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MG MGB Technical - how to determine top dead center ?

hi
79 mgb

how can one person, alone, determine top dead center ?
one can't really rotate the crank shaft and observe the motion of a probe inserted in the #1 spark plug hole.
even two people have trouble getting an accurate reading.
is there a trick ???
or, does one simply advance the timing until it pings and then back off a bit , and never know where top dead center is ?

john
john sutter

Line up the marker on the harmonic balancer and the pointer on the timing cover.

To actually set the timing, buy a timing light. Put some white paint on the pulley mark and the pointer, start the engine and see where you're at. Then turn the (previously loosened) distributor until you get the required amount of advance. Distributor can be quite loose. Don't forget to tighten it up again though...

Then drive the car, find that it pinks, and turn the distributor forwards until it doesn't!

Neil
Neil22

John You screw in the piston stop till it hits the piston a little before TDC. Mark the crank pulley or timing disc, then turn in the opposite direction till the piston hits the stop again and mark. Half way between the two marks is exactly TDC. Have all the spark plugs out and only turn by hand, you wont damage anything. When actually checking timing positions turn only in the direction of rotation. To do this you do need a screw in piston stop which can easily be made from an old spark plug body. If you need a picture lit me know.
Denis
Denis4

Hi John,

How I did it was to use a welding rod with two bends in it. The first bend was about three inches from one end at ~90 deg, the second bend about 4 inches from the first bend, again at 90 deg to form a hook..
Thread this rod down along side the engine and insert the hook into #1 spark plug hole. Then lying under the front of the car with your spanner, to turn the engine, and a marker pen, rotate the engine till the rod just starts to move. Mark this spot on the pulley. Now do the same on the other side of TDC and the true TDC is halfway between the two marks.

Herb
Herb Adler

I'm with Neil. I just did this a couple days ago while setting valve lash. Find the timing notch on the crankshaft pulley and make a chalk mark on the front of the pulley so you can see its location better. Move the crankshaft until the notch lines up with the bottom-most timing pointer. This puts pistons 1 and 4 at top dead center. One revolution of the crank puts them at top dead center again, but with the valves in different positions. For a lazy mechanic like me, this is the easy way to adjust valves. At TDC, four valves are in position for adjustment. One more revolution and the other four valves are ready to adjust.

If you don't happen to have a 1-3/8" deep socket to turn the crank, you can raise one of the rear wheels off the floor and use it to turn the engine. -G.
Glenn G

It depends how accurate you want to be. Adjusting valves or even setting timing, the pointer is close enough for some, but the OE timing marks are not accurate enough for things like dialing in a cam or even accurately knowing what your timing is. Once the actual TDC is found and marked it is there forever so its worthwhile. Over the years I have checked many engines and its rare to find one close, their usually 2-4* out.
A simple stop can be made from an old plug, the 3/8" stud is protruding 7/8". A few minutes work.

Denis

Denis4

The factory mark can also shift if the pulley is delaminating. Marking this for future use is pretty pointless, but it *will* tell you if the factory is right or not. But these days, with modern fuels in high compression engines it's more about setting it as high as it will go without pinking rather than the manufacturers figure, which you will be lucky if you can reach even with the 98/99 octane (UK) fuels available.
Paul Hunt 2010

You don't need to be very accurate with crank position when checking valve clearance/lash as the valves are fully closed for about half a turn of the cam.

For timing, as John suggested at the start, and as Paul says, you're going to have to fiddle with it anyway. The markers are close enough to start that process off.

If you're dialling in a cam, you'll need a more accurate method ie cylinder head off and a dial test indicator. I agree that the markers on the pulley and cover are probably not spot-on, but I don't think that you're going to get within 2-4 degrees feeling for a piston fouling against a stop or looking at a bent bit of welding rod.

It's horses for courses I guess!

N

Neil22

A piston stop is as accurate as it gets and is quick and easy. Why would you want to pull a perfectly good head is you can do the job well without. The secret is to screw the stop in so the piston hits well BTDC (20* is good) Denis
Denis4

I use Deins4's method. It gets you bang-on centered.

When you paint the timing mark on your pulley - extend the
painted mark fully across the front face of the harmonic
balancer. This way if it de-laminates and slips, you'll be
able to see it.
Daniel Wong

Neil,
Cam manufacturers believe the positive stop method is accurate, in fact they sell a kit for the purpose.
See http://cranecams.com/?show=article&id=3
Regards,
David
David Overington

David,
Fair point...!
Neil
Neil22

This thread was discussed between 09/03/2010 and 12/03/2010

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