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best way to determine TDC for leakdown test


vairter

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I'm working on doing a leakdown test to determine whether I have a blown head gasket leading to coolant loss.

 

There seem to be at least 3 ways of setting the cylinder to TDC:

 

(1) remove the valve cover and infer the valve positions from the lobes on the camshaft

 

(2) put a screwdriver or coat hanger down the spark plug hole and watch the motion of the screwdriver to set TDC

 

(3) buy a top dead center locator that threads into the spark plug hole and find TDC as the midpoint of the crank pulley positions for upstroke and downstroke contact with the locator

 

How precise do you need to be in setting TDC for this purpose (i.e., making a leakdown measurement to confirm a blown head gasket) and do all 3 of these techniques accomplish this with adequate precision (newbie here) ?

 

Also do you always see air bubbles in the radiator if the head gasket is compromised, so that it is a definitive determination?

 

 

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You don't have to be precise at all, just have to have valves closed. Option 1 would be the easiest way because there you'll see that the valves are closed. Option 2 would work but it may be a bit difficult with tilted motor and then you have a chance to hit the "wrong" tdc and have valves open. 

 

Headgasket may also blow from one cylinder to another, in which case there's no bubbles.

Racing is Life - everything before and after is just waiting!

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No need for that.

Take off your distributor cap.

Rotate crankshaft so that TDC mark on front sprocket  pulley aligns with indicator on chainhousing and rotor points to the little mark on the rim of the distributor (where lead to plug cyl 1 is)

Do leakdown on cyl1

Turn crankshaft 1/2 turn (clockwise if your'e standing in front of the car looking to the rear) and test cylinders according to firing order (see valve cover)

Edited by uai
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Feel.

 

Since 1 and 4 are at TDC, and 2 and 3 are 180 out, there is a 'loose'

spot in rotation.  You do your best to get into the middle of it, and then

mark the pulled (or something) as to this best guess.

 

Then slowly add air pressure.  If you're right, you just found TDC for #4 as well.

 

If you're wrong, the engine will spin.  If you see which way it went, you can go back

to your old mark and adjust a little way one way or another.

And try again.

I have a timing pointer that aligns with the pulley, and marks.  that works 92% of the time.

 

Don't forget to take the ratchet off the front nut, or you can get clocked when the engine spins

and it flies off.

 

I've never held with holding the crank- once you have marks, 

it's easy after that.  

 

And a big old Ferd V8 is not nearly as difficult to do, it turns out.  And leaks a lot more!

 

t

7%

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"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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