Checking TDC

ISTR you can get some sort of gauge you stick down plug bores.

A dial gauge with a plug adaptor. Standard way of measuring TDC for timing a two stroke... or at least it was 20+ years ago.

Whether this would easily access a v8 head is another question.

I've got a dial gauge. Where would one get a plug adaptor?

Yes - the plug is at quite an angle to the piston crown. Presumably works best with a vertical plug?

I'd try a motorcycle repair place- this is (or was in my motorcycling days) a standard bit of kit.

I did make something from an old spark plug to get one of the old CZ/Jawa or MZ engines going once. Just break away all the ceramic and the little electrode and I made a plunger from a bit of brass. The dial gauge was held in a clamp to the head fins and its pin rested on the plunger.

You should still be able to see TDC but any reading of distance from TDC would be wrong.

Brian

In message , "Dave Plowman (News)" writes

The old fashioned way was to take an old spark plug, remove everything except the metal housing, and smear water/washing up liquid (or a child's bubble mix) across the top, and use to replace one of the plugs. Turn the engine, and when the bubble is at maximum size, the piston is at the top of the stroke.

The message from snipped-for-privacy@italiancar.co.uk (SteveH) contains these words:

Called a pencil!

that is a nice way!!

there is no need for any real complication though, no dial gauges, but you do need a degree disk attached to the front pulley and a bolt about 2 inches long welded or screwed into an old plug body. put the degree disk on with a pointer roughly showing tdc, turn the engine back from tdc till you can screw the stop plug in, then turn the engine forward till the bolt in the stop plug just touches the piston crown, note the exact degree shown, turn the engine most of a turn BACKWARDS till the stop again touches the piston crown. you now need to work out the mid point between your degree readings to arrive at your tdc mark. remove the stop plug, turn the engine to your calculated tdc point and mark it or bend your original tdc plate till it is accurate, then do a double check.

on v8s I always used to advance them a lot from standard anyway, if you are racing them a static of 36 degrees works really well

mrcheerful

Hmmm, why do I suddenly have the urge to take the plugs out, squirt a fairy liquid mix in the holes, and turn the engine over on the starter?

:-))

Steve

Because you're a big kid, like me.

Remember the 'Secret Life of Machines'?

They were explaining how an engine worked and had a rail of injectors rigged up off the engine spraying bursts of petrol past a flame. A spectacular sequential flamethrower. Lovely.

You can download the whole TV series, BTW.

"shazzbat" wrote in news:edeeag$qv7$ snipped-for-privacy@inews.gazeta.pl:

A bubble blowing machine !!! Luverly :-)

Bit of welding rod shoved down the spark plug holes.

Watch it move as you raise the piston and mark the pulley when it stops moving. Continue rotating crankshaft and note when it starts moving again. TDC is the midpoint between the two marks.

Dave Plowman (News) wrote: Try with a piece of stiff wire. Watch it VERY carefully as you rotate the crank and it will give you a close answer. I got mine to within 1 degree doing this.

Becuase the spark plug isn't vertical this would be a pretty inexact way. It would need something with no sideways movement.

Providing the tip doesnt move on the piston surface this would be prefectly ok to use. Use a pencil with a rubber end to reduce slippage and repeat it a few times to be sure.

I'm really not convinced. At the angle involved any slight sideways movement where the 'plunger' touches the piston would translate into several degrees.

It's not easy it's true, but it's not impossible either. In the case of most engines the spark plug hole is large enough for you to angle your bit of rod to get it 'almost' in line with the bore. And making the measurement several times will allow you to be sure you haven't made a mistake. You can usually feel if the rod has slipped and start again.

Although admittedly I haven't had occasion to do this for twenty years or more. :-))

Steve

It doesnt really need to be inline with the bores to be honest, though it does help it not to slip. You will feel it slip providing you dont put excessive force on it or go in ham-fisted. As mentioned, multiple measurements and careful not to touch the pencil on the side of the plug hole if possible and you'll end up with a very good degree of accuracy. Otherwise maybe a laser range finder? You can buy them in B&Q but they are pricey. Aim them down the plug hole and keep it steady as you rotate the crank?

Ultrasound maybe - taking the earliest rebound signal - but not infrared or laser. These latter two require a flat surface *perpendicular* to the incident ray to bounce back to the receiver sensor.

That's why laser warfare in space is fraught with danger: either the object with mirror protection bounces the ray away or the source gets itself destroyed on the rebound, if the mirror happens to be orthogonal to the powerful beam..

Ultrasound wont work, the entry hole is too small and echo insider the chamber will confuse it. Laser (infrared or visible wavelength) is perfectly usable in this situation, using phase difference calculation

- so long as the incident spot can be seen the calculations are totally reliable (quite rare you get something that laser wont be visible on) There is no need to have a perfectly flat surface or perpendicular. The very fact that you can see the laser spot from a laser pointer on the a wall when you shine it at the wall shows that *some* light is reflected directly back at the observer, due to the roughness of the surface. This is all the sensor needs to calculate. Perfectly smooth would be aweful for measurement unless you could get the detector bang online with the incident spot.

Laser warefare in space? What planet are you on? We dont need high powered lasers - using a visible wavelength, low power laser and modulating its amplitude with a known frequency you can determine the exact distance of the dot it projects onto an object providing the dot can be "seen" by the phototransistor (eye if you will). This technology is currently for sale in your local hardware store, works perfectly without mirrors or intergalactic space juice.