I was wondering if there is any machanism preventing a camshaft against turning during service. It may happen that, when you turn off your engine, the camshaft stops in a position where lobe opens a valve. Then a tension from spring occurs between rocker and camshaft. When you unscrew a tensioner, a timing chain becomes loose.

What prevents camshaft from being turned by the rocker, especially in 2v engine, where the second lobe doesn't push the other rocker in the same time and can't stop the possible movement.

I was looking for some kind of blockade near camshaft or the rockers.

What I've found was this small mechanism:

enter image description here

When attached to the rockers, it looks like that: enter image description here

But I couldn't find any information about it's function and I can't imagine how it works.

  • do you think it might depend on what engine you are talking about?
    – agentp
    Commented Feb 19, 2017 at 1:21
  • I don't see how this works to stop the cam from turning to install it you'd need to pull off the cap and punch out the dowel pins from what it looks like in the picture. Can you link to the product?
    – Ben
    Commented Feb 19, 2017 at 8:06

2 Answers 2


While I don't have an answer regarding blocking or stopping the cam, You can work around it without too much trouble.

With the chain still in place, You can mark on the crank and cam gear teeth and mark the chain around those teeth. This way you will know how many links should be between the marked parts of the pulley when you are done. Taking lots of pictures is always good.

Here is a hastily drawn example. you mark your teeth (the red) and mark the chain where it goes around that tooth (green) enter image description here You take the tensioner off, it shifts a link or two. Before you tighten everything back up, line it back up with both sets of marks. You might need a helper to hold a wrench on the cam pulley if its holding a valve open


What I have found with some OHC interference engines is that there are special dealer tools to hold the cam. They usually bolt to the head and have pins that slide into holes on the cam gear. I usually just build a tool that does the same thing when needed. If you post a pic of the front of your engine, we might be able to point out ways to attach such a device. I will try to find a pic showing what I'm talking about.

[Edit based on comments]

'is dismantling the tensioner, without blocking the timing gear first, safe?'

Probably, but this is a very engine specific question. I can't say there isn't an engine out there where the piston can hit a valve if the timing is 1 tooth off.

'is there a mechanism which allows me to do it if engine stops in the position described in my post?'

I'm not aware of a commercial tool that would allow you to lock the engine at any arbitrary point. Again, you could probably rig up something. Or you could rotate the engine to try and find a sweet spot where the forces are in balance.

  • Hey. I am 100% aware of how to prevent timing belt or timing gear from unintentional turning. Maybe I wasn't clear enough. in different words, my question would look like this: is dismantling the tensioner, without blocking the timing gear first, safe? And is there a mechanism which allows me to do it if engine stops in the position described in my post. I am asking it because dismantling the tensioner itself is quick and easy, but if there is a need of blocking the timing gear, then it's getting more time consuming. Commented Feb 17, 2017 at 20:41
  • @KrzyßCebuloix Why wouldn't you set cylinder 1 at TDC before you unscrew the tensioner? Then the valves would be closed on cylinder 1. If there aren't timing marks pull the plug and use a boroscope or a dowel if you can.
    – Ben
    Commented Feb 17, 2017 at 21:21
  • Thats' how I always do it, but it requires unveiling timing gear. That made me concerned what if i just remove the tensioner. And if it happened that in this particular type of engine i can't do it, because it can be dangerous, i still would like to know how this black mechanism works. I've found that it's called "stopper plate" so I assume it stopps something. But do you know what and how? Commented Feb 18, 2017 at 14:13

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