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The Wikipedia entry for the Ford Cologne 4.0L SOHC describes the use of a "jackshaft":

It uses a jackshaft in place of a camshaft to drive a timing chain to each cylinder head. Three timing chains are used, one from the crank to the jackshaft, one in the front of the engine to drive the cam for the left bank, and one on the back of the engine to drive the cam for the right bank.

It isn't easy to follow the description or visualize what's going on.

What is a jackshaft, and what exactly is it doing here on this engine?

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@Zaid Thanks for contributing this knowledge! I just want to post my own description because it took me more than one read-through to understand your answer.

My understanding of the situation:

  1. Ford had invested in manufacturing the original 4.0 block several years ago.
  2. There was no reason to design and invest in a new block.
  3. There was already a hole in that block to put a camshaft into.
  4. Ford wanted to use overhead cams.
  5. In order to drive those new cams, Ford would need to move things around on the front of the block.
    • Ford didn't like that idea.
  6. Ford stuck a straight bar (jackshaft) through the original camshaft hole.
  7. Ford stuck sprockets on the front and rear of the jackshaft.
  8. The crankshaft drives the jackshaft just like it did with the original camshaft: with a short chain.
  9. The jackshaft powers one of the new cams with a short chain that is attached to the front.
  10. The jackshaft powers the other cam with another short chain attached in the rear.

A picture is worth 1000 words:

Drawing of the chains.

  • 3
    Them paint skillz! – JoErNanO Nov 5 '16 at 9:44
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Much of the information in this answer is adapted from this excellent forum post


In the generic sense

A jackshaft refers to an intermediate shaft that transmits rotation/power from one shaft to another. Per Wikipedia:

A jackshaft, also called a countershaft, is a common mechanical design component used to transfer or synchronize rotational force in a machine. A jackshaft is often just a short stub with supporting bearings on the ends and two pulleys, gears, or cranks attached to it. In general, a jackshaft is any shaft that is used as an intermediary transmitting power from a driving shaft to a driven shaft.


On the 4.0L Cologne SOHC

This particular engine is adapted from an overhead-valvetrain (OHV) design, where a single camshaft actuates pushrods that control valve timing.

OHV configuration

As overhead-camshafts make the single camshaft used for the pushrod-engine redundant, designers decided to replace it with a "blank" shaft (jackshaft) that would then transmit rotation from the crankshaft to the rotation via timing chains.

This parts diagram is very useful in describing this rather quirky configuration. The jackshaft is in the middle of the V; the timing chains (Left Hand Front Cassette, Right Hand Rear Cassette) connect it to the camshafts for each bank; the jackshaft is chain-driven off the crankshaft.

Cologne SOHC Engine Diagram

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Your explanation of the application in the Ford motor is spot on. To answer the original question of "what is a jackshaft?" in a broader context: a jackshaft is simply a shaft that transfers rotational movement from one path to another. Basically, it's a spacer between chain and/or belt drive circuits.

Here's a few pictures that illustrate the concept. enter image description here enter image description here enter image description here enter image description here

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