Hi folks, a friend of mine asked me for help with his Dnipro MT final drive a few days ago and while fixing it I decided to dedicate a separate article to this rather small, yet extremely important part of all heavy cossack motorcycles (Dnipro MT, Ural, K-750, M-72, etc.). Hope this article will help you take care of the final drive on your motorcycle and prevent any problems with it in the future.
As you know, there are 3 motorcycle final drive systems: chain drive, belt drive and shaft drive. All 3 types have their advantages and disadvantages, you can find lots of info about this topic on the Internet. In our case we are talking about the shaft drive system. The rotation from the gearbox is transmitted to the driveshaft and from the driveshaft to the final drive unit. Among the advantages of this system I can name the cleanliness, because the final drive unit is properly sealed and dust and dirt can’t get inside, maintenance-free and high durability and reliability.
Speaking of the final drives that are installed on heavy cossack motorcycles, there are 3 types, depending on the final drive ratio.
Final drive ratio is a correlation between the turns of the driveshaft and the rear wheel of the motorcycle. To calculate it you need to count the number of teeth on the crown ring gear and divide it on the number of teeth on the small pinion gear.
The bigger the ratio is, the easier it will be for the engine to rotate the rear wheel, but the smaller maximum speed your bike will have. Thus, the biggest part of all heavy cossack motorcycles with sidecars are equipped with the 8/37 sets, which provide maximum torque. There are some models with the 9/35 sets. As a result, they have a higher maximum speed and slightly decreased torque, which is nice for paved roads. Yet, such motorcycles are rather an exception, because the 9/35 sets are more common for solo Dnipro MT and Ural motorcycles.
Generally, if your bike has a sidecar and you prefer riding off-road, the 8/37 ratio is the best option for you.
The 9/35 ratio is good for those who use their bikes without sidecars or for those who ride with sidecars but mainly on paved roads. From my experience I can say that the combination of the 9/35 set and 5-speed gearbox is ideal for solo Dnepr and Ural motorcycles.
As for the 10/35 ratio, it’s meant to be used on solo motorcycles only. I can think of two possible reasons to use this ratio.
The are a number of common problems with a final drive reduction unit you might face. Here are detailed descriptions of these problems and ways to get rid of them for good.
Problem #1: oil leaking from the final drive reduction unit.
This problem occurs when the sealing between the reduction unit and its cover isn’t tight enough. The problem might be connected to poor quality of these parts (i.e. the contact surface might be not flat enough, which was pretty common for soviet-made parts), the sealing product might lose its characteristic, the nuts that keep the cover might get loosen…
Solution: once the final drive reduction unit is disassembled, you should check the contact surfaces. If they are not very flat, you can make them flat (by using sandpaper or asking some metal works specialist for help)…or you can leave it as it is, because if you use a paper gasket and a proper sealing product, this should be enough to prevent any leaking (there is no oil pressure here, so super high class sealing isn’t necessary). As I’ve already mentioned several times in my articles, I use the Victor Rainz sealing product, which is used to seal Mercedes Benz cars, but I’m sure you have some other high quality sealing products in your country. In case you want to get the Victor Rainz sealing product, here it is for sale on amazon. If the contact surface is flat, you don’t even need to use a paper gasket. Once you apply the sealing and close the reduction unit, tighten the nuts well. Now when everything is ready you can fill the reduction unit with oil and … here comes an extremely useful advice from me.
Thus, there is no chance for leaking when the motorcycle is parked. As for the oiling, these greases are absolutely fine for this purpose and everything inside the final drive reduction unit is oiled properly. The very same method is used in angle grinders.
Problem #2: some nuts can’t be tighten properly.
Obviously, there are 3 possible reasons here: a screw thread got damaged on the nut, stud or inside the reduction unit housing.
Solution: If the nut is damaged – replace it with a new one. The same situation is with the stud. Yeap, these threaded studs can be simply screwed out from the final drive housing and replaced with new ones.
If the screw thread in the housing is the cause of the problem, it’s no tragedy and you should not buy a new housing.
The first option is to screw out the old stud, clear the hole from old thread and cut new thread of slightly bigger diameter. This can easily be done using a tap of the right size. Once you cut a new thread, you will need a new stud one end of which will match the new thread while the other end will have the same size as the previous stud had to fit the hole in the reduction unit cover.
If you don’t like to deal with metal and don’t want to thread anything or you lack the needed stuff for this process near at hand, there is another option for you. You can take a special metal adhesive and use it to stick the stud with the housing. I’m not sure if I used the correct terms to describe this process, so here is a link to the products I’m talking about. If the adhesive is of good quality, using it will be enough to keep the stud in place. There is no high pressure inside the final drive unit, that’s why there is no need to put too much force when tightening the nuts on it. It’s much more important to use the right sealing product.
Problem #3: strange noise inside the FD unit or the rear wheel got jammed.
I’m afraid this is a bad news for you. If you hear some noise, you should stop immediately and check what is wrong. Otherwise, you will definitely make things worse and might get the wheel jammed during the ride, which is especially dangerous if you ride a solo motorcycle (without a sidecar). Plus, the earlier you notice the problem and stop riding, the less parts will be damaged, i.e. you will have to buy less spare parts and spend less money on repairing the final drive unit.
The wheel jamming and strange noise can be caused by several issues:
Solution: the only way to know the exact cause of the problem is to disassemble the FD unit. If one of the gears is damaged, you will have to replace it. You shouldn’t continue using a gear that has at least minor damage. The same situation is with the bearings. If there is no visible damage to the bearings, try to rotate them – every bearing should rotate smoothly and silently. If you feel anything strange (crunch, gap…sorry, I can’t find the right terms in English) while rotating a bearing, it should be replaced.
As for the third issue I mentioned, this happens when tap bolts are used inside the FD unit. The upper part of the bolt SHOULD NOT have threading. When it does, the threading on the upper part of the bolt gets smashed with time and as a result the bolt becomes slightly smaller than the hole it sits in, i.e. a small gap appears between the bolt and the hub. And when this happens with all the bolts, the hub starts to hit the bolts, which results in destroying the bolts and making them literary lose their heads. That’s why it’s so important to use hex bolts, which don’t have treading on their upper part.
If you are among those folks who want everything to be absolutely perfect, you gonna need this info. As in case of most mechanisms with gears, it’s important to have the right surface of contact between the gears. The best way to check this is to use paint. You should paint the teeth on the crown ring gear, assemble the final drive, rotate it a little, disassemble the final drive and check the paint print on the small pinion gear. Here is now an ideal print should look like.
If you are lucky to have the ideal print, nothing is needed, just assemble the final drive and get your bike on the road. If not, here are the details on how to fix this issue and place the gears properly.
As in case of any other parts for Dnipro, Ural and alike cossack motorcycles, you have 3 options:
Hope this article gave you the information you were looking for. If you still have some questions, please ask them in the comments below and I’ll answer them.