My '19 plate Tuono became slightly incontinent a few weeks ago with a small amount of coolant pooling in a V profile on the side of the block just below the thermostat housing. I did some digging and whilst not a frequent issue it's also not unheard of, seems the cold weather can distort the plastic stat housing causing it to weep from the join of the two halves. There's quite a lot of discussion with owners in N.America & Canada about theirs leaking over the Winter months. Luckily AP stock the stat housing so with part of the Xmas budget redirected one arrived a couple of days later.
My method to replace was:
- Drain coolant and flush system.
- Remove small amount of bodywork to access undoing rad cap & top hose.
- Stat housing has 3 hoses secured with sprung hose clips as you face it from the R/H side, North, South & West.
- South pops off easily, West requires carefully pushing/wiggling the stat housing Eastwards and the North hose remains attached.
- I took note of top hose orientation to other cables etc then sprayed some WD40 onto the top hose bends. A bit of wiggling and the stat housing and hose slipped out via the triangular gap in the frame where the housing is located. Once removed remember to mark the top hose to stat housing orientation for fitting to the new item.
- Refitting with new was just the reverse of the above although I used Jubilee clips for the West and South hoses in case this ever needs doing again.
- Rad took 2.4ltrs of coolant , I added some coolant, rocked the bike from side to side to burp the system then added some more until full. Expansion tank took 200ml.
- Ran it up temp, rode it and checked levels, 24 hrs later they seemed fine from the initial fill.
On examining my old stat, 10K miles old, you can see the join slightly bowed, enough I guess to allow seepage between the two halves and 'O' ring on the engine side of the unit, out of sight. The two housing halves are bolted together with two bolts 180deg apart, my non engineering brain questions whether that's really the best design but I'm not gonna lose any sleep over it. For V4 owners this maybe something to keep an eye on periodically.