A little BCD help please
 

Any idea what the most likely BCD would be for the rings in the images below?

I measured the radius using a compass. The radius measures 37mm, suggesting the 74/110 BCD combination.

However, all three rings attach in the same BCD. So all three would be 74mm. Could that be correct? I'm not immediately finding 42-tooth rings having a 74mm BCD. Yet that's what I'm pretty sure I'm seeing on the bike.

Some here have more experience than I on these older models. Any help in nailing down what I'm looking at is appreciated.

http://bikeforums.net/attachment.php...hmentid=463171

http://bikeforums.net/attachment.php...hmentid=463172

http://bikeforums.net/attachment.php...hmentid=463173

Your pictures are consistent with a 110mm BCD. 74mm BCD rings generally mount on a separate bolt circle, drilled and tapped into the arms closer to the spindle than the 110mm mounting. Not all 110mm BCD cranks have the 74mm BCD mount points; you'll want to look at the back side of the crank to find them.

Thanks John. I've looked again. I can't measure anything that resembles 110mm. If I draw a 110mm circle, it's clearly too large to fall on all the bolt centers. It's even a larger circle than is needed to go around the outside of the five arms.

There's only one circle too. There is precisely one set of five bolts and all three rings attach to those five. One diameter is used for all three rings.

Am not saying that 110mm is wrong. I'm just struggling to "see" it.

You are correct. All three rings mount on a 74mm BCD. This is an "oddball" crank that Specialized used on MBs for a short time at the turn of the century. It didn't last long, and chainrings are no doubt scarce. Maybe an old time Spec dealer? Even if you could find a 42Tx74 ring it'll be a bugger to make it work. Chainrings for that crank were designed specifically to work together and other combos of rings won't without some creative spacing, bolts, etc. Time for a new crankset?

Ah, reddog3, thanks for that. Mystery solved then, and leave it to Spec to do something oddball like that.

Yes, probably a new crankset. Had been leaning that way anyway, because it's probably close enough to the same money as replacing all the rings.

It had just been bothering me a lot that I couldn't put my finger on what I was looking at in terms of those chain rings. Appreciate the help.

+1, I dont see an Outer Bolt circle at all ...consider buying a whole new crank-set and BB and replace the whole thing..

it was done that way for Price, to lower the whole bike cost to make it more affordable.

often do do similar there are Riveted rather than bolted on crank-chainrings.