I did not go into what must be done to change the 3YF transmission into a 4SU unit.
3YF is the XTZ , the engine used in all MuZs, 4SU is the Belgarda SZR and the engine used in that bike which was assembled by Minarelli in Italy. Minarelli also builds the new 4-valve XT660 engine.
In that posting, you can see the different ratios and that the only difference between the 3YF and 4SU is 1st gear. Everything else is identical.
So, for those MZ riders desiring the better cluster (that the Skorpion should have had to begin with!) the easiest way to go is to obtain an SZR cluster thru eBay or similar source. They are not offered very often, however, and the price can be unrealistic as are the prices of many XTZ parts at eBay.
Lacking the complete cluster, one can order the mainshaft and loose 1st gear for the 4SU from any Yamaha dealer:
Mainshaft: 4SU1740100 list price 125.00€
1st. gear: 34K1721100 list price 40.00€
This is a 4SU cluster. The mainshaft is at the top, 1st gear at the right on it. This gear is milled directly onto the shaft. That is why the mainshaft must be exchanged. 2nd gear at the far left is pressed onto the mainshaft. This presents the main problem when rebuilding the 3YF cluster with the 4SU mainshaft. A suitable press is mandatory and one must make (or find) a piece of steel pipe into which the whole mainshaft will fit, allowing 5h gear -second from the left - to rest on the rim of the pipe. Thus set up, the shaft can be pressed thru the 2nd gear and fall free.
All the parts can are then assembled in the same oder onto the new mainshaft and the 2nd gear pressed back on. For this job, it is best to have a different piece of pipe into which only the shaft itself fits, allowing 1st gear to rest on the rim. Being observant of course, one will have measured the axial tolerance of 5th gear to turn freely as well as the exact amount the shaft stands proud of 2nd gear. Turning a small piece to fit exactly flush with the end over the shaft before disassembly will make assembly nearly fool-proof.
I have reassembled several Skorpion clusters with the 4SU mainshaft and it is not higher science but you do need, as I said, a suitable press and the mandrills to insure the the parts are moved straight. It is not necessary to heat the gear at all.
As I have repeatedly written, it is a must-do for anyone riding a Skorpion hard. Apart from needing to the break the engine completely, the actual job takes only about 30 minutes, a couple of cheap and easy pieces of pipe, a lathe and the press.
The total cost for the parts (here in the EU) ±170€ plus the scraps of pipe. 200€, even 250€ are thus acceptable for a used cluster in good condition if one does not have the machineshop at hand.
If and when one does get into this, one could consider going for a shorter 5th gear; see chart below.
Slipstream offers (or at least offered the pair of gears for 270£. Here in Germany, Joachim Pethke has made pairs and such pairs do turn up in the ESA (european supermono assoc) scene.
I am using a used pair myself.
Obviously, the pressing is the same to change 5th.
At one time, OVER made racing clusters for the XTZ engine, used both by theirselves and marketed as Yamaha works clusters. These cost 2500DM in the 90ies. They now generally go for over 2000€.
Koehler here in Germany made clusters for MZ and these very occasionally turn up. I have installed one.
Slipstream offers (-ed?) a close ratio cluster as well, on my dated price list at 1500£.
Again, the ratios of the OVER and Koehler clusters as well as the shorter 5th gear are on that chart:
I do not recommend either of the race clusters for street use; I have extensive experience with the OVER myself.
For serious track outings, tho, they cannot be beaten if the motor has been freed up and the ignition suitable without rev limiter.
For street use and the rest of us (including me now) the longer 1st of the SZR together with a shorter final ratio opens up a whole new world of fast Skorpion riding - and big game chasing.
Oh, and one more thing, another important must do for anyone running the 3YF or 4SU hard:
One must replace the gear on the crankshaft for the balance shaft with either that of the new
XT660 part no. 5VK-E1536-00
or of the Raptor 700 pat no. 1S31153600
Both fit, Shown in the picture here:
is that for the Raptor. The XT660 gear is a 1:1 replacement; the thick steel washer behind the gear must remain in place.
The Raptor gear is as much thicker as the washer is thick, so the washer must be removed. The gear as far as size and module is identical. I prefer this solution.