Top Dead Centre

[trw99]

Blimey!

I am not an engineer in any sense of the word. (I have often noted that Elan owners do have some sort of engineering background, however!). I was swapping out my dizzy cap and leads and found that the engine was not at all happy thereafter, such that I put back the old leads and cap. Previously the engine had been running fine.

So I did what I'm sure we all do, use the search function here and I find I likely need to find TDC and re-set the timing. I have never done this myself before. Well, a pint of beer, packet of crisps and a packet of nuts later, my head is spinning. And not from the beer.

Type TDC into the search box and you'll understand. So many varying opinions and ways and views and differences of opinion!

I did find one reasonably simple (and therefore most suited to my equally simple mind!) solution. I replicate it below, fully in the knowledge that a, you will all think me simple, b, you have a better method to find TDC, c, that I have added to the confusion to be gained from anyone else searching on how to find TDC and d, I may have set the hare running - again!

If I remember correctly this came from the much-missed Steve Waterworth:

"Another trick to find top dead center.

1) Turn engine so cyl 1 is well before TDC
2) Inset a stick in cyl 1 spark plug, lay a straight edge across the cam cover, mark the stick even with the straight edge.
3) Now also mark the pully.
4) Rotate the crank, the stick will rise as it approaches TDC. Keep turning till it goes over TDC and starts coming down. Stop when the mark on the stick aligns with the straight edge again. You are now the same number of degrees AFTER TDC as you were BEFORE TDC.
5) Mark your pully again.
6) Half way between these two marks is TDC.

The problem with measuring with a gauge or ruler is dealing with Dwell. The piston will remain at TDC for a few degrees. The measurement trick eliminates the dwell issue. My crank pulley was spot on after this process."

Tim

[JohnMorin]

Hi Tim

I'm a bit confused as to why you needed to be so exact with TDC when I assume you are installing the distributor. As the ignition timing will not be TDC you will probably be using the marks on the front cover to set it, the TDC mark on the front cover should be sufficient for distributor installation.

I have only used exact TDC, found using a dial gauge, to do camshaft timing.

Can you explain your process please.

Thanks

John

[alanr]

Your method of detemining TDC is quite adequate if a little crude. However I think you are thinking far too much on what is involved in just changing a dizzy cap and leads!...I think you just have the wrong dizzy cap...or at least a different cap to the one you took off!
I suspect that the dizzy cap you have bought is not the same as the one that you previously had when your car was running fine. It probably looks the same on a quick exterior glance but is actually different inside.

Look inside the new dizzy cap and check that the 4 contact segments are in exactly the same place, as your old dizzy cap, in relation to the position of the cap as when it sits on the distributor base as your old distributor.
There are variations of the same style of dizzy cap that have the rotor contact segments in a slightly differnet position. This will affect your ignition timing and I believe is what your are experiencing.

Alan.

[john.p.clegg]

I agree completely with Steve, even aiming to get the marks approx. 90 degrees before TDC and 90 degrees after TDC , this is where there is the most piston travel per degree and will get your TDC setting more accurately than any other method.

John

[Craven]

Dip stick method is fine, make sure you are on the compression stroke for N0 1, but use only the UP position Max as your timing point only, won’t mean much to you but with these pistons the dudgeon pin is offset so the down stroke of the piston and up stroke are not symmetrical around TDC.

[Andy8421]

I have just been through this hoopla as part of my attempt to figure out what cams I had.

My dial gauge only has a limited travel (10mm) so even with careful trimming of the brazing rod I had attached to the dial gauge to go down the piston hole, I could only measure 10 or 15 degrees either side of TDC to average before the piston went down too far.

I estimated TDC using the dial gauge without averaging points either side - just to get the protractor in the right ball park. I found that after having a few goes at it, it was relatively easy to anticipate the middle of the 'dead band' and establish where TDC was. I zeroed the protractor indicator (bent my bit of sharpened welding rod), and went through the process of measuring either side of TDC, then averaging to find 'true' TDC.

My initial attempt at finding TDC was within a degree of having gone through the averaging process. Given the inaccuracies of the whole song and dance, it would be fair to say that the two methods gave pretty much the same result, and there was not much benefit from averaging two points vs simply estimating the point of maximum piston travel.

It may be that it was much easier to do this as I had the engine on a stand, I could stand right in front of it and could keep turning the engine over until I felt I had it right. Possibly much harder to do in the car.

The two point averaging process was definitely helpful when trying to establish MOP of the cams. The travel of the follower is much less than the crank, and given the 2:1 reduction from the crankshaft, I found that trying to estimate MOP vs using 2 points and averaging could differ by up to 5 degrees or so.

[Andy8421]

Dip stick method is fine, make sure you are on the compression stroke for N0 1, but use only the UP position Max as your timing point only, won’t mean much to you but with these pistons the dudgeon pin is offset so the down stroke of the piston and up stroke are not symmetrical around TDC.

This got me thinking.

I believe you are correct, TDC will be when the little end, big end and crank main bearing are in line. If the gudgeon pin is offset, then TDC will occur when the crank isn't in line with the bore. As you point out, the up and down stroke won't be symmetrical.

Not sure that is significant for small deviations either side of TDC if that is being averaged to find 'true TDC', and I am afraid my maths is so rusty I am not up to figuring it out any more.

[512BB]

I am at a total lossTim, as to why you are trying to find TDC when setting the ignition timing. I have written about this and how to do it several times here, but it seems to get lost, goes over peoples heads or just dismissed. The figure you want is 12 degrees BTDC on the pulley, on the compression stroke for your BV engine.

I have also found a way to static time so accurately that I do not even bother to get a strobe out any longer. If you need some help or guidance getting it right, you are welcome to pop round. Just don't drive off in the wrong car!

Leslie

[alanr]

I really don't understand this thread at all...
The OP is only changing a distributor cap?...Providing he has the correct replacement cap, or the same cap as his old one, he doesn't need to mess around finding TDC or anything like that at all. With the correct distributror cap it is a 10minute job!

Alan.

[Craven]

The problem with measuring with a gauge or ruler is dealing with Dwell. The piston will remain at TDC for a few degrees. The measurement trick eliminates the dwell issue. My crank pulley was spot on after this process."

I think Tim’s post is just a critique on how us old hands approach a basic requirement in setting up timing be it ignition or valve.

[trw99]

Mmm, not a critique as such, just a note to say that for my simple mind doing the research became a little boggling!

Anyway, I appreciate the replies elicited and thank you. I do not actually need to find TDC and re-time my engine, but I was looking into what I would have to do in the event. I have been swapping out the usual ignition items, since I did not know what age they were for sure. For example, the dizzy cap still has Lucas imprinted on it and is a dark brown in colour!

Tim

[john.p.clegg]

Leslie , Alan.

I think Tim posted...

" So I did what I'm sure we all do, use the search function here and I find I likely need to find TDC and re-set the timing. I have never done this myself before. Well, a pint of beer, packet of crisps and a packet of nuts later, my head is spinning. And not from the beer.

Type TDC into the search box and you'll understand. So many varying opinions and ways and views and differences of opinion! "

John

[bitsobrits]

For example, the dizzy cap still has Lucas imprinted on it and is a dark brown in colour!
Tim

It's not the age, it's the miles, as the old saying goes. An unbroken ancient cap that doesn't have significant arc (burning) marks at the spark transfer points is likely just fine. Modern replacement caps and rotors have a pretty good probability of being inferior to the ancient originals as there are a lot of parts being made these days that aren't made to the OEM specification. So don't throw the old cap away!

[512BB]

TRW99 wrote: 'I do not actually need to find TDC and re-time my engine, but I was looking into what I would have to do in the event. I have been swapping out the usual ignition items, since I did not know what age they were for sure.'

Well, you would certainly want to change the points and condenser if they were an unknown quantity, and for that the ignition timing and engine wants to be set at 12 degrees BTDC on the compression stroke. Nobody in their right mind would carry out that task with the dissy in situ, as it is a simple task to pop it out, do the necessary, and replace. I have found that to get the dwell to spec on my analyzer, the points need to be set to 13 thou., or the dwell angle is to low. I don't know what others have found, maybe my analyser is out.

Leslie

[Andy8421]

I have been swapping out the usual ignition items, since I did not know what age they were for sure. For example, the dizzy cap still has Lucas imprinted on it and is a dark brown in colour!

Tim

Tim,

There is an awful lot of rubbish out there. I went through a phase of having 'distributor cap of the week' as that's about how long they lasted. Rotor arms were another source of much hassle.

I now buy all of my ignition components from 'The distributor doctor' as I have found the quality of his parts comparable to OEM. When I spoke to him last, he mentioned that his competitors have started copying his signature red rotor arms with inferior products, so now he stamps his with 'DD'.