Hi All

I've been reading up on cam timing but have a few questions re fitting cam sprockets please as I am a complete novice. Info for DOHC is particularly what I'm interested in.

I am ok with what the crank/cam shaft relationship is - i.e. when the valves/open and close in relation to the stroke of the crank.

I'm also ok with the fact that for an overhead cam, the sprocket for the belt/chain must be aligned with the cam so that when the crank turns the valves are opened/closed at the correct time. It seems to be done normally with marks on crank pulley and sprockets against marks on the block - is this accurate ?

What I can't get my head around and haven't found in a search is how do you make sure the cam is in the right place (or should that be correct degree angle)before you fit the sprocket ?

Some websites suggest finding TDC on cylinder 1 and then measuring the valve lift but how do you do that with the head on ?

thanks in advance
Paul

Methods vary on many engines the timing belt or chain is fitted at half stroke not at TDC---- you have to exactly follow the manufacturers timing instuction for a particular engine or if using an aftermarket cam the cam manfacturers timing data.

quote:

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Originally posted by pdm
Some websites suggest finding TDC on cylinder 1 and then measuring the valve lift but how do you do that with the head on ?

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Thanks for both responses.

Bang on with that link - thanks Mike. Now to get one with some more reading.

The system for determining crank and cam positions varies from engine to engine.

The method of determining TDC by measuring when the piston is at its maximum travel is very inaccurate.

For an engine at top dead centre with 80mm stroke the piston movement for 1 deg of crank movement is about 0.007mm.

For this reason some manufacturers, notably Ford, use a special pin inserted in the side of the crankcase, the crank is rotated until the crankweb rests on the pin
Using a pin the difference in pin length for 1 deg crank angle is approx 0.6mm
So the system is very accurate.

Alternatively the crank pulley can be uniquely keyed to the crank at manufacture and the timing angles marked on the pulley.

Which ever way it is done the manufaturer has to provide way of determining the crank/ cylinder block relative positions. Once you have that position you can use angle turning gauges to put the crank in any known position.

On the other hand measuring the valve lift with a dial indicator gives an accurate indication of when the valve starts to lift.

For any engine the manufacturer will have provided a way of positioning the camshaft so that with the crank in its correct position the belt or chain drive can be fitted knowing the timing is correct.

If you use aftermarket cams then these are set using the cam manufacturers data and very often an angle gauge to determine the exact angles that, say inlet 1 commences opening.

So if the manufacturer helps you out with TDC, you can then do the cams as per the link.

One more question I have re the mg link - the middle gauge being used to find TDC - what is that resting on ? The piston itself ? In that case how does the engine maintain compression ? (or is that a really silly question).

thanks again for your help.

Ok sorry - ignore last question - it's through spark plug hole isn't it -doh.

Told you I was a novice.

Oh well I'm picking up a £10 practice engine next week - that'll probably make all this a lot clearer (unless its welded itself together inside !!).

Thanks everyone.

The main reason it is confusing is because everything is moving relative to everything else.

Once you have fixed the crank in a known position say no 1 TDC then all you have to do is relate the no1 cam(s) on each shaft to crankshaft positon.

If you are going to play around with an engine then that is the easiest way to learn. Make notes of each setting and then see if you can replicate them.
Use a simple protractor on the crank and a DTI on the valve and see if open and close angles are consistent with the manufacturers data.

If you are just re-fitting a standard cam just follow the workshop manual.

Be aware the bottom sprockets on some engines (particularly Rover K and some Vauxhalls) are notorious for having some angular float which make accurate timing difficult.

Rover K engines are timed at mid stroke --- easiest way to check this exactly is to measure down the cylinder bores to ensure the pistons in all four cylinders are at exactly equal height.

If using vernier cam sprockets I would first time the engine up as per standard --- and only after that accurately adjust the cam timing using the vernier settings.

[Edited on 25/9/09 by britishtrident]