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1974 1/2 MG- Top dead Centre and overdrive

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J

John H.

Freshman Member
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Good day,

First time on this forum and looking for some tech information.

Timing marks on the 1800 I am re-building are very hard to read and I am looking for an easy way to determine Top Dead Centre? Any ideas?

Also, I purchased a 1978 Overdrive with the control on the shifter to install during the re-build. I do not have a wiring diagram to hook this up and was looking for someone that may have one?

Thanks for all your assisitance
 
Hello John,
the usual way to determine TDC is to use a degree wheel and a dial indicator. Set the degree wheel on the crank and make up a pointer. Using the dial gauge to meausure a set distance down the bore either side of estimated TDC noting the degrees, then divide the wheel reading in half to get TDC.
If you are very careful this method can be adapted, using a physical object (say a large nut) and a straight edge across the bore and mark off the crank damper where the nut just touches the straight edge as you rotate the engine. Divide the two marks in half and that is TDC. Mark the engine accordingly.

The inportant (very) thing to ensure when wiring an Overdrive is that it must not be able to engage when reverse gear is selected. Usually there is a switch on the 3rd\4th gear selector which stops the Overdrive being engaged unless in those gears. (Top gear only on some Jaguars due to the engine torque being so large.)

Alec
thirsty.gif
 
Isn't there a notch on the flywheel and metal pointers to line up? Is all of that gone? The previous post is accurate but seems involved if you don't need to. I cleaned mine off and painted notch and pointers white for starters and worked from there. Also makes setting timing easier. If the first cylinder is at the top of it's stroke (wire coat hanger end inserted in sparkplug hole) and the valves are both closed (valve cover off so you can see them) you are roughtly at TDC then line up notch on flywheel with 1st pointer. Also...at that point if you pull the dizzy cap, the rotor should be pointing at the sparkplug wire location that goes to cylinder one. About 1-2 o'clock position when looking at top of dizzy cap.
 
Hello JTB,
yes it may seem involved, but if the marks are lost the method outlined is the only way to get an accurate mark. You only have to see how far the crank revolves at TDC to see how little movement there is at the piston, i.e. that is at the most inaccurate place to do a measurement.
By the way if your engine has offset gudgeon pins you are in for a harder time!

Alec
cheers.gif
 
Thank you for your help. I will check this. The problem is that the book I have the front cover in the book has the marks at the 6:00 o'clock position and there are three with one being longer. My cover has 5 marks on the cover at the 11:00 o'clock postion and all marks are the same size. I have done the piston #1 at top and valves closed with valves on #4 floating a bit. There is also a 1/4 on the flywheel, not sure what this represents for position.
John
 
Piman,
Guess I am so visual that I got lost in the description. Your description isn't bad...I just have to see it....kind of a learning defect, I guess.

John,
On mine, and I think on all of them, the pointer or mark furtherest to the right is TDC with the ones to the left being 5 degree increments before TDC. So if you set the timing initially (static) at 10 degrees before TDC you should be able to start the car then adjust timing from there with timing light (dynamic).
 
Hello JTB,

it is something that is so obvious if you do it but difficult to put into words.
If you think of the crank pin, at TDC it is at the top, so movement around that point is horizontal, so a relatively large rotational amount has very little vertical deflection, hence the inaccuracy of determining TDC by feeling or measuring the piston crown position even with a dial indicator.

Alec
cheers.gif
 
Hello JTB,

it is something that is so obvious if you do it but difficult to put into words.
If you think of the crank pin, at TDC it is at the top, so movement around that point is horizontal, so a relatively large rotational amount has very little vertical deflection, hence the inaccuracy of determining TDC by feeling or measuring the piston crown position even with a dial indicator.

Alec
cheers.gif
 
Hello JTB,

it is something that is so obvious if you do it but difficult to put into words.
If you think of the crank pin, at TDC it is at the top, so movement around that point is horizontal, so a relatively large rotational amount has very little vertical deflection, hence the inaccuracy of determining TDC by feeling or measuring the piston crown position even with a dial indicator.

Alec
cheers.gif
 
Piman...
Yeah...will just have to do it to understand. I know what you mean....unless you have the stinkin pictures to indicate what you mean so you don't have to do a page long glossary it is hard to be clear with it. Thanks for the info..it's printed and saved so I can have a look when I get my car back and am down there mucking with the timing. Putting in a Pertronix ignitor (and carrying my points for backup) so will need to set timing once more at least.
 
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