Checking TDC

We are talking about static measurement here. I don't think there is any change of phase until there is a Doppler effect.

You'd think wrong. Phase changes with distance, *Frequency* changes with Doppler.

I think you've lost sight of the fact I want an *accurate* indication of TDC - not approximate. The engine timing marks already give an approximate indication. Or maybe even an accurate one - I simply want to check them.

Take the head off. Tis easy then.

Ah, yes....you are quite right. This was learned 48+ years ago. A lapse of senility here. :)

I think a tight tolerance 'plunger' that screws into the plug 'ol will do the job ok. Seems Draper sell one for only a few quid - p/n DRA31491M

"Dave Plowman (News)" wrote in news: snipped-for-privacy@davenoise.co.uk:

Even on an offset head, a piece of coathanger wire will tell you, just turn the engine over manually till it's as high as it will go then turn the engine back the other way. Do this a handful of times and the reading off the coathanger will be the same once its stopped slipping around. Worked for me on a Triumph engine with an offset plug.

As i said, its perfectly accurate so long as you are careful - its as accurate as your vision is, so down to about 20 microns pencil movement assuming no slip ;) And the laser rangefinder would be more accurate than you could ever use.

However what are you wanting to get the timing so completely perfect for, im curious.

And added to that, how do you know what it *should* be? And under what conditions?

And how much difference do you think 1/4 of a degree makes? how about a whole degree? Could you tell?

I don't see how something which can slop around in a hole at an angle to the part it's touching can give an accurate reading - although it would be better if the piston was near the bottom of the stroke.

Maybe if you could guarantee its going to find exactly the same spot on the piston. But you couldn't.

I'd like an accuracy of within 1 degree, which shouldn't be hard to achieve with an accurately machined 'plunger'.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Coyoteboy" saying something like:

I would think the humble ball bearing fired from a passing Shuttle by a catapult-equipped astronaut would do sufficient damage at a closing speed of several thousand mph.

There's a difference between accurately finding the top of the stroke and measuring what the stroke is.

A pencil, rubber end first, won't slip appreciably once it finds it's comfort zone and should be easily good enough to see 1 degree movement.

So make it a mount which is an interference fit down the plug hole. Still easier than making your plunger.

Want to bet the timing marks are that good already? I know they are wide lines, so just line up the edges.

However I'm still curious as to how you know what it *should* be?

Because A) you are only working at a TINY TINY fraction of the stroke - rocking it + or - 10 degrees will see virtually no slop or movement, the pistons travel is sinusoidal so around TDC the piston position change relative to crank rotation is fairly minute....

You dont need to. You are looking at a relative measurement, not an absolute one. Even if there were 5% error in the range reading when it was 5 degrees off that is irrelevant because it will be the same either side of TDC and the TDC point will be the minimum range, assuming you dont let the measuring device slip. You arent looking for 5 degrees, in which case you would need absolute positional accuracy, you are looking for a zero crossing point which means no absolute measurement is needed to find the crossing to a very accurate degree. If i gave you the voltage output from a pressure sensor in the head, for example, but gave you none of the pressure/voltage characteristics of the sensor - you couldnt tell me at what point in the stroke the cyl pressure hit

True, but im still missing why this is required. Is the requirement for tuning your spark timing or valve timing? And what are you using to tune these, or are you just trying to hyper-accurately set up the stock ignition system?

:) I like the thought of that - back to basics at the height of technology :)

The message from "Coyoteboy" contains these words:

Trouble is the aiming is far from basic. To shift to a lower orbit you need to fire against the direction of orbit - to slow down. To rise you need to fire fowards. To move forwards you need do all sorts of complex things. Hitting a target in space isn't terribly simple even if they're not far apart. Once they're twenty miles apart it belongs to a class of problems known as "Sodding hard to do in your head on the fly".

"Dave Plowman (News)" wrote in news: snipped-for-privacy@davenoise.co.uk:

Granted I've only done it rebuilding bike engines, but I found something like a length of welding rod up to the job, even on a Triumph engine where the spark entered at a 45% angle. The weight of the rod keeps it in place, and a mark on the rod sighted against a convenient point of engine ensures it is acccurate. Rotating the piston backward and forward a few times ensures everything has settled and you can see the high point before the ros starts going down again. If you need more accuracy than this, some kind of micrometer gauge may be needed. Like this perhaps?

Guy King wrote: Once they're twenty miles apart it belongs to a class of

I find these things fascinating. Its amazing how much the human brain can actually do, especially at short range, - its part of what my research is trying to emulate - unfortunately i end up getting bogged down in 1s and 0s to really see the overall picture. Comments like yours re-inspire me!

The error cause by slop in the spark plug hole would be less with the piston near the bottom of the stroke.

But it can slop around in the plug hole? Nor would I be keen on turning the engine with it in place - I'd remove it and replace so it measures on the downstroke only. The angle would be such with the piston near the top you might break a pencil.

I've discovered they can be bought for only a few quid. Nor would I rely on a laser type - domestic or DIY ones simply ain't that accurate.

It's an often mentioned fact that the Rover V-8 marks are sometimes just plain wrong - nothing to do with how you read it which is easy enough.

Perhaps I'm just curious?

The rod or pencil moves up and then down. The zenith coincides with TDC. To obtain a repeatable result, all the variables should be kept constant, and that is where "careful not to touch the pencil on the side of the plug hole if possible" (re: previous post) is incorrect. The pencil tip albeit with rubber slides along and is being dragged along too. The usual concavity of the top of the piston does not help. The top portion of the pen held in the hand or grasped between the fingers would wobble, an amplified movement of its lower tip by virtue of the position of the fulcrum, i.e. the hand. Unless the pencil is *pressed against the side* of the cylindrical hole, and is allowed only to move up and down with no other 'degree of freedom', the zenith will be all over the place. Furthermore, how can one be absolutely certain that the hand/fingers, i.e. the fulcrum, hasn't moved a fraction in the execution? I can assume one hand is holding the pencil and the other hand is attempting to turn the pulley. It is a very sloppy technique.