I've just got some 15 mm wheel spacers for my aftermarket alloys on the front wheels and they came with 42/45 mm tapered bolts depending on where you measure from . 

The OEM wheel bolts for the standard alloys are 27mm radius bolts . 

So 42/45 mm bolts you think should be OK ryt??

 

But when I put the longer tapered bolts in with the 15mm wheel spacers in place on the front they lock up the wheels , so they must be a bit too long , but I don't see how when 42/45mm should be fine with 15mm spacers ?? 

 

The alloy wheels came with bolts that are 33mm long , which where too long without the spacers and where locking up the wheel , but I've had to use them with the 15mm spacer , my worry is there too short . 

 

Anybody else had this problem ?

 

My car is 2003 Skoda superb 1.9 130 PD awx.

 

Could it be the tapered bolts tighten further into the alloy wheel meaning there is more thread which reaches through the wheel? 

 

What length bolts should I order for the front ? 

 

Also I have 20mm spacers on the back but they had radius 50mm bolts in for standard alloys .

But I think if I order 50mm tapered for the 20mm spacers I might have the same problem as I have on the front ..

I can't answer this, but I can see a mistake in your assumptions. That is why I can't answer this. You have assumed that the depth of metal from the bolt land on the wheel to the back face is the same on the standard wheels and your aftermarket ones. It may not be.

I know I have sorted this now , you just assume that the bolts supplied will be correct , but yes the alloys only take up 7mm of the bolt when fully in .