Saturday, June 6, 2009


I mentioned the OVER works cluster I have.
In the past I have often recommended changing the XTZ shaft for that of the SZR in order to gain a longer 1st gear.
The two new engines mentioned in the last posting also have been changed. One of them has the SZR shaft and first gear, the other has an ex-MZ works racing cluster made by an east German machine shop for MZ. It differs slightly from the OVER.

Here are all four clusters:

You can see how rediculously short 1st gear is in the XTZ engine. Belgarda changed that for the SZR, leaving the rest alone. Both racing clusters are very different from stock, with extremely long 1st gears and close ratios. The OVER cluster is most extreme, being even longer in 1st and shorter in 5th. There are only about 800RPM between shifts which is perfect for racing but can be very trying in regular driving and downright exasperating in traffic. Speeds below 25MPH are only possible with a slipping clutch.
Having tested the ex-MZ cluster as well, I can say that it is only slightly better on the street. Both shift by themselves, fast and effortlessly, up and down without the clutch.
This of course has to do with the higher quality of the cluster itself, but more importantly with the small difference of speed between the two shafts. That is the reason why the SZR cluster also shifts better than the XTZ. The layshaft is driven by 1st gear, so the extremely low 1st of the XTZ makes the speed difference large. The slightly longer 1st of the SZR greatly improves the shiftability, and the extremely long 1st of the two racing clusters makes shifting nearly unconscious.

The OVER cluster is different in another way as well.

Here you can see both an SZR at the bottom and the OVER cluster at the top. You can see that the gears look different, and that 4th and 5th have coarser, stronger teeth and are considerably wider than the stock. The dogs are different as well.
The ex-MZ cluster does not use two different modules.

The OVER is no longer made and the ex-MZ was never offered as such. Reportedly, the machine shop which made them would make more if demand arose. Slipstream does offer an racing cluster. All of these, foremost the OVER, are or were very expensive. Slipstream also offers a shorter 5th gear.
More important though, for the "normal" Skorpion rider is the modification with the SZR shaft and 1st gear. This is a simple and worthwhile modification. For me a must-do.


I haven't written anything for a while; not a whole lot to tell really, at least not with regard to this blog.
I did build up two moderately tuned XTZ engines for Skorpions and also got around to a first stage of my pet idea, an air-cooled Skorpion. The original Seymour Powell design project had the 4-valve air-cooled Rotax engine which MZ was already using in their so-called Classic at the time.
I chose to stay with what I am familiar with, i.e. with Yamaha and assembled a bastard engine utilizing the XTZ crankcase of my racing engine with its one-off welded crankshaft with Carillo, the OVER racing cluster, and my own rollerchain timing gears (December 2007). On top of that I put a TT cylinder, bored out to 98mm with a Wiseco racing piston and a TT head with minor porting and a Megacycle 266/20 cam and RD's new Beehive springs.
These are very impressive to me. The pressure curve seems flatter, more constant and the dry engine turns over lighter by hand than with the more usual racing springs. The springs are 30% lighter even with steel retainers than the previous sets with Ti retainers according to RD.
Basically the engine looks like this now with a Grizzly manifold and Mikuni TM40/6:

Not needing a water pump, I turned an aluminum cover for that. The water pump drives off the crank at 1:1. I plan, when I find time(?), to make a timer with pickup for this position so that I can use any after market ignition and set the static advance as I see fit. That will also allow me to adapt a smaller lighter flywheel since the OEM pickup will no longer be necessary.

Installed in the bike, we now have this:

The oil cooler is from XJ600, the leads are new as is the direct feed to the camshaft. the exhaust manifold is from Devil for a TT. Suprisingly, it fits the Skorpion frame almost exactly.

For the time being, however, I have adapted the Skorpion's original Lenhart und Wagner exhaust system to fit the TT head.
There will be a SR Racing can eventually matched up to the headers shown above.

As with my Yamaha, I use an Ignitech Sparker TCIP4 programmable iginition box. Both of the aforementioned XTZ engines also use this box.
The bike started right up and all the new oilsupply lines are tight; everything is dry. Now to test ride and above all to break in the new barrel, piston and cam.

During the winter I will dismantle this engine again and install a new crank. This is what my pet idea is really about. The new crank will have 10mm LESS stroke, have a 5mm longer conrod and be lighter. Barrel and piston will remain as they are now, reducing the capacity to 560cc from the 630 I now have. A much lighten balancer shaft will also be installed, together with a solid drive gear for it. Both of the new engines mentioned above have these solid gears, one of them a lighter balancer and that one in particular is "smooth as silk." The lighter generator will also contribute (I hope) to making an engine with a much higher usable rev-range and ( I hope again) not much less performance, if at all.

The Czech ignition box has proven to be a real improvement. The advance curve of the DENSO box is too extreme of anything bu the stock engine and even the stock engine benefits from less advance above 1000RPM. In the stock Yamaha SZR, I have the box set up to a maximum advance of 28º at 6000RPM instead of the 37º of the OEM box. The limiter ist set to 9000 and I regularly rev the stock engine in 1st, 2nd and 3rd over 8000. Instead of wasting money on having the OEM box modified as most do ( and I did as well when I didn't know better!) it is definitely better to buy the Ignitech Sparker. The price is another argument for the Sparker and the support from the company couldn't be better.

Like my earlier blue Skorpion, this bike also has the Brembo TZR wheels that the Skorpion prototypes had: 3"/17" front and 3,5"/17" rear. I am testing the Haidenau Cup racing tire in 110/70 and 130/70. The first few miles were very encouraging: light-footed as a bicycle.

I will update about the aircooled Skorpion as time goes on. Next week I will definitely get it on the scales.