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MG MGA - finding TDC with a degree wheel

The cylinder head and timing chain cover are removed and a degree wheel is on the crankshaft. A dial indicator is reading #1 piston position, set to read a little below TDC. Turn the crankshaft clockwise to contact dial indicator on the compression stroke, set dial to zero, mark the location of the pointer on the degree wheel, and then turn crank anti-clockwise. Now my question -- do I turn the crankshaft anti-clockwise to get the second reading while piston is again on the compression stroke? Doing that puts the two pointer positions about 30 degrees apart on the degree wheel, does that seem about right? I understand that splitting the difference between the two marks on the degree wheel is true TDC.

Thanks, George
G Goeppner

If you turn it a/c you end up on the power stroke, but that's immaterial.
As long as you have the same depth reading (just starting to lift the indicator) then you split the degree reading for TDC.
It doesnt matter whether you are 30 deg apart or 45 or whatever. Just don't make it too close or you will lose accuracy.
Art Pearse

This only works on engines where the cylinders are central over the crankshaft.
Art Pearse

George. The piston does not care what stroke it is on. The cam controls what is happening in the four stroke cycle, not the crankshaft.

I prefer to turn the crankshaft in a clockwise (viewed from the front) direction because there is no chance of the crankshaft pulley bolt coming loose--you are turning it in the direction to tighten it. Also, when degreeing in a cam, rotating clockwise, then anti-clockwise can induce some possible minor miscalculation due to the timing chain being slightly flexible.

To find true top dead center, mount your dial indicator pointer downwards over the cylinder. Bring the piston up near the top of its travel and move the dial indicator downwards until it touches the piston. Rotate the piston until the dial caliper shows its highest reading and zero the movable scale on the indicator. Rotate the piston downwards, then bring it upwards until the dial indicator reads -.001". Mark your degree wheel in line with whatever you are using for a pointer. Rotate the piston, turning the crankshaft in the same direction, until you again read -.001". (This assumes that you set the scale to zero properly and, if that is done, you will see the needle read -.001", move to the zero mark, then drop again to the -.001" reading.) When you just hit the -.001" reading again, make a second mark on your degree wheel. True top dead center is midway between the two marks, after the first mark and before the second mark.

Les Bengtson

I hate to disagree with you Les, but as you know, at exactly TDC, there is no vertical motion as you rotate the crank, so this is a very insensitive place to be measuring vertical motion. Surely it is better to back off to sat +/- .100" or even .200" and split the readings from there? Having said that I confess I have never dialed TDC, I used a piston stop, which stops the piston about these distances from TDC.
There is .001 - .002 play at the big end, so trying to measure at the +/- .001 is not a good idea, IMO.

Art Pearse

And I'll disagree with Art and back up Les!

Any clearance at the big end is filled with oil and is immaterial for TDC marking. The drag of the piston at TDC is minimal - you are more likely to get significant drag and thus squeeze out oil in the BE bearing the further you go from TDC.

One thou either side of TDC is fine and is normally 10 degrees or so on the dial.

ALWAYS go clockwise - NEVER turn the crank back unless you go at least 90 degrees and then go clockwise again to the mark. The slack in the timing chain will screw up your readings!
Chris at Octarine Services

OK, forget the backlash issue for a moment. Your accuracy in using a dial is say 1 thou.
Look at the table below which is the piston drop below TDC vs the crank angle for a 1.75" throw.
If you are dialing .001, the sensitivity is 0.8 deg between 1 and 2 thou and 1.9 deg between 0 and 1 thou.
If you dial in at 0.1", the sensitivity is only 0.1 deg per thou.
I rest my case

Deg dial
0.00 0.0000
1.00 0.0003
1.94 0.0010
2.74 0.0020

19.46 0.1000
19.56 0.1010
Art Pearse

Also, the timing chain doesn't enter into it for finding TDC.
And George doesn't have it installed.
Art Pearse

In fact, you could simply use my table by dialing down from the piston high spot. Eg. dial down 0.100" and you are 19.5 deg BTDC. No need to do both sides at all. But don't do it at .001 down.

Art Pearse

Art. Never done this "in real life" have you? Your theories remind me of Alcibiades when he landed on Sicily and was told his invasion force had to leave because, "You are not allowed to be armed in Sicily". I imagine he was amused by the official as I am by your responses here. You demonstrate what the immortal Homer Lea called "The Valor of Their Ignorance".

The piston will "dwell", for several degrees of arc, at both the top and the bottom of its travel. Thus, things like .100" below the height of the block have very little meaning. First, the piston will not, unless the block had been "decked" ever rise to the top of the block. Even if it should, the dwell would be some ten degrees, perhaps more. All of this makes your calculations less than accurate. The piston has no effective movement, over several degrees of arc, at both the top and the bottom of its movement.

To proclaim some degree of accuracy, such as .100" equals some degree of arc, such as 19.5 degrees only works if you have accurately determine the true Top Dead Center in the manner that I have described and Chris has confirmed. Even then, using your method, I would not be sure that the true top dead center--of the crankshaft, had been determined.

Remember, we are, here, using an indirect measurement--we are attempting to determine the top, dead center of the crankshaft by measuring the piston travel--which is articulated through the lower end of the piston rod and the upper end of the piston rod.

George, if he uses my method, as confirmed by Chis, can truly determine the true Top Dead Center of his crankshaft.

The immortal Homer Lea referred to people who "In the Valor of their Ignorance" made well meaning mistakes. I would suggest that you have made a splendid argument for a mathematical model that does not adequately model what is happening here.

Your entire argument demonstrates that you have never put a dial indicator on a piston and, factually, determined the actual top dead center.

If you have the opportunity to do so, please take the chance. I have done so and found it to be a worthwhile experience.

We all, you, me, and everyone else, are, at best, still students.All I have learned, over a number of years, is how much more I have to learn.

Les :)
Les Bengtson

Les, despite your literary credentials, you have not read what I said.
I never referred to the block deck as the point of measurement.
And I am not so ignorant of engineering issues as you suggest.
I do not doubt your expertise; I only offered what I think is an improvement to the method, which you have failed to comprehend.
You can find the highest travel of the piston using the indicator and either note the reading or zero the scale. At this point it does not matter if the crank is floating a degree or three.
Turning the crank until the piston drops by 0.100" below the highest point will in fact give you 19.5 deg BTDC, and the float you mention becomes insignificant, geometrically.
It is simple geometry; please tell me how I have mis-calculated! I could have picked an even 20 deg and a different movement.
It is also irrelevant that I have not dialed a piston before. (I used a piston stop) I am familiar with, and have used such an instrument otherwise.
The whole point of my post is that the float in crank angle at TDC is why you should back off from that area during measurement.
Using a reductio ad absurdum argument, it would be more accurate still to dial in .0005" either side. Why stop at .001?
Art Pearse

Thought you guys maybe interested in these photos of "degreeing the cam" on my coupe (MGB 5 brg engine) this week (with the head off).
My method to get TDC - bring No. 1 piston to best guess of TDC - set degree wheel to zero on the pointer - set up dial gauge on piston top (crankshaft centreline) and zero - turn crank clockwise until piston displacement is 0.100" record reading on degree wheel (say 20 deg) - turn crank counterclockwise until you you get piston displacement 0.100", record reading on the degree wheel (say 5 deg) - then do the calc 20-5 = 15, divide by 2 gives 7.5 degree - adjust the degree wheel TDC mark by 7.5 deg. This is true TDC (check again to make sure you have equal deflections each side of TDC )
Hope this is clear.

Mike Ellsmore

This is a photo of me degreeing the cam after I had found true TDC. My cam (45-75,75-45) requires 0.114" lift on No 1 cylinder inlet valve cam follower at TDC on overlap (info found on your cam card). Overlap is where the exhaust valve is closing and the inlet valve starts to open. To measure the lift set up the dial gauge on No 1 inlet valve push rod and zero when piston is at BDC, bring to TDC and note reading - in my case it was 0.125". To get back to 0.114" slacken the adjustable cam sprocket set screws (remove one completely) and then tap the sprocket back gently to get desired reading. Tighten screws and check again (make slight adjustment if necessary).

Mike Ellsmore

Photo of the adjustable cam sprocket.

Mike Ellsmore

Mike, good method. Looks like you agree with me that .100 travel is better than trying for .001.
The throw on the MGB engine is also 1.75", so you could also use my table.
At some point you have to transfer the degree wheel tdc to the crank pulley and its pointer?
Art Pearse

Worked out well for me I think. I used the "clockwise" "anti clockwise" approach, with dial indicator on the #1 piston. TDC ended up about 1mm from the TDC mark on the pulley, this with a new cam, pistons, late MGA timing cover with rubber seal, and MGB harmonic balancer. Spot-on I'd say...thanks to all for the stimulating discussions.

G Goeppner

I have rebuilt 100s of these engines - whether you use 1 thou, 100 thou or 1000 thou is immaterial as TDC sits in the middle whatever measurement you use. As long as you are either side of the dead dwell at TDC it will work and yes I often use .5 of a thou.
Chris at Octarine Services

Chris, of course it "will work". You make readings, split the diff and make your mark. Job done. My whole point was that you can only measure to say +/- 1 thou on the gauge and the same the other side. Maybe you can do a bit better, but whatever, the measurement is not perfect and the degree errors per thousandth piston movement are much greater near to TDC. Do you not agree? If you use .100, the error is only 0.1 deg per thou. So why not split a bigger difference? It is simple geometry.
Art Pearse

I am with Chris on this - it doesn't matter what displacement you use as long as you are past the dwell of the crank on TDC (10-12 degrees ), the accuracy of the dial gauge and the pointer/protractor is the same regardless of the displacement you use. I use 0.100" as is a full sweep of my dial gauge. Make sure the pointer is firmly attached - my pointer is the thickness of 1 degree!
Yes Art, you have to transfer the adjustment figure to the degree wheel - just loosen the wheel and move the TDC mark away from the pointer by the adjustment amount and tighten the wheel - making sure not to move crank. You then recheck where TDC is - in the example above you would have 12.5 deg both sides of TDC with 0.100" displacement clockwise and anti-clockwise. Check and recheck is the trick.
Mike Ellsmore

The real test of precision of a measurement is to repeat it several times, recording the different results.

I would suggest that dialing in TDC using a large displacement leads to better repeatability than dialing in using a minute displacement, for the reasons I have stated.

Perhaps either Les or Chris could do this test, in the name of science.

A 1 thou error when measuring .100 displacement leads to a +/- 0.1 degree error ; the same error when measuring .001 displacement leads to a crank position anywhere from TDC to 2.7 degrees away.

Please do the trig yourselves if you don't agree!
Art Pearse

Let the experts give some advice:
Gene Gillam

Thanks Gene.
I quote from ref 2 and again rest my case!

"NOTE: The exact travel of .100-inch below T.D.C. is not important. Any check point between .100 and .500 will give good results, as long as you check each side of T.D.C. equidistantly."

Ref 1 uses a piston stop which would typically stop in the range mentioned.
Art Pearse

I can easily read a dial gauge down to a tenth of a thou (full scale deflection is only 15 thou) and more to the point I can turn a crank smoothly near TDC so I land exactly on a thou or half thou mark on the dial - the pointer is the same width as the marks so I guess I am getting better than a tenth accuracy.

I guess the reason Isky & Crane use .200 is that it is safer to use with a positive stop than a small distance off TDC as the mechanical advantage obtained near TDC could result in damage to the piston.

It is that same mechanical advantage that makes setting near TDC easier and thus more consistently repeatable.
Chris at Octarine Services

This thread was discussed between 15/08/2013 and 26/08/2013

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