If I can add a little more info to this thread as I'm helping (or is it hindering!) my son in this process.

The Engine in question is from a MC28 R3S and we have no history on it.

However, I was wondering if this was a common problem with the Dry Clutch set up (wear wise)? as I notice that DMR-JAPAN (see Part E5-21028 Figure 1) do a complete Collar replacement set and wonder if there are a wear issues with the collars wearing and not throwing the Inner Clutch Basket away from the Outer Clutch Basket, or am I missing something very simple? (!!Was always told to look for the simple things first!!)

Additionally, I noticed that the Outer Clutch Basket on the MC28 R3S Engine may be from an earlier date (Part No 22101-KV3-780) as the one fitted (see Figure 2) has no Castellations [well Castellations of a sought], although, I was led to believe these parts are Interchangeable!!!

I have now managed to get a new Outer Clutch Basket, although it was advertised as Part No. 22101-KV3-780, I believe it to be the later Part No. 22101-KV3-980 (see Figure 3), as it has the castellations.

Apologies, for butting in on this thread, but we are at an impasse as to moving this forward.

Many thanks

PS: Can I take the opportunity in thanking everyone in the help received so far, as although it’s been a struggle to get this far (Bike has been in boxes all over the place in the past 25+ years) as without all the help from the Forum and the Forums Knowledge Base it would have been an impossibility.
Here’s hoping to another NSR250 to return to the roads sometime soon.