Friday, June 5, 2015

Setting The Valve Timing

<This post is part of the 'Winter Project 2014/15 - New Engine Rebuild' project>
I must admit that I was a little worried about this part of the project as valve timing always seemed to be a bit complicated, but like any other subject after a bit (actually a lot) of research and learning, it's nowhere near as tricky as I first thought. In fact, I'd even go as far as saying, it's a bit of a doddle when you know how. 

There have been times in the past when I have started a job by steaming in, getting stuck and then getting the manual out only to find that I'd made a critical mistake and had to do everything over again. Not this time though, this time I did things properly. I opened up Dave Vizard's Big Yellow Bible (well it's mine actually) and read the chapter on valve timing several times. I also watched a number of the many videos available on youtube on the subject before I even looked at the engine.

In this particular edition, I found
pages 336 and 337 particularly helpful.

From what I've read and watched it all boils down to measuring how far round the crank rotates between two critical points. The first is Top Dead Centre on piston 1 and the second is the point at which maximum lift is achieved on the inlet valve of piston 1. One thing that's important to know is which way to rotate the crank. All the info I'd read assumed you'd already know, but I didn't. From watching the video at the end of this post, of the starter motor driving the flywheel, I came up with a mantra to remember as I kept forgetting.

The crank rotates: "UP the FRONT, DOWN the BACK". That's up the front of the engine and down the back of the engine. Knowing this, it doesn't matter which end of the crank you are working with, it's always correct.

Stuff Needed
For this job, I had to buy a Dial Test Indicator (DTI). Well I say I had to buy one, I've actually wanted one for a while as they're very useful and irresistibly gadgety, so this seemed like the perfect excuse to get one. I paid about £17 for one from eBay that is accurate to 100th of a mm and it even came with a magnetic stand .

Next thing that was needed is a protractor to go on the nose of the crank to measure the rotation. I saw these for sale at a few Mini specialists and they were about £8-10. Not a huge amount I know, but for me it seemed too much for a plastic disk, so I decided to made my own instead. I searched for an an image of a protractor in the internet, printed it on some card and cut it out. Total price: zippo really, just the cost of the card and toner!

This gets mounted to the nose of the crank, behind a pulley and a piece of wire can be used as a pointer.

The last bit of stuff needed before I got started was to put the cam follower and pushrod in place for the inlet valve on piston one, this is the second hole of the piston closest to the timing end of the engine.

Finding TDC
The first job then is to find Top Dead Centre (TDC) on piston number 1, but because this is a 4 stroke engine though, there are two TDC points within the 4 strokes, so it's important to make sure it's the correct one. This is something else that just seemed to be assumed knowledge as it wasn't obvious in the guide I read. Anyway it needs to be the TDC just before the intake stroke and not the TDC just before the power stroke.

But how do you know the difference? This foxed me for a while also, but after a bit of experimentation it was pretty simple to tell. If you turn the crank (up the front, down the back) from a TDC point and the intake pushrod doesn't lift, it's the wrong TDC and the crank needs rotating round to the next TDC point.

Once the correct TDC point is roughly established by hand, the DTI tool can be magnetically mounted to the engine block so the plunger is sat on piston 1.

Ignore the extra pushrod, it does nothing.
Slowly advancing the crank past TDC, the pointer on the dial indicator will rise, hesitate then go back. Then reversing the crank, again the dial indicator will rise, hesitate and fall again. Now the trick is to find the exactly point in the centre of the hesitation in the dial indicator.

The way I do this is to choose a number on the dial indicator that is near to the hesitation point, then I rotated the crank past TDC and the needle responds by going up, hesitating and going back down again. I stop rotating when my chosen number on the dial indicator is reached and then make a note of the angle on the protractor.

Next I rotate the crank the other way, past TDC and again the needle on the dial indicator rises and falls and I stop it returns to my chosen number and again make a note of the new angle on the protractor. The centre point, or true TDC, is the number between these two angles.

A simple calculation will work it out. Add the two numbers together and divide by 2. Once this true TDC angle is known, the crank can moved around to that point on the protractor.

The last part is to bend the wire to point at 0 again on the protractor and now we are set. It sounds more complicated than it really is, but trust me, it's not.

Finding maximum lift of the inlet.
The DTI tool now needs to be moved and adjusted so the plunger is sat on top of the previously inserted inlet pushrod.

Now it's time to rotate the crank (up the front and down the back), I found it easier to attach a socket and wrench to the clutch end as it gave me more torque than trying to rotate the pulley. As the crank rotates, the valve will lift and the DTI will spin around until eventually it stops. The point at which it stops is when the valve is at maximum lift.

Again a little to-ing and fro-ing of the crank around this point and taking angle measurment, I can work out the max lift point as the cam follower rides over the lobe of the cam. I measured 121° just after hesitation and 93° just before. So 121 + 93 = 214. Dividing that by 2 gives 107°, meaning that there is 107° of crank rotation between TDC and the max lift on the inlet.

Comparing this value to the specs for my camshaft (which are in this post here), I can see that I should have an angle of 110° between TDC and max inlet lift, so I'm 3° out.

To deal with this discrepancy, I need to buy a 3° offset woodruff key. There are a selection of woodruff keys available ranging from 1° to 9° degrees. The only downside is that they are quite expensive for what they are, a standard non-offset woodruff key is less that £1, whereas the offset ones are nearer £10, but as I wan't the timing spot on, I'm going to have to get one.

Here's the whole process in a video...

As mentioned above, I was a little worried before I started that this would turn out to be a bit complicated, but I really feel pleased that I have managed to de-mystify what I thought was a very complex subject. Once I got the proper tools and got into it and practiced a few times, it turned out to be pretty straight forward:

  1. Find TDC on piston 1 
  2. Zero the protractor
  3. Advance the crank to max lift on the inlet
  4. Work out how many degrees the crank has rotated
  5. Compare to the spec for the camshaft
  6. Correct with an offset woodruff key if necessary

Obviously if I had splashed out and fitted an adjustable timing kit, then there would've been no need to get an offset woodruff key as the offset could simply be corrected by adjusting the timing gear, but hey-ho, you get what you pay for!

*** UPDATE * * Tuesday 16th June **
The new 3 degree offset woodruf key arrived today and after fitting it, went through the timing process again and got the magical 110° number I was after.

Here is a video of the details...


For updates, stick a 'Like' on Wayne's Mini Progress facebook page

No comments:

Post a Comment