Seal Frustration

I have assembled a number of Montesa engines since my first Impala.  In the last few years I have found that the shift shaft seal, #0099.2232, on about one half of my rebuilds would pop out when closing the central engine cases.  This usually required the re-splitting of the cases with all of usual problems in reassembling the gears, shims, and gasket.  I would occasionally press the cases together and the lip of the seal would turn out on the shift drum.  I could ease the lip back with the aid of a used and rounded small common screwdriver.  This was not the best plan because the tension spring in the seal may not seat.  If this seal doesn’t do its job then transmission oil will contaminate the primary oil and cause the clutch to drag excessively and reducing the lubrication of the transmission.

This is caused by a sharp edge on the shift drum.  The new seal is smaller on the lips than the worn seal that was removed.  This will wear in with use but as a new seal it presents an edge that will not easily slip over the edge on the shift drum.  There are a number of steps that must be done in order for the seal to slip into place when assembling.

You will need a few items:

• A medium carborundum whetstone
• 280 grit aluminum oxide coated abrasive
• Fine, single cut, 8 inch round file
• Multi-purpose grease
• Patience

Steps should be taken in order:

1. With the engine cases separated and clean and with the seal removed, file a slight chamfer on the edge of the seal-receiving hole.  It only needs to have the sharpness removed and shouldn’t be more than 1/32” in width.  It shouldn’t have any roughness.  A fine file should be sufficient but it can be sanded lightly.
2. After cleaning press the seal into the hole without lubricant.  It should be dry.  If you have chamfered the hole properly the seal should go in without tearing.  Drive it in until it bottoms.
3. Remove the shifting forks from the shift drum noting how they came off for later assembly.  Also inspect the shift fork guide pins for excessive wear.  Replace if necessary.
4. The edge on the shift drum that must be eased is located behind the gear on the shift-plate end and behind the groove of that gear. Using the whetstone, round the area on the drum.  You can check progress by touching the edge lightly and running it around the drum.  Polish it with the abrasive paper.
5. Clean all areas of abrasive.
6. Pack the lip of the seal with grease.
7. Reassemble the shift forks to the shift drum.
8. You are now ready to close the cases.

If you have a metal lathe then mounting the drum in a 3 or 4 jaw chuck and spinning the drum while using the whet stone and abrasive can do the job quickly.

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