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TDC on a Rover 4.6 V8


garrycol
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I need to set up a trigger wheel with respect to top dead centre on my Rover 4.6 V8. Now these engines do not have TDC marks on the pulley or a pointer.

I have the spark plug removed to observe the piston and the rocker cover removed so I can observe the valves on number 1 cylinder.

The problem is that with the piston at the top and the valves closed, the crankshaft can be moved through about 15 degrees with no observable movement in the piston. Likewise, the engine has never run after being built and whie it does turn easily by hand it is still a bit tight and initially rotates in a jerking fashion making it hard to determine when the piston is at the top.

Observing the piston movement up and noting where it stops moving while slowly rotating the crankshaft until the piston starts going down you can estimate the half way point which should be TDC but is a bit hit and miss. DEach time I do this I get a slightly different position for TDC.

Is there a way to determine exactly where TDC is rather than the hap hazard way described above.

Thanks

Garry

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Run it around clockwise until you stop feeling upward movement and make a mark somewhere convenient on the pulley. Then anticlockwise similarly and make another mark. Do this a few times and you should find it lands at the same two points every time. The point in between the two marks will be TDC.

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Run it around clockwise until you stop feeling upward movement and make a mark somewhere convenient on the pulley. Then anticlockwise similarly and make another mark. Do this a few times and you should find it lands at the same two points every time. The point in between the two marks will be TDC.

This is how I do it:

The tool is here: http://www.summitracing.com/int/search/part-type/piston-stops

But you can get them over here as well, or like me, make one, I just hollowed out an old plug and put a bolt with a nut either side of it through the plug.

Thanks for those comments - i did a google on buying a piston stop tool locally and nothing came up so I might have to make one myself. (until mentioned I had never heard of one of these before)

Cheers

Garry

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If you have a DTi and steel disc (ideally with degrees on it) you can find the exact TDC.

If you have the kit, then why not do it!

If you had a cable tie with graduated marks on you could then work out a set piston height each side of TDC and then you can work out exact TDC.

Hopefully this is making sense!

Steve

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Hopefully this is making sense!

Steve

Not really -

DTI ????

Kit - ????

The piston movement up an down about 15 degrees either side of TDC is not really measurable with anything other than scientific instruments so I am not sure that marks on a cable tie poked through the spark plug hole at a 45 degree angle will show much - and yes I tried it earlier so I know it does not work.

Thanks for the input.

Cheers

garry

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Not ideal for setting up a trigger wheel perhaps but good enough to get going is a 'field fix' method some may find of interest:

Oil poured down the plug hole will give enhance the movement of the piston and give a more accurate limit either way than a screwdriver or something down the plug hole. Even dial indicators are a pain because of the angle of the hole. Once you start playing with your map the TDC will just be a reference point.

The knock sensor will sort the timing out for you once it's running.

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Thanks for all the input and advice. This engine is going into an older vehicle and I do not have the 4.6 flywheel and crank sensor etc. As it is going to be controlled by Megasquirt 3, it was cheaper and simpler to go with the recommended components such as the pulley trigger wheel that go with MS3.

Of course, if I had thought about this when assembling the engine I would have done this when when putting the timing gear in and before the heads went on where it would have been obvious where the piston was at TDC and both exhaust valve lobes on the cam in the closed position.

cheers

garry

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No one locally sells the tools so I made my own.

20160331_181433_zpsieny1usr.jpg

Because of the shallow angle of the spark plug hole the piston comes up very close to TDC before touching the end of the tool but it should still be doable. I will set it up tomorrow after I have made up a pointer which will need to bolt off the timing cover near the Edis 8 sensor.

Cheers

Garry

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removing the head is best but a lot of faff. I've only ever done it with a dial gauge and head off but I reckon the following may work:

- take an old spark plug and drill a hole in the centre so a thin rod e.g. piece of coat hanger wire can slide freely

- fit an angle gauge to the crank - you can find pdfs to print off on the web

- take two points, before and after TDC where the rod is same distance - a dial gauge would be best and mark the angles

- TDC is midway between the two marked points

you wind the engine in one direction to avoid any backlash

as you already noticed, at TDC, engine can turn a few degrees without any piston movement hence why you need two points.

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Me? Im guessing piston stop is bolt on end of spark plug that stops piston?

I suppose there is small chance of leaving tiny indent in the piston that but at tdc crank can move a some degrees without piston movement, hence you need to take two positions. As explained here, scroll down page.

http://www.burtonpower.com/tuning-guides/tuning-guide-pages/camshaft-fitting-timing.html

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Using a stop you rotate one way, then the other to get your two marks, or am I missing something?

I'd have thought residual oil or assembly lube could easily effect a DTI reading, but fairly certain it wouldn't make a difference to a piston stop.

Oh, and if you make a mark in the piston with a piston stop, you DESERVE the engine to blow up :P

That said my old series ran for many thousands of miles with a small bolt shaped impression in one of the pistons -it had fallen out of the carburettor butterfly.... No issues, and still going strong as far as I know :)

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I smashed the middle out of a sparkplug (wear goggles) then put a piece of fuel line up the middle with a round head bolt through the middle (m5 I think) then used continuity beep on multimeter to tell me when the piston touched. Did it in both directions and tdc halfway between.

Andy

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Using a stop you rotate one way, then the other to get your two marks, or am I missing something

Yes. Backlash.

There is tiny clearance at both ends of con rod as well as crank bearings

Turn it one way the clearance us taken up, turn it in reverse there will be tiny gap that needs to be taken up so esults in no con rod movement. Is it significant? Dunno, my guess is maybe 1 or 2 degrees on new engine, depends on how accurately you want to set the timing.

It is a faff but is for a reason.

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But the backlash will be there in both directions, surely?

For example. Take left hand and touch end of thumb with end of first finger to make a circle.

Take first finger from right hand and put it in the circle.

Now move right finger in a circle clockwise.

At all times your right finger is pulling on your left hand.

Now move right finger anti clockwise. Unless you have fat and stumpy fingers, your right finger moves across the gap before it touches your left hand.

This is the backlash, albeit on a much bigger scale, obviously dont do this in public else youll get strange looks.

There will be tiny gaps in thr places mentioned (needed for lubrication) so the reverse movement will need them to be taken up before the piston moves.

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I understand what backlash is, and appreciate that when you turn back around to go the other way there will be a flat spot.

However, when you go clockwise, hit the stop, all backlash will be taken up.

Then, when you go anti-clockwise, hit the stop, all backlash will also be taken up.

So.... it leads to perfect symmetry if you ask me.

I could appreciate in running, that perhaps the oil film in the big end bearings may not be 100% distributed evenly, but then we are talking fractions of thou, I would think? Manual lists it as .6 through to 2.2 thou.

As you are hitting a stop, of course these clearances are closed up.

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