Many engine components are a mystery until you experience problems with them, need to research them and then get your hands on them. This was the case for the SR250 clutch. There is nothing special or unique about it, it follows the same recipe as any motorcycle multi-plate wet clutch design. That is, a clutch basket with a sandwich of friction plates and steel plates pressed together with a pressure plate by a set of springs, with an actuating mechanism to relieve the spring pressure and therefore friction, allowing it to slip/spin freely. In the case of the SR250, the confusion comes when ordering the friction plates and the steel plates…
Hot tip: work on the clutch without dropping the oil by leaning the bike over 30-45 degrees.
We wrote quite an extensive article about the SR250 here and mention many of the similarities and differences between the different SR250 models. One of those differences happens to be the clutch. The first model SR250s, that is, Japanese produced bikes sold in the US from 1980-1983 had true 249cc (250!) engines, as opposed to literally all SR250s sold outside the US which had a 239cc engine. The difference is a 75mm piston as opposed to a 73.5mm piston. Anyway, the 249cc engines must have had slightly more torque than the 239cc versions because they were built with a 11 plate clutch (6 friction, 5 steel) and the 239cc versions were built with a 9 plate clutch (5 friction, 4 steel). This decision was likely made to save costs.
SR250 clutch parts with different style clutch plates.
Same same but different! 9 plate 239cc version to the left, 11 plate 249cc version to the right.
All SR250 models share the same clutch parts numbers for the basket, the inner hub, the pressure plate and the adjustment mechanism. To allow the use of these same parts but allow a different number of pressure plates and steel plates, different plates were made at different thicknesses. Below is a table listing the different parts numbers for the different clutch plate parts and their thicknesses.
Different plate thicknesses add to a different total thickness.
Friction plate: 5831632100 type 1.
Friction plate: 5831632100 type 2.
Friction plate: 5Y11633101
Steel plate: 3J21632400
Steel plate: 3601632500
So the key takeaway here is to not mix and match parts from the two different versions. This sounds like a no brainer right? Well it would be if it weren’t for the different listings on different websites - which makes it seem like the parts are interchangeable when they are not. In terms of performance, in the case of the 239cc engine, a 9 plate clutch will suffice/not slip. It will also handle a decent increase in power as long as the springs are upgraded - which provide extra plate pressure. Jadus has a kit for that here. If the engine has any further modifications, like a bigger cam or piston, the 11 plate, 249cc clutch parts should be installed to avoid slippage.
9 plate, 239cc clutch pack.
11 plate, 249cc clutch pack.
Custom 11 plate clutch pack with one thicker steel plate - read below why.
In the case of the 100mph SR250 engine, where a lot of engine work has been done, an extra step was taken… On closer inspection, the clutch basket has room for/allows for a slightly wider clutch pack width - see above and below. Now knowing all the different widths of all the different plates, we could build a pack that is slightly wider than both the stock 239cc pack and the stock 249cc pack by swapping out one of the 1.2mm steel plates with one 2mm steel plate - gaining an extra 0.8mm width. It may be possible to swap two plates but we will save that test for another day. By increasing the pack width, you also increase the spring pressure - because they will now be deeper into their working range and therefore have a higher rating.
Here we can see there is probably room for one more thick steel plate.
Here you can see the highlighted 2mm steel plate in the last position.
And to conclude all of this about different clutch plates, below you can see what the clutch pack looks like with the wrong combination. It is much narrower than it should be and fills much less of the clutch basket. This is when the 2.8mm friction plates are used in a 9 plate set up, along with the 2mm steel plates - adding to only 22mm (nominal). Embarrassingly, this is the exact mistake we unwittingly made and we promise you, your clutch will slip like crazy if you do this!
Wrong clutch pack combo.
Too much space left in the clutch basket - meaning less spring pressure/less clamping force.
One last note on clutch parts… We have not encountered this with any of our SR250s, but sometimes a clutch basket will be worn and either need to be repaired or replaced. The wear occurs on the guide surfaces where the clutch plates move, opening and closing - see the images below (this clutch basket is considered to be in good shape). The aluminium surfaces can become worn and jagged, making it difficult for the plates to move freely with clutch engagement and disengagement - resulting in either a jammed or slipping clutch. These irregularities can be carefully filed flat again but it requires care and precision to not remove too much material and to keep the surfaces parallel.
Wear marks on SR25o clutch basket.
These surfaces must be smooth and parallel.
If you are experiencing clutch problems yourself, hopefully this article will help and if you want to inspect your clutch and or replace parts, our how-to video may be helpful. Check it out here. One thing we missed in the video was the alignment of the pressure plate and the basket - see the arrows in the image below. This is not really an issue though because it only physically goes on one way - as shown in the video.
SR250 clutch alignment marks.
If you have any questions about the article or the SR250 clutch, shoot us an email or hit us up in the comments below.
Happy wrenching and happy riding!
Jadus