So after watching the video linked by SwooshDave and from this diagram, is it the parts labelled in the above diagram "Dog teeth" & "Ring with internal dog teeth" that are making the grinding sound in my Mini when trying to downshift 4th to 3rd?
I did try a bit of double-declutching as recommended by some fellow local Mini enthusiasts...maybe improved a wee bit. I find if I'm really quick on the downshift, no clashing. If I hesistate, like happens when approaching a corner and braking with clutch pressed, before engaging again, then pretty nasty clashing.
Interestingly, the last time I was out on a mountain climb run, I performed about 4 total downshifts and absolutely no clashing happened. Beginning to wonder if it has something to do with oil temps and how the synchro cone friction is affected.
Need to do it's first oil change since purchasing a month ago as oil looks very clean but of unknown weight/type. Apparently Mini's like similar weight as our bikes, 20w50. For those that don't know the Mini's have engine/gearbox in a shared oil bath, much like most modern bikes. So you can't use specialty GB oils and the engine oil needs to be selected based on GB requirements. Valvoline VR1 with ZDDP is considered quite good (as it is for our Commando's), but that stuff is hard to find locally and typically costs $12-$15 a liter. I'm going to use Castrol GTX 20w50 and picked up some ZDDP additive to try with it. There is also a Lucas brand classic car/hot rod oil locally with specifically declares very high zinc content and at a good price. May try it one day.