John,

I've seen that listing for quite a while, but haven't heard of anyone with experience of the .40.

Their other engines seem to be "inspired" by PAW's layout, and I have seen comment somewhere, here, perhaps, that they are modest in power, serviceable but a bit heavier than the comparable PAW engines, if I recall. The photos of the .15 and .19 Auroras I seem to remember on Carlson's site confirm the similar appearance to PAW engines. Ed's catalog pages used to have a grid with some of the specs and performance 'stats.' Some of those may have been a bit optimistic, for some of the engines.

If the data grid is still there for the .40, I'd be most interested in its weight, first, and then the stated RPM range and power numbers... Engines designed as diesels need to be more sturdy than glow engines, so weight can easily get excessive at a larger displacement. Diesels also vibrate a bit... A larger diesel may shake more and a bit harder. PAW .29s and .35s aren't too bad, at the right settings and loads, but that may not indicate anything about any other "larger" diesel.

Parts replacement is always a consideration, but generally, diesels seldom break anything in reasonable use. Aurora engines, unless they say otherwise, are most likely iron piston in steel - or, at least iron - sleeve, so there should be adequate oil% in the fuel, and all or most as castor. Sleeves and pistons can wear out, but it usually takes a long time, in reasonable use, with good oily fuel. I doubt you'd find a replacement set, but if things go right, you'll probably be tired of looking at an engine "that old" by the time it happens.

I'm almost certain that things you might mislay, like the prop nut and washer - possibly also the head hold-down bolts - will be some kind of standard international thread - IOW available just in case. Spraybars and needles can be swapped in from any source so long as they fit, and the jet hole(s) can be about centered across the intake throat.

If the price looks good to you, and you have a project for an engine that should swing a 12-6 or so prop around 9,000 RPM, an Aurora 6.5 might be an interesting, and not disappointing experience. So, best of luck, either way.