OK to substitute larger power resistor for nozzle?

I am building the Prusa Mendel i2 and it's almost complete, but yesterday when I turned on the heat to the J-Head nozzle for the first time, it didn't heat at all. I measured the resistance and it is close to a short (0.3 Ohms - the spec is 5.6 Ohms or 6.8 Ohms). I ordered a new power resistor but it won't come for over a week. My year-long senior project is due, and I have to present it to the entire school, on 5/4. My local electronics supply store does not have the part - they only have power resistor in the hundreds of Ohms. Question: can I use a much larger power resistor temporarily to demonstrate my printer? Will it harm the j-head nozzle?

No, it won't work.
The 5.6 ohm resistor generates 25W to heat the J-head, the 6.8 ohm resistor generates about 20W. If you subsitute a resistor in the hundreds of ohms range, it will be lucky to generate 1W.
(the power generated is V*V/R... with V being approximately 12V).
If you can't get a resistor of 5.6 or 6.8 ohm, then you need something close. A higher value will take longer to heat (too high and it won't get hot enough.. but I don't know at what value that is).
A lower value resistor will heat up faster.. but there is a danger it could fail.
If you can get something close, give it a try. But certainly 100s of ohms won't work... in fact, I would try to avoid going much over 10 ohms.

By the way, are you sure you measured it right?
0.3 ohms definitely sounds like a short (most of that 0.3 ohms will be the leads and connectors of your multimeter). I ask just because it is unusual for a resistor to fail into a short circuit, they usually go open circuit when faulty.

Overnight mail might get you the part in time

Most people use cartridge heaters now, not power resistors. They are widely available on eBay. Will your Jhead accommodate one?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thank you all very much! I'm getting a replacement of the correct part from the seller. (My nozzle does not appear to have a large enough hole for a cartridge heater.)

Update: the power resistor didn't fail short. My wiring was faulty so that the resistor leads were shorting out on the nozzle.