Lightweight MZ Crankshaft
While working on my MZ race bike we modified a brand new crankshaft to lighten it and fit a stronger conrod. Here is how we did it…
Start with a standard MZ crankshaft
One of the great things about MZ racing is that most of the parts are available relatively cheaply. A good quality standard MZ crank, which we have to use to comply with class regulations, was bought on eBay for about £85.
Machine the inside
Shown here on a specially made stub arbor in the lathe, the bob weight was completely removed to leave a flat surface with just the small upstand around the big end being left.
I started by taking most of the metal off with the angle grinder, the surface was hard so the grinder made getting through this a lot easier.
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Remove corners
Next I cut the corners out of the wheels. As with the last stage I removed most of it with the angle grinder and just cleaned up the surface in the milling machine.
Fill in the corners with aluminium
With a two stroke it’s important to keep crankcase volume to a minimum in order to keep primary compression up. For this reason my next job was to replace the steel I have removed with aluminium, which is about a third of the weight. Here you can see the corners filled in with aluminium blocks that are screwed down into the flywheel. Left oversize at this stage.
Turn down the aluminium
After screwing the blocks on for the last time, with Loctite, I put the flywheels back onto the stub arbor and skimmed the faces flush with steel.
Fit the inner packing plates
The next job was to fit the inner packing plates to fill in the gap between the two wheels when the crank is assembled. I pressed a length of aluminium into the hole left by the M8 bolt that holds the corner piece on, then drilled and tapped through this rod from the side.
The top two bolts you can see on the flywheel hold the plate on and stop rod working its way out under centrifugal force.
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Finally the main shaft was held in the lathe and the OD turned down to match the original flywheel.
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Final assembly
The complete assembly is pressed together using a Yamaha conrod and big end kit. The standard conrod is fine on a road bike but this engine will be revved a lot higher and a lot more often, so the Yamaha one lasts better.
Note the sleeves that are pressed into the flywheels first to make up for the smaller Yamaha crank pin. New phosphor bronze shims are also inside to get the assembled width correct with the required 0.6mm end float for the conrod on the crank pin.
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I have checked the balance factor and will see how the bike performs once it is back together. If there is to much vibration there will be some trial and error to get it right. Once I’ve settled on a figure I will share it here.
UPDATE - There was a lot of vibration to begin with so the crankshaft was stripped again. I added a steel disc opposite the big end (after boring a hole in the aluminium) to rebalance the crank assembly. I then measured it again at 67%. The bike has been raced again like this and the vibration issues have gone.
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This was all done in a rush between two race meeting so I didn’t take any pictures. Next time it is apart I will take some pictures. Hopefully you will have to wait a long time for that though!
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