Front timing chainring getting loose
 

I noticed some odd noises near my front bottom bracket, and discovered that my front timing chainring is coming loose from the crank. :eek: What has been happening, is that the chain ring bolts have actually been grinding away radially, so the bolts no longer fit snugly in the holes. Now that they have gotten loose, they are probably chewing up the holes in the crank arm and the chainring as they move on every rotation of the pedals. (I can't confirm how much/little the crankarm/chainring holes have gotten chewed up, since, the bolts/nuts have been so mangled that they are frozen and can't be tightened or loosened. FWIW, the crank arm/chain ring are about 800 miles old.

I'd like to get this fixed ASAP, but am not sure what all to do:
1) I'm guessing I'll have to drill out the old bolts, carefully, and replace them, unless anyone knows a better way to apply enough torque to them to get them to move. They are the style where the bolt uses 5mm hex and the female uses a large flat screw driver to get started, but the bolt tightens into the screw drive channel, so to loosen them, all I've got is a 2mm-ish slot on either side of the nut to prevent the nut from turning.
2) I've got new bolts on order, but I'm nervous about what I should be replacing. If I'm lucky, the crankarm and chainring won't need replacing, and new bolts will do it. BUT, if the assembly still wiggles when I install the new bolts, is there some step I can take to secure things other than replacing the crankarm and the chainring?
3) Is there any way to use digital calipers to measure the holes in the crankarm/chainring to reach a better decision on whether or not to replace them? IOW, if the holes are supposed to be 16mm, but have grown to 16.05mm, what is the line where I should replace them? (I just made up the numbers for that example)
4) Is this a random thing or could I have done something stupid to cause this? I was the one who installed the new parts 800 miles ago. Anything I should do this time to prevent this from happening again?

This is incredibly low mileage for such a failure. Have you tried giving it some time with penetrating oil?

What year, make, model of tandem cranks did you install?

Can you post a photo of the 'mangled' chain ring bolts & crank?

Short Answer: If the bolts truly won't unthread you are correct in that you (or a machine shop) will need to either split or drill-out the bolts. If the bolt holes in the chain ring and crank arm have been elongated or enlarged then, yes, you'll need a new left-front timing crank and chain ring.

There is actually a special tool made to fit the back ("female side") of chainring bolts. It's a two-pronged screwdriver sort of thing. See it on Nashbar's site at http://www.nashbar.com/bikes/Product...1_10000_200497 Other companies make them, too, including Park. Your LBS should have the tool for sale or be able to remove the bolts for you using their shop tools.

A cheap tool to have in the toolbox, and makes life a lot easier. Using this tool and a hex wrench, I'd be surprised if you aren't able to remove the bolts. Otherwise, drilling them out shouldn't be too difficult.

I also can't imagine chainring bolts grinding away at the crank, but stranger things have happened.

What size/type of cranks are they? I have some NOS stock front tandem cranks in my parts box that I could sell you pretty cheap if they fit. They are Shimano Deore DX, 175mm length, 110 BCD with 38-tooth chainrings.

With only 800 miles on the bike, see your dealer. Should be covered under warranty.

The tandem's not new, just the crank arms & chain rings....

It doesn't jump out at you until you get to the second sentence in bullet #4.

Thanks for that info. Got one ordered.

Sugino XD600 175mm/110BCD, square taper. I'll know if they need to be replaced as soon as my order with the new bolts and bolt wrench shows up, and I'll post picts once I get it all apart.

Are the bolts too 'long' and allow the chainring to not be fully tight? Without seeing it, it sounds like the bolt was the wrong size (a double ring bolt was used instead of a single ring bolt, or the washer was missing).

Outside of that, make sure the bolt/chainring doesn't move once they are tightened down and you should be okay.

If a good soaking with penetrating oil before using plus the new tool doesn't give you enough leverage or if the new tool won't stay seated, you should be able to 'carefully' use a pair of Channel Lock pliers to hold the lip of the female nut against the chain ring.

However, you'll want to put a small piece of metal, wood or plastic between the face of the chain ring and the pliers jaw flat to keep the pliers from maring the finish of your chain ring. Use a hardened steel allen wrench on the screw head. Worst case scenario, the bolt head's allen flats will strip-out, in which case you can either attempt to use an easy-out extractor or go back to the original Plan B: split or drill out the bolts. Frankly, I'd be surprised if the Channel Locks + steel allen head would not work but, again, until I can see just how mangled the bolts are all bets are off.

As for determing if the chain ring or crank arm bolt holes were deformed, just check them against the stoker's timing crank and chain ring holes. Your new chain ring bolts should fit up with the same amount of tolerance; there's typically some play. If the new bolts can be torqued-up and create a firm press fit with the cranks, even if the holes are a little deformed, you should still be OK using the old cranks and rings. However, you'll need to spend some time making sure that you get the ring centered so there aren't any excessive high or low tension points in your sync chain as you rotate the cranks. If you find that while riding the chain rings shift off-center, you may need to revisit the crank arm and/or chain ring replacement scenario.

As for how common this is, as noted in another recent thread on chain ring bolts coming loose we've had it happen a few times, but not where near to the extent that you've just described. It typically happens when new chain rings, bolts or new cranksets are installed and you neglect to check the bolt torque after the first couple rides. Now, if chain ring bolts come loose you'll typically hear a clicking noise that stays in sync with the rotation of the cranks as the chain ring deflects away from the crank arm under load on each turn of the cranks: that's your first clue that something's not right. This can sometimes be mistaken for a bottom bracket creak. Again, the solution is to always recheck chain ring bolts after a recent installation and/or to chase down any unusual noises that come from your tandem, as those are usually precursors to bigger issues down the road if left uncorrected.