5 replies to this topic

[mister bridger]

Planning on fitting a 266 cam to my 998 A+ shortly. Is the dot to dot method sufficiently accurate or can someone point me in the direction of a better way? My Haynes manual isn't very helpful on this

[firefox]

This is off my re-build thread from cooperman i dont think he will want to type it again hope it helps
easily on and off the crank and cam when lightly oiled. This just makes the job so much easier.
Now bring pistons 1 & 4 to absolute top dead centre (TDC). At this point the woodruff key in the crank should be vertical. Now, without the chain on, slide the crank sprocket onto the crank and the cam sprocket onto the cam.
On the front face of the cam and crank sprockets you will see a small indentation or 'dot' as we'll call it. If you were to draw a straight line from the centre of the crank through the 'dot' on the crank sprocket then the line, if continued, should pass through the centre of the cam sprocket. Check this first. Now turn the cam sprocket and hence the cam until the 'dot' on the cam sprocket is also on that theoretical line. Thus the two dots are close together and on a straight line through crank and cam centres. Use a 12" straight edge to check this by eye.
The position you have, known as 'dot-to-dot' timing, is now correct.
All you now have to do is to take the sprockets off, fit the chain, and re-fit the sprockets in exactly the same position and the cam is timed to standard tolerances. Check with your steel rule again to be sure it's correct.
If you are using a single chain, this in now the time to fit the tensioner. If it's a duplex, or double, chain, don't fit the tensioner at all.

However, the manufacturing tolerances of crankshaft, both sprockets and the camshaft can all add up to some amount of cam timing inaccuracy. In fact I've seen as much as 9 degrees which will have some impact on the power output, but it will still run.

To eliminate this error you can time it more accurately using either a vernier sprocket of an offset woodruff key.
To do this you need a dial test indicator (DTI) and a 360 degree protractor. AndyMiniMad has posted a photo of how to mount the protractor and the initial position of the DTI to check the accurate position of TDC.
So, establish the actual absolute TDC by putting the DTI plunger onto the top of a piston and turning the crank until you get the greatest DTI reading. Now back the crank until the DTI reads 3 thou (0.003") before TDC and then turn the crank until it's 3 thou after TDC. The position halfway between these two positions is absolute TDC. Mark TDC on the block against the protractor and set the TDC mark, or zero degrees on the protractor to this mark.
Now put a push rod into no.1 intake push rod hole. It's the second push rod hole from the front of the block. Ensure the push rod is fully down.
Re-position the DTI so that the plunger sits into the cup at the top of the push rod.
Rotate the crank clockwise until the push rod has moved to its highest position as shown by the DTI reading. Zero the DTI at this point. Rotate the crank anti-clockwise until the DTI has gone down by around 0.010", then rotate it forwards until it reads 0.005" below the figure at max. lift. Note the number of degrees on the DTI. Now rotate the crank clockwise until the DTI reaches zero then goes down again to exactly 0.005". Note the number of degrees. Add half the difference between the two figures in degrees to the lower figure. The is your actual cam timing which with a Rover cam should be 110 degrees ATDC and with a Kent cam should be 106 degrees. It is necessary to ensure that the push rod goes as far down as it can at all times as the cam follower can stick and give a false reading.
I usually do this measurement 3 times and take the average to get best accuracy.
So long as the figure is within 2 degrees either way, I normally leave it 'as is' for a road engine. More and I correct it with an offset key.

The angle measured was the crankshaft angle, but it's corrected at the camshaft. The cam rotates at 1/2 engine speed, so the offset needed is 1/2 the error measured at the crank. If the cam full lift is reached at less than the desired crankshaft angle, the cam must be turned fractionally anti-clockwise, so the offset key step must be offset clockwise, i.e. by moving the cam sprocket clockwise, this has the effect of moving the cam lobes anti-clockwise since the chain pitches are the same. I hope this is easy to follow, it's not as difficult as it sounds. Vice-versa if the cam full lift is reached at a greater angle of crank rotation than is required.
Once timed in accurately it will not be necessary to fit a different offset ley when you change the timing chain later on. Only if the sprockets and/or cam are changed will it be necessary to re-time to this accuracy. Just dot-to-dot will be fine in future.

[mister bridger]

Thanks, that's what I was after.

[firefox]

Thanks, that's what I was after.

You are very welcome

[Cooperman]

I'm glad I didn't have to re-type all that! Thanks Firefox.

[firefox]

I'm glad I didn't have to re-type all that! Thanks Firefox.

Ha ha ha my words exactly, just a little payback for all the help you gave me