Would you ride a bike that has a crack rear derailer pulley ? Will this damage the chain or would you just replace the pulley?

It won't hurt the chain.

I did ride a Nuovo Record derailleur once with a cracked pulley. I thought I lost a lot of shifting precision, although I didn't ride the bike long enough to get a good feel for it.

Let me get this straight. You have a broken part on your bike and you're asking if you should replace it?

Cracked derailleur pulleys are common, at least for Campagnolo Nuovo Record and similar derailleurs.

So, I think it is a valid question of what will happen if one continues to use them.

Unfortunately, my limited testing did not go well, at least if the crack is pretty bad.

Tacx makes some great replacement pulley wheels that can fit older stuff. I use them on my Dura Ace 7400 RD and I love them. I wouldn't ride cracked and broken components at least not once I figured out they were cracked.

Worst case, the crack could cause the pulley to break apart and lock up the drive train while riding. It may not be very likely, but why risk it when replacements are readily available?

Exactly what happened to a friend of mine that I rode Eroica Gaiole with in Italy. He ended up stranded because his chain kept ejecting itself from the derailleur cage and attempting to commit suicide in the spokes (the chain, not the friend). No brainer...it's broken...fix it before it leaves you stranded or causes a whole bunch more damage.

If you take a close look, a lot of used Campy rear derailleurs from the 70's-80's will have Suntour or Shimano jockey wheels.

I am glad to hear that it was the chain trying to commit suicide and the not your friend !

It's a shame the pulleys do not last. I have cracks on my campy pulleys and my sealed suntour pulleys.

cracked SunTour sealed pullleys??!!
Dang... I've had the Campy ones crack and replaced them with either Bullseye or the gray/silver sealed SunTour pulleys. If I had to guess, I would have guessed that SunTour would have done a better job of engineering than Campagnolo.

As noted earlier, Tacx makes some nice replacements. I recently purchased some Tacx T4000 pulleys that seemed well suited (dimensionally) to my SunTour and Campy derailleurs. Pretty inexpensive, especially compared to NOS Bullseye!

Steve in Peoria

Older Shimano pulleys also crack, and get a lot of miles put on them prior to failure.

You don't want to buy a new set for $4? https://www.ebay.com/itm/2PCS-Bike-1...item2a7fd7eea0

And NO! you can't ride with a broken pulley despite people here saying otherwise. The pulley will come apart and the chain will destroy the rear derailleur.

This is because they are exactly the same size and cheaper - not because they were any better. You can presently buy better idler pulleys from China for $4 a set. They do take forever to get here.

The better part is people saying - "Go ahead and ride it. It won't hurt a thing."

The pulleys in the link have 11 teeth. The vintage pulleys have 10T. Are you positive the 11T pulley will work with vintage deraillers?

10T ally are available and exact to the original Bullseye design. Super low cost ($10) vs. the greedy sellers of 'so called' NOS vintage Bullseye ($80 pair!). I'm doubting they are actual NOS bullseye.

All colors plus of course silver available. E the Asian source for the 10t. Great product and service.

I am positive about nothing but assume they would. However you can buy original equipment for less:
https://www.ebay.com/itm/5pcs-Mounta...r=640981854167

Thanks for everyone's help. I will change out the crack pulleys with the asian ones since they are inexpensive to replace.