 40mm for the front and 48mm for the rear?

I'm having a hard time finding 48mm thread length bolts. Most common options are 45mm and 50mm and to a lesser extent 47mm. With my last W12, the Achtuning reseller in WA shipped me 40mm and 45mm bolts for use with 12mm/20mm spacer purchase.
2) Is the 3 mm deficit in the rear bolts significant enough to want to avoid it this time around or is it insignificant considering the lesser load on the rear axle?

3) Would the the extra 2mm on 50mm thread length bolts scrape or otherwise cause problems if installed on the rear axle with the 20mm spacers?

Finally, all spacer suppliers/resellers I've found just ship solid, one piece lug bolts. Search results seemed to indicate lug bolts with moving ball seat washers are desirable when securing aluminum wheels. In my case, the lug bolts will be securing OE 20"x9" seven doublespoke forged wheels.
4) Is there any reason I should purchase bolts with moving washers or are these types of bolts totally unnecessary in this application?

ModestW12

Perhaps that was TL;DR (too long; didn't read). If you aren't sure opinions are welcome!

1) Can someone confirm I have the right stock wheel bolt specs?

2) Does anyone think the extra 2mm on 50mm bolts would bottom out or scrape on something?

3) Or is it negligible to have 3mm less thread bearing the load than stock? Articles in the internet seem to indicate 18mm of the 28mm stock bolt is load bearing, so 83% of stock.

dvs_dave

Sounds like you're making things very complicated for yourself. Just buy spacer sets with extended bolts of the correct length included. 100's of options on eBay and it's what I did. They work well.
See this thread https://www.audiworld.com/forums/a8-...2886787/page2/

ModestW12

That's exactly what I'm trying to do.

I already bought the spacers but not the bolts because the lengths the vendor offered were not the ostensibly correct 48mm for the rear axle.

My question is which length of extended bolts should I get? The only ones I can find (that aren't $8 a bolt) are either 3mm shorter or 2mm longer.

That cost is not a barrier but also not trying to burn money considering last time Achtuning sold me the 45mm and I didn't have issues. But it seems 45mm is less than stock as far as load bearing lenght and don't want to push my luck running underspecced bolts.

MP4.2+6.0

Variously:

You can get washered lug bolts from H&R in some sizes. I use them when available. They do cost more.

Yes, you basically add the spacer width to the original lug bolt size to get the new one.

When in doubt, follow the H&R instructions about the minimum number of threads/turns you need for a given diameter bolt.

No, no experience with bottoming the bolts on mine, that is with H&R DR model spacers. I did bottom them on DRA's, but that is a different design and one I would not recommend for D3's from my trying those too.

dvs_dave

Get the longer bolt option. There is plenty of space behind the hub to accommodate a few mm of excess lug bolt penetration.