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Maybe this is a silly question, but what is the technical definition of the "front" vs. the "rear" of the engine?

On my vehicle, from the context of part names, the "front" is the side with the timing belt and the other belts, the alternator, compressor, p/s pump. The "rear" is the side with the flywheel, clutch, transmission, VTEC solenoid (it's a Honda).

Relative to the vehicle itself the "front" is on the driver's side and the "rear" is on the passenger side.

Which side of an arbitrary engine is the "front", in general? E.g. is it always the timing belt side? The side opposite the transmission? The side with the crank pulley?

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    I've made some assumptions along the way, but this has clarified things for me. I mean, I guess I intuitively knew this stuff, but really, great question. – Pᴀᴜʟsᴛᴇʀ2 Aug 14 '16 at 20:50
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    Something to note, many Honda engines run anticlockwise, so the gearbox is on the other side of the engine bay compared to most transverse mounted engines. – RemarkLima Aug 14 '16 at 22:05
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The rear of an automotive engine is the side the power is taken off of. The side where the clutch or torque converter attach. The other sides are in relation to the rear: the side opposite the rear is the front, and the left side of the engine is the side on your left as you look at the engine from the rear, and likewise the right.

I would guess that the "front" of an automotive engine is the side that was usually on the front in "traditional" rear wheel drive cars. Otherwise I would think it would make more sense to name the engine sides in a less subjective way (e.g. "power deliver side" and "accessory side" or some such nomenclature). But when engines usually sat in a vehicle in a consistent manner "front" was a lot less of a mouthful.

I don't know, but I suspect that on an aircraft engine it is reversed and the front is the side the propeller is on.

You can't go by things like the timing drive – some engines don't have them (two strokes for example) and there is nothing that says that they have to be on one side or the other – BMW builds engines with the timing drive at the rear for example. I suppose the crank pulley would usually be on the front.

  • I.e. no matter what, the "rear" is always the side with the shaft that ultimately drives the wheels? – Jason C Aug 14 '16 at 20:25
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    Yes. So, for example on the original SAAB 900 the front of the engine faced the rear of the car and the rear was just behind the radiator. Changing the clutch was very easy. – dlu Aug 14 '16 at 20:27
  • tcmlink.com/pdf2/maintenancemanuals/x30014/x30014.pdf seems to confirm what you say about aircraft engines; it's from a Continental O-300 (from a Cessna 172, circa 1955, apparently the most popular aircraft in history according to Wikipedia), all the parts labelled "front" are on the propeller side (e.g. the "front main bearing" on pp. 4-5). Interesting. – Jason C Aug 14 '16 at 20:46
  • I was pretty sure about aircraft engines, but didn't think to use the Google. There just aren't enough pusher planes to make it the other way around. – dlu Aug 14 '16 at 20:49
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    On both piston and jet aircraft engines, the "front" is the part nearest to the front of the plane. But people still get confused and ask questions like "does this engine rotates clockwise when viewed from the front mean when you stand in front of the plane looking at the engine or when you sit in the pilot's seat looking forwards?" – alephzero Aug 14 '16 at 21:22

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