Bike Repair – Checking Drivetrain Wear

Cyclist checking drivetrain wear

Our 16-step guide shows you how to examine your transmission for damage and replace any worn parts.

Get cleaning

The first thing to do is to clean the chain and transmission. There’s not much difference between almost worn and worn, and having gunge stuck in the chain links isn’t going to make the precise measuring of the chain link pitch any easier. Besides all that, who wants to get covered in oil while they’re working on their bike?

Measure the chain wear

Take your chain wear indicator – we’re using the Park Tools version here, but you could also try one from Rolhoff. Insert the short measuring finger on side A into a link. Allow the other end of the measure to drop between the links. On an unworn chain, the long measuring finger will not fall between its links – as in this case, you have less than 0.75mm of wear, so you’re good to ride.

Signs of wear

If, when you’ve followed the same procedure using side A of the tool, the long measuring finger falls between its links, this indicates that you’ve got between 0.75mm and almost 1mm of wear. You’re still good to ride, but be aware that the chain is stretching so you’re already adding extra wear on the soft alloy chain rings. Be prepared to replace the chain as soon as is practical to do so.

Double-check

If your long measuring finger has fallen between the links, switch the tool over and measure the same set of links using the finger on the other side. If, when the small finger is in position, the long finger still falls between the links, then you’ve got more than 1mm of chain wear. This is a serious issue and the chain must be replaced. Failure to replace a chain that’s worn to this level will damage your transmission system irreparably.

Remove Chain

Shimano chains require the use of a chain splitting tool to break them. As the chain you’re removing is worn out, you don’t need to worry about being too gentle – just split the chain and let it run through the transmission. Lay it out on the floor in a straight line on a plastic bag to avoid spreading grease and getting fluff in the new chain in the next step.

Measure chain

Take your new full-length chain and lay it out flat and straight next to the old one, so you can see how many links to remove to make it the correct length. You’ll notice that at the halfway point, the old chain begins to show its stretch as the links no longer line up. Make sure that you take this into account before splitting the new chain.

Fit new chain

SRAM chains use a splittable Powerlink system that makes fitting (and removal) a tool-less affair. Shimano chains require a special joining pin (which comes with every new chain), so insert it through the corresponding link plates and push into position with a chain tool. Snap off the lead section using pliers. If the newly joined link is a bit stiff, grasp the chain firmly each side of the stiff link and give it a slight flex. This usually frees the link.

Check chainrings

Chainrings have a hard time. They’re made from soft alloy to make them light, but this also means they’re getting constantly mashed by the steel chain. Check your rings for hooking caused by chain stretch. Also check for bent, gouged or missing teeth – the chainrings can also wear out from nonchain-related illnesses too.

Get the right BCD

Make sure your replacement chainrings share the same bolt circle diameter (often abbreviated to ‘BCD’ on the packaging). Also check they have the right amount of arms – it’s easy to pick up the wrong one in the shop. You might want to consider making a change to harder chainrings or different gear ratios at this point.

Remove worn chainrings

Use a 5mm Allen key (some new cranks use Torx T-30) to remove the worn chainrings. Occasionally you may find that the two threaded chainring bolt halves have corroded together. You may need to use a special tool called a Shimano TL-FC20 (Park Tools also makes a good one). This will hold the slotted side of the nut nicely and allow easy extraction.

Get cleaning

Clean the crank arm’s chainring fitting tabs with a clean cloth. These get quite grubby through daily use, and any dirt left on them will interfere with the ability of the rings to be completely in line. Besides all that, it’s just good bike manners to put things together cleanly. Also check the crank arm bolt holes for wear, especially on carbon arms.

Chainring alignment

Make sure you’ve got the new chainrings correctly aligned. The overshift protection pin (a small pin on the outer chainring) should line up behind the right-hand crank arm. This ensures all the shifting ramps and pins are in the correct place. You might laugh at this, but we’ve seen it done.

Add anti-seize

To avoid the issue of having the chainring bolts seize in future, it is a good idea to protect the threads by coating them with a smear of anti-seize compound. If you haven’t got any, buy some, and a dab of whatever grease you’ve got will do in the for the time being.

Check cassette

Worn cassette clusters are easily spotted the same way as chainrings. Look for widening of the pitch between the teeth, particularly in the middle five sprockets, and any other bent, missing or otherwise damaged teeth. Remember, though, that sprocket teeth aren’t uniform from new, with some lower and more twisted to provide you with faster and quieter shifts.

Remove cassette

Using a chain whip, cassette removing tool and a large adjustable spanner, remove the cassette as shown. Occasionally the cassette lock ring can be stubborn to remove. Rather than applying more pressure gradually, try applying some strong sudden force to the chain whip and spanner. This can often be effective in ‘shocking’ the lock ring into coming free.

Fit cassette

Line up the narrow spine on the freehub body with the narrow spline on the cassette and slide them into position. The larger sprockets will be joined together; the smaller ones will go on singly. Make sure you get them in the correct order and the right way around, and also ensure that the relevant spacers are in the right places. Replace and torque the cassette lock ring to 14Nm.

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