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MG MGA - TDC

How do you know if you're at TDC on the compression stroke vs TDC on the exhaust stroke, just by observing the #4 cyl rockers?

Thanks!

George
George G.

TDC on the exhaust stroke is when the exhaust valve closes and inlet valve just starts to open "the valves are rocking" as they say! TDC on the compression stroke is after the inlet valve has closed ( this is when the valves on no 1 cylinder are rocking) - turning the engine in clockwise direction.
Mike
Mike Ellsmore

The easy way to find TDC is to take out the #1 plug and put your finger in the hole while cranking the engine. When you feel pressure on your finger this is the compression stroke. You can then use your hand crank or wrench to move the engine to TDC. I put a screwdriver in the hole and watch for the point when the screwdriver starts going down. Move the engine backwards to find TDC.
JEFF BECKER

Depends why you want TDC. If you are timing, this is a bad way as there is no up/down motion at TDC and you will be several degrees off.
Art Pearse

TDC compression stroke on number 1, line up timing marks for TDC and look at the rotor arm to see if its pointing to 1 or 4 and if it points to number 4 turn it over 360 until the timing marks line up again
Andy Tilney

I have checked TDC on No 1 cylinder with a dial indicator on the top of the piston and with degree wheel on the crank. Then fitted the timing cover to check whether indicator is correct - it was spot on!
George, why do you want to know TDC on no 4? Timing or engine assembly?
Mike
Mike Ellsmore

Mike, I want TDC for #1 for timing.

George
George G.

Use a piston stop if you want to be accurate
Art Pearse

George, do you want it for camshaft timing or ignition timing? For camshaft timing its best to use a degree wheel, if it is for ignition use the valve rocking method or as others have suggested then use a dial-back timing light to set dynamically.
Mike
Mike Ellsmore

Jeff. I saw your hint using a screwdriver.
Just last week an old friend of mine with an early Morgan(triumph powered) used a screwdriver to check piston position. He got called away and on return he hit the starter and proceeded to destroy one piston and severely damage the valves on #1 cylinder.He had left the screwdriver in the plug hole.
A set of liners with new pistons is almost $500.00.
That's just the beginning of his costs for a moment of inatention.
Sandy
edith sanders

It is OK to use dial indicators (or screwdrivers) through the plug hole only if you have a vertical path through it to the piston and you are setting some position BTDC such as ign timing. The drop distance from high point can be calculated geometrically from the crank throw and rod length.
But you cannot locate true TDC this way.
And I don't think through the MGA plughole you can pass straight down vertically.
Art Pearse

Thanks all. The engine has been rebuilt but not started. I was doing this to check the orientation of the distributor drive gear (I was off by one tooth), and to install the distributor with the rotor at the plug #1 position. The harmonic balancer crank pulley was slightly advanced to the timing cover pointer. All worked out well!

Thanks!
George
George G.

This thread was discussed between 27/06/2017 and 01/07/2017

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This thread is from the archive. The Live MG MGA BBS is active now.