When to Rebore a Motorcycle Cylinder

As part of HappyWrench, I receive quite a few questions via email.  One that I received recently that I thought I should address is “When to Rebore a Motorcycle Cylinder.”

Generally, it comes down to two things – piston clearance and shape.

Clearance

Clearance alone (i.e. the clearance between the outside of the piston and the inside of the cylinder), if not too far out of specification, can often be addressed with an oversized piston, new rings, and a quick cylinder hone.  That said, there will come a point where you can’t go to the next oversize because the next oversize doesn’t exist.  This will result in you having to replace, re-sleeve, and/or rebore your cylinder or cylinder sleeve.

Shape

Usually the bigger issue when it comes to when to rebore a motorcycle cylinder is shape.  Cylinders wear unevenly during operation such they become (1) out-of-round and (2) tapered.

Out-of-round occurs due to the fact that the pistons and rings wear more against the front and back of the cylinder (versus side to side).  This creates an oval shape to the cylinder over time.

Taper occurs in the cylinder due to the simple fact that the pistons travel and wear more against the top of the cylinder than the bottom (i.e. creating a fatter cylinder at the top versus the bottom).

Don’t get me wrong, both of these problems can actually be solved with a high-end cylinder hone and the patience of a Buddhist monk.  However, it is simply more practical from a time and cost standpoint to take the cylinder to a shop and get a professional rebore.

Again, let me emphasize that a cylinder hone is not meant to bore a cylinder oversize or correct a cylinder that is out-of-shape.  That said, it can be done using a high-end cylinder hone if you are really stubborn and patient at the same time.  You will need hours to get the job done and should be done a tiny bit at at time – basically, hone a little, then measure, hone a little, then measure.  I promise that a cheap hone will make a cylinder that is out of shape worse, so stick to the high-end.  Start at the bottom of the cylinder and work your way back up.

Best advice I can give is that if a cylinder is misshapen, take it to a shop with a boring machine.  I am DIY-er, but there are also times when it is more cost efficient to get it done by a pro.

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