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Early Honda CX500s and GL500s came with rear engine covers that had a hole for the mechanical seal that was 27.8mm in diameter. Later bikes - and all of the 650 variants - had a hole that was 28.3mm in diameter. The mechanical seals that match each size are not compatible, and the smaller size is no longer available. That means that if you have a bike with the smaller size hole and seal you need to either take out the guts of the old seal and replace them with the guts from a new seal, or enlarge the hole to accept the currently-available seal size.
Enlarging can be achieved a number of ways - any good machine shop should be able to enlarge it precisely - but many people have found that they are able to do a good job with a small flap wheel in a drill or rotary tool (e.g. Dremel). Many people have found the Dremel 504 to work well (eBay US). It's 28.6mm in diameter and 4.8mm in height. If in doubt, leave it to the professionals.
Make sure you are working on the correct side of the rear engine cover when you are enlarging the water pump seal... you wouldn't be the first one to neatly enlarge the oil seal side, which is something you definitely do not want to do. The correct side:
You also do not want to enlarge the hole beyond the point that the weep hole sits, or else you will interfere with the oil seal. The weep hole far edge (when measured from the mechanical seal side) is:
8.05mm or so. You don't need to go further than that. Try reinstalling the oil seal - it'll act as a guide or bump stop to hopefully prevent you from going too far, though I certainly wouldn't rely on it solely as you may well push it out of place.
One challenge is keeping a drill straight - you need to ensure that you are enlarging the hole evenly:
A drill press can help here. However you proceed, do so slowly and briefly before stopping to measure:
Clean away the swarf regularly:
Measure at several points across the bore, and make sure you are not just measuring the outside edge - you don't want to make the hole conical! Keep it up in slow, brief sessions until you reach the desired diameter:
Slow and careful is the order of the day. It's much harder to put the swarf back onto the rear cover than it is to remove it. Once the hole is big enough, make sure you thoroughly clean the swarf out of the weep hole before you install a new oil seal or mechanical seal. Once you have done all of that, install your new seals!
If you are doing this as part of a triple or quadruple bypass procedure, head back here to the main bypass page for the other articles.
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