MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - timing marks are off?

Hello all. I am trying to set my timing, and i noticed that my TDC mark is off. Here is waht i did. Removed #1 plug and inserted a straw, rotated the engine until the straw was at it's highest point. Looking underneath i notice that the TDC mark on the pully is about 1/2" or maybe 3-4 teeth to the left of the TDC mark. Can i correct this, or is it just a case of this being the new TDC pint and I should make my mark here?

Thanks

Ken
70 B
Ken Harris

Ken. The proper method of finding top dead center (TDC) is by using a dial indicator and a circular protractor.

You have a fairly long, in terms of rotational movement of the crankshaft/non-movement of the piston, dwell when the piston is at the top of its stroke, but the crankshaft is turning. About thirty-forty degrees of arc if I remember the last time I did it correctly. You have to set the dial indicator at zero when the piston is at TDC. Then, you attach your protractor to the crankshaft. Turn the engine until the dial indicator reads about .001" less than the zero mark on the face of the gauge. Zero the protractor while continuing to rotate the engine until you are over the top and the piston has again moved .001" down the bore. Note the position of the pointer on the protractor. The actual TDC position is one half way between zero position and the second position.

That is the only method of obtaining an exact top dead center value. Any other method will be a less accurate approximation, with some methods fully capable of demonstrating the type of inaccuracy you describe. Until you can accurately determine the true top dead center, I would not worry about any "problems" with your timing marks except if the inner and outer pieces are rotating in relationship to each other. In that case, the harmonic balancer needs to be replaced or rebuilt.

Les
Les Bengtson

Quick, dirty, accurate method of Les's correct one:
Bust the center out of a spark plug, braze or glue a bolt in, sticking out about 3/4"-1", where the electrodes used to be. Screw it in #1. Turn the engine gently 'til the piston contacts the stop, mark pulley at the pointer. Turn the engine backwards until the piston hits the stop again, mark pulley. TDC is exactly halfway between the two marks. Useful to put a new pointer somewhere you can see it - up on the timing cover - and mark for that too - but be sure to distinguish the two sets of marks!

As Les says, if the old TDC marks disagree with the new ones, either you did something wrong, or the damper is coming apart. That's way evil - fix it.
FRM
FR Millmore

thanks guys, but upon rechecking things it appears everything is normal.

Ken
Ken Harris

Alternately, you can ascertain true TDC without a
dial indicator and with minimum surgery by using
a piston stop, a degree wheel and two short
pieces of stiff wire.

Fabricate a piston stop from an old sparkplug.
Basically, you gut-out a sparkplug and epoxy
(or weld) a short metal rod through it's threaded
hex body. The metal rod should be long enough
to protrude well below the combustion chamber
and make good contact with the top of the piston
a few degrees below top dead center.

Remove all of the spark plugs in order to make
rotating the crankshaft easier, by hand.

Set #1 piston near to BTDC (bottom dead
center), and then insert the modified sparkplug
(piston stop) into the #1 sparkplug hole.

SLOWLY rotate the crankshaft by hand until you
can feel that the piston has made contact with
the piston stop. You do not want to dent the top
of the piston with the piston stop!

Attach a degree wheel to the harmonic balancer,
and attach a stiff wire onto the engine front plate
so that it points to "0" degrees (TDC) on the
degree wheel.

Now, SLOWLY rotate the crankshaft in the opposite
direction until the piston contacts the piston stop,
yet again.

Attach another wire to the engine front plate so that
it, too, points to "0" degrees (TDC) on the degree
wheel.

Now...rotate the crankshaft in the opposite
direction so that the two wires are simultaneously
pointing to equally opposing degrees.

Example: one wire will point to + 18º, and the other
wire will point to - 18º.....or... +22º and - 22º;
...or +14º and -14º...etc.

When the crankshaft is positioned so that the
wires are pointing to equally opposite degrees
- your crankshaft will now be at true TDC.

======

Tip: Paint the TDC line onto the harmonic
balancer so that it continues across the front face
of the balancer (both outter ring and hub). This way, a mis-aligned line will indicate if the
harmonic balancer has shifted out of position.




Daniel Wong

Daniel, I know your method is accurate, but how do you get a degree wheel on the crank if the engine is in the car? I made a stop like FRM described years ago, the only difference is I never had a welder so I threaded the old spark with a tap, screwed in a bolt and sawed it off to the desired length. I rounded off the end of the bolt in the stop so it doesn't damage the piston. I have always used marks on the balancer as FRM described unless I have the engine out of the car so I can install a degree wheel.

Ken, good to hear you have it fuigured out.

Clifton

Clifton Gordon

Well, Daniel said the same thing I did, except for the degree wheel, and the complexity and error probabilities introduced by alla them numbers! . You can easily calculate the distance in degrees at the OD of the damper if you want or need to. For IGN timing purposes, the spacing of the OE pointers will do, and can easily be transferred to another location.
FRM
FR Millmore

This is an addendum to the procedure I described
earlier.

REMOVE the piston stop before making the final
crankshaft rotation to determine TDC!

(...ooops!) Sorry for any confusion.

Clifton: To mount a degree wheel in-situ:

I sacrificed one on my degree wheels by greatly enlarging the center bolt mount so that the crank bolt wrench socket can pass through it.

I attached the wheel to the harmonic balancer
with double-stick mount magnets.
Daniel Wong

If you took the car to a mechanic to do this he would do it FMRs way as its more accurate and takes five minutes. you dont need a degree wheel to see if its out and as FMR said the distance out will tell you how much. Denis
DENIS H

This thread was discussed between 23/04/2007 and 24/04/2007

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now