Related to this question: What would happen if you tried to start an engine with the cam gears meshed

Why don't valvetrains use an idler gear between the cam gear to keep the cam gears in sync, to simplify timing? Through this one could either have a chain running to one camshaft and a gear keeping them in time or a chain over both gears with service then simplified such that you can just time the crank to one moving part instead of two.

I can see the concerns being more moving parts and therefore more places for wear to occur, and weight/inertia.

Would this setup work? What would be the drawbacks? I have seen setups with crank chained to one gear and that gear to the other, but not with a gear intermediate.

  • I think the main reason is because they (the 2 cam gears in a DOHC design) are usually quite close together. Where are you going to put that idler?
    – Zshoulders
    Commented May 11, 2017 at 20:10
  • I would imagine a V setup, the two cam gears close to each other and an idler set between and below them.
    – Ceshion
    Commented May 11, 2017 at 20:11
  • I saw a cam setup like this once. I forget what it was. Something European, 60's-70's, 4 cyl, RWD, DOHC. The timing chain ran between the crank, the distributor and a idler gear for the cams. My memory may not be 100% correct
    – rpmerf
    Commented May 11, 2017 at 20:31

2 Answers 2


Bottom line is: It's been done.

Here's one for a small block Chevrolet engine:

Edelbrock Accu-Drive Gear Drive on Summit Racing

Just as you describe, there's an idler gear (well, maybe two) which keeps things in time.

These gears are used mostly in racing applications. They aren't usually used in street applications for two reasons:

  • Cost - These cost anywhere from four to ten times the amount of a regular timing chain setup.
  • Noise - Due to the fact the gears are cut straight, these make a lot of noise.

The reason racers use them is because unlike a timing chain, there's no slack. Timing is kept in check very well. Also, as timing chains wear out, they stretch. This changes the timing just enough it can affect performance. When you are racing, especially bracket racing, dialing in your performance is a must. Using the gear set over a chain can provide you a little extra insurance to know things will be the same again and again.


Short answer: follow the money.

Long answer. Cutting gears and ensuring the mesh correctly is incredibly expensive. You've got to rough form, final form, heat treat then final form again. Then you have to emplacement the mating gears to extraordinary tolerance. Add extra bearings for the idler... never mind the real estate inside the engine for the idler and cover. Oops. Got to add rotational inertia into the conversation... and it's adverse impact on fuel economy.

Umm. Just no. What exactly are you getting for all this money? How much do you want to spend on your next car?

Why would anyone possibly do that?

  • It was mainly curiosity, but I did think of that it would make the engine easier to service if you only have to time two points vs three
    – Ceshion
    Commented May 11, 2017 at 20:18

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