Over the years I have rebuilt anywhere from 1 to 5 engines per year for my various Montesa motorcycles. I have found that rarely has any engine been unmolested by either my earlier attempts at rebuilds or the attempts of others. As I now am rebuilding someone else’s engine on occasion, I felt that others might benefit from my experience. Over age the dissimilar metals of steel and aluminum will corrode and make extraction and assembly of the fasteners difficult. One of the things I now do to make the rebuilding process easier is to clean the threads by chasing them with taps and dies. Once done the engine can be more quickly reassembled.
This is not meant to be a comprehensive information sheet on metric threading as pertaining to the Montesa engine. It is meant to help you in maintenance and repair of your engine if you decide to do it yourself. Be sure to use correct tools in removing fasteners. Use two nuts tightened together to extract studs.
There are a number of threads that can be cleaned up so that the fasteners fasten and the torque torques. Montesa uses the following taps and dies on their earlier engines and continues them through most of the model line. A hand-tap wrench is helpful and can accommodate, in most instances, the 5mm through the 8mm tap. A diestock to accommodate the threading dies is also useful however you can get by without either and use a 6-inch adjustable spanner (Crescent Wrench) in place of the hand-tap wrench and socket wrench or larger adjustable spanner for the die if you purchase the hex form of die.
5mm x .80 thread pitch Tap
6mm x 1.00 thread pitch Tap & Die
7mm x 1.00 thread pitch Tap & Die
8mm x 1.25 thread pitch Tap & Die
10mm x 1.50 thread pitch Tap
12mm x 1.50 thread pitch Tap
14mm x 1.50 thread pitch Tap
1. 5mm – stator.
2. 6mm - shift cam plate adjustment stud, stop, main bearing seal cover plate (s), intake manifold.
3. 7mm – primary cover screws, case bolts & nuts.
4. 8mm – cylinder sleeve nuts & studs, ignition cover retaining stud on some models.
5. 10mm – head fixing studs/nuts.
6. 12mm – early primary drain plug.
7. 14mm – fill & drain plugs, shift detent.
Most hardware stores should be able to help you in locating the correct taps and dies. If you don’t wish to purchase all the taps and dies listed then you can usually get by with just the 5mm tap and the 6, 7, and 8mm taps and dies. Each item should cost between $4.00 and $6.00. Figure around $35.00 for a set.
A spot of anti-seize on the threads before assembly will make it easier to work on the engine at a later date. Do not use anti-seize on the cylinder base studs. Use LOCTITE Threadlocker when inserting the stud into the cases.
Happy restorations.
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