Did campy change their chainring bolt diameter?
 

So I have a white/black label 11s UT crank with a worn 53t ring. I bought a new replacement 52t 11s Campy chainring to replace it but when I went to mount it to the crank, the chainring bolts were sticking out a couple mm from where they were with the previous chainring. Sure enough, I measured the bolt holes compared to my previous ring and they are slightly undersized.

What gives?? Why would they change the bolt dimension within the same 11 speed generation? I understand if they did it when they changed to 12s, but this is just annoying. Luckily I can return it, but i dont really enjoy taking apart chainring bolts multiple times.

FWIW, my original chainring is labelled "EPS" and this new one is labelled "XPSS"

Maybe the 52 is for a 36/52 compact crankset and your 53 is from a traditional 39/53 road crankset?

Were you referring to bolt circle diameter or bolt hole diameter?

The 50/34 and 52/36 use the same bolt diameter, whereas the 53/39 uses a different one, at least on the 5-arm version. The 4-arm cranks all use the same bolt pattern.

Its the bolt hole diameter. BCD is the same for both.

This must be it.. Is there a reason they did that? Paging OP

You're still not being very clear - are these the 5 arm cranks or 4 arm cranks? There are two BCDs for each - 5 arm cranks used either 135mm BCD (53/39) or 110/112mm (52/36 or 50/34). 4 arm cranks use either 146mm outer/112mm inner BCD, or 12mm outer/96mm inner BCD.

For all 5 arm cranks, the outer chainring bolt holes are 10mm with 12mm countersinks, and for all 4 arm cranks, the chainring bolt holes are all 8mm with 12mm countersinks. These two hole/countersink sizes have been industry standards for decades.

These are the 5 arm 135bcd. For the bolt holes, theres the two diameters like you describe where the lip of the bolt sits against the smaller diameter of the hole. Thats the part that is smaller on the new chainring, so im guessing its the 8mm even though its 5 arm.

Now I'm confused. Some 5 arm cranks have 8 mm holes in the arms And they utilize these smaller chainringbolts:

Is there a a reason for the 2 different bolt diameters? Why aren't the same? Why the complexity?

I think you might need to the measure the bolt holes. I'm pretty sure that all Campagnolo outer chainrings have 10mm bolt holes (and 12mm counterbores).

Those bolts are for carbon cranks. Carbon has a high strength/weight ratio, but a low strength/volume ratio. To preserve strength of the arms at the bolt holes they use smaller holes in the arms, which leaves more material around the bolt holes. So these bolts have 10mm shoulders to engage with the traditional sized chainring bolt holes, and then neck down to 8mm to fit through the crank arm holes. It's actually a fairly elegant solution - these bolts (which come with the cranks) makes these cranks backward compatible with chainrings with traditional bolt holes.

Just confirmed the new chainring measures 12mm and 8mm. I dont remember what my original ring was(and of course its all bolted up on the bike now), but I know it was larger.

Okay, I've been doing some digging around in the Campagnolo Small Parts catalog and I think I know what's going on.

The original 11spd 135mm BCD cranks used chainrings with the traditional sized bolt holes (12mm/10mm). Aluminum cranks used traditional 10mm chainring bolts, and the carbon cranks used the bolts in the picture above to step the bolts down to 8mm in the middle.

But for 2011 to 2014, Campagnolo changed the chainrings to us a straight 8mm bolt. The outer chainring apparently had an 12mm/8mm bolt hole, and the inner chainring had a threaded 8mm hole, so that the bolts screwed directly into the chainring (I've heard of these, but haven't actually used them).

So, it looks like you may have a 2011-2014 outer chainring, but 2010 or earlier bolts (and inner chainring). So you may need to get a new outer chainring of the right type/vintage. The other option would be to get new bolts plus a new inner chainring.

The part numbers for are as follows:

2010 and earlier:
52 tooth chainring: part no. FC-SR052
39 tooth chainrng: part no. FC-SR039
chainring bolt set: part no. FC-SR100

2011 to 2014:
52 tooth chainring: part no. FC-SR152
39 tooth chainring: part no. FC-SR139
chainring bolt set: part no. FC-SR200

The problem with the earlier carbon crank nuts/bolts was that the 8mm nut portion had to be tapped for a 6mm alloy bolt, which had very limited tightening torque resistance (so people often broke them or stripped them).
Specialized S-Works carbon cranks had the same issue (I readily stripped more than one, and was unhappy to find that the bolts were only sold in sets of five, for around $50/set).

So Campag's newer bolts are 8mm, and screw into 8mm threaded holes in the inner chainring, for much greater torque and tightening tension before anything strips or breaks. The holes in the crankarm remain at 8mm.

get a new inner ring and the matching bolts, this happened to me too.