Actually you don't, but one can cheat a bit. If one cuts a small amount from the water pump one can sneek a new hose on. Worst that can happen is that you screw it up and have to replace the pump. I wouldn't remove the pump to replace the hose and put the old pump back on anyway. Take a hack saw blade and start with a small amount.

In the mean time I'd find some duct tape till you can get the proper hose.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician