What is the duty cycle of model # 0300841, and how large an auxiliary tank could it support? Thanks…Frank K.
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That species would normally be no more than 50%. “How large an auxiliary tank could it support? ” Depends on what you mean by support. It would likely fill a 120 gallon tank in a half hour, but might kill itself doing so. And most tanks aren’t cheap, so it may be a better deal to get a bigger machine.

The question is what is your intended use, says Doug from s.d.ca?
And Frank – if you are considering buying this, I’d stay away on the simple ground that the manifold is a unit casting, and apparently unavailable as a distinct part, or even as part of an assembly.
You would be able to replace it with pipe fittings, if needed, but that’s a pain.
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Doug, I would primarily use air tools like angle grinders, buffers, and drivers 3/8 & 1/2. Almost always only one tool at a time. I already own the machine. Just underestimated my requirements. Frank.
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You can forget running a buffer as it stands. Grinders are probably OK at least for smaller jobs. Ditto for the wrenches, assuming you’re mostly changing tires.
Here’s some info on what to expect –
https://www.about-air-compressors.com/buying-an-air-compressor/
or
How to choose a compressor from Campbell Hausfeld
You might also consider buying an additional compressor and running them in parallel.
And here is some info & questions related to your question about adding a tank –
https://www.about-air-compressors.com/coupling-two-air-tanks-together/
https://www.about-air-compressors.com/add-another-compressor-tank-to-an-existing-air-compressor/
Feel free to come back if that doesn’t answer your question.
Good luck.