radiator cap and coolant expansion tank questions??

whats the relief pressure for the radiator cap. im wanting to find a cheaper aftermarket equivalent.
preferable one that's already in the country i need it asap.

started the bike up yesterday and after 10 min of idling the bike was real warm and temp gauge 3/4 of gauge, fan came on and then all of a sudden the coolant just started boiling out the little breather hole in the expansion tank in the rear just behind the filler cap. made a real mess.
(mid topic question why did they put the expansion tank waaaay back there??)
from my understanding of coolant systems my coolant is brand new, fan kicks in at right times, radiator fins good condition, could be thermostat faulty,its never been changed in its life im assuming so that will get a birthday i got the part just gotta fit it. only thing left is radiator cap which is what keeps the whole system under pressure so the coolant doesn't boil off so if that's not working im assuming that's probably what caused the water fountain.

would this assumption be correct or is there more to it?? <!-- s:-? --><img src="{SMILIES_PATH}/icon_Eyecrazy.gif" alt=":-?" title="Eye Crazy" /><!-- s:-? -->

 

ok the radiator cap did have a faded number, it was 0.9 bar
is this the correct pressure cap for this bike. my mate thought it was a tad low but were do you find the relief pressure for the cap yamaha specifies for this bike.

i called yamaha asking how much for a new one and the greedy little beggars wanted $92nzd or $71.46AUD. what the heck yamaha are you purposefully trying to chase customers away <!-- s:sniper: --><img src="{SMILIES_PATH}/icon_snipersmilie.gif" alt=":sniper:" title="Sniper" /><!-- s:sniper: -->
i hung up in disgust and called the more reasonable guys at repco who had the equivalent 90kpa and size rad cap for $20.
they also had 2 more caps with higher pressures. 100 and 110kpa respectivley.

 

don't know why it came boiling out. assumed the rad cap relief valve open dropping the pressure to atmospheric causing the coolant to boil.
overflow hose that goes from radiator cap to expansion tank is fine and in the correct place doing its thing.
expansion tank was on low level when started and high level when fully heated up. that's how its supposed to work yes.

 

wow so i can have a rad cap anywhere between 75-103 kpa weird i thought they would give a specific value so i suppose going to the 100kpa wont hurt then.
i will replace the thermostat and the rad cap just to rule them out.
question: when filling the coolant system from empty (drained eg replacing old fluid) how do you know when you have put enough in there's no dip stick or sight glass anywhere. do you have to measure the quantity out from the manual before you put it in and would you have to compensate for the water that's stuck round the engine block or is that not worth worrying bout.
i probably did have to much in the coolant sys but its never happened before so well see.

as for a crack in the block i don't have the tool to test that but i will rule out the other three options before i look at that.

gratefull for the help dave.