Bullet Proof An Evo Transmission

Article refactored

This article is an old version and has been refactored into Evo Two Speed Transmission Improvement and Evo Two Speed Transmission Maintenance. The original old article below has been kept in order to maintain a historical record.


The Evo 2 speed transmission is not only a marvel for being the first in the industry. It is also reliable, easy to maintain and relatively easy to understand (in comparison with complicated CVTs found on larger scooters). There are, however, a few failings of this transmission that will not become apparent until after long periods of use.

Though nothing will replace preventative a maintenance schedule, there are certain modifications which can be made to the transmission in order to ensure that the transmission lasts the life of the scooter.

Weak Spots

Shielded Bearings

The transmission comes with a full set of high RPM shielded bearings which are designed to handle the high RPMs that the transmission will likely experience. For whatever reason, Puzey design did not see fit to anticipate that the transmission may get dirty during its life time.

Shielded bearings are great for areas where the bearing will experience higher RPMs, but not so great where dirt is involved. Although the shields will block most dirt from entering the bearing, they will eventually fail due to the introduction of dust between their races. When one of these bearings fails, serious damage can be done to the transmission including the destruction of cams and shafts supported by the bearings.

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Left: dirty shielded bearing next to seized bearing. Middle: this bearing seized and destroyed the top shaft in the picture. Right: destroyed cam from a bearing seizure.

Aluminum 2nd Gear

The transmissions 2nd gear housing and clutch system is a work of modern art. Not only is this gear system designed to handle effortless shifting, but the clutch plates are designed to be replaceable for cheap after considerable use.

One problem a 2nd gear could run into in the long run is the fact that it is aluminum while the replaceable clutch "catch" is steal. After considerable use, the replaceable clutch "catches" will begin to bite into the not so easy to replace aluminum lower 2nd gear.

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Notice the dents caused in the inner of the gear by the clutch catches

2nd gear clutch degradation

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Oil spun into a clutch plate after around 1500 miles.

Over the course of the life of the scooter, the 2nd gear clutch system will slowly degrade. This is due to perfectly normal wear of the clutch catch pads and the introduction of contaminates to the clutch plates.

The lower 2nd gear pulley houses a large 6204zz shielded bearing. This bearing does not get stressed much since it actually spins along with the shaft and gear while you cruise in 2nd gear, but it does slowly leak its own lubricant through the shields on either side of your clutch. This will slowly make your clutch less and less effective and will also decrease the life of your clutch catch pads1.

The loss of clutch pad material and introduction of oil can make effectively shifting much more difficult. The clutch will eventually require more force to engage than your thumb or the [bowden cable] can handle.

One-way 1st gear bearing

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The lower 1st gear in the evo transmission

A crucial part of the functioning of this transmission is pressed deep into the lower 1st gear. The one-way bearing converts a ridiculous amount of force into driving power while in first gear and spins cleanly while in 2nd gear. Should this bearing fail in anyway, your transmission will be destructively compromised.

Not only could the bearing itself fail, but a rumored possibility (reference?) is that the bearing could break away from its press fit in the aluminum gear.

In the case of failure, replacing this bearing is nearly impossible for two important reasons. Finding a suitable bearing (HF3020) is one heck of a challenge and you will need a hydraulic press both to get the old one out and the new one in. Even replacing this bearing could remove enough aluminum from the gear to limit the life of interface between the gear and bearing.

How to fix the problems

Buy a set of sealed beargings

In order to replace all of the bearings in the transmission, you will need 10 bearings.

  • 4 x 6000 10x26x8 Ends of top shaft and belt guide (C4-5 3COB06000A 10x26 BEARING)
  • 2 x 6001 12x28x8 Inside cams of top shaft (C4-3 3COB06000A BEARING 28x8)2
  • 2 x 6002 15x32x9 Inside caps of bottom shaft (C6-2 3COB06002A BEARING 30x15)3
  • 1 x 6204 20x47x14 Inside lower 2nd gear (C8-3 3COB06204A BEARING 20x47)
  • 1 x 1910 10x19x6 Inside cover clutch plate (C8-5 3COBL1910A 6x19 BEARING)

Buying a decent set of sealed bearings should run you somewhere around $50. Installing these bearings should be generally straightforward except in the case of a couple press fitted bearings. For a trick to remove the most difficult of these bearings, see the howto.

Fix bearing/anneal clutch plates

Making sure your clutch continues to grab smooth and strong is a relatively simple process.

Replace with sealed bearing
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6204 sealed bearing ready to be pressed into the 2nd gear housing.

The best way to deal with a slipping clutch that requires too much force to engage is to clean it, but in order to assure that the problem won't just occur again, you should replace the shielded bearing inside with a sealed bearing. If you have already completed the first step, you shouldn't have any worries.

For a cool idea on how to press out this tight bearing, see the oven trick article. The article demonstrates with a wheel bearing, but the same theory applies. You should be able to push the old, shielded 6204 bearing out with your hands (inside of oven mitts) if you perform the trick correctly.

Drain grease from bearing
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shielded bearing in gas

Another quick fix to this problem which doesn't require buying another bearing (and several racers swear by this) is to just soak the oil out of the 2nd gear pulley bearing. This can be effectively achieved by soaking the bearing and housing in gasoline overnight. Once completed, just blow out the remaining gas with compressed air or let it dry. Since the bearing doesn't have to hold much weight, it won't break down from the lack of lubrication.

Anneal and polish the clutch plates
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clutch plate being heated to soften metal

You can effectively make the clutch plates "stickier" by heating them up and making them softer. This can very simply be achieved by heating them up until they are cherry red, letting them cool and reinstalling them in the scooter. A butane torch or gas stove is usually the most effective method of doing this. The weakening should be mild enough to not effect any of the other properties of the plates.

See the how to article.

Buy a second, 2nd gear

At the time of the writing of this howto, there are no steel/reinforced 2nd gears available to counteract the wear caused by the clutch "catches." My best suggestion is taking shifting easy and rotating the gear once in a while to even out the wear to bot sides of the inner notches of the gear.

And, if they are available, get a second one in case you eat the first.

Replace the 1st gear with a metal sleeved gear

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This 1st gear has a steel ring pressed into it to improve the bearing-gear interface.

There are a few different 1st gears floating around for the Evo transmission. The 1st generation gear with a weak bearing, the 2nd generation gear with a strong bearing and a few modified gears that are lightened and/or given a steel sleeve for the bearing to grab.

If you can come across a steel gear with a strong bearing already press fitted, you'd be set for life. Otherwise, there will be no replacement for the maintenance of cleaning and regreasing the bearing.

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