how critical is TDC

4tford

4tford

Well-known member
I am ready to install a distributor and wiring in an 8ba. I ran a hose from number one spark plug hole to a bottle of water. Rotated engine until the bubbles in water stopped. Is that close enough. Never gone passed a short block before. Thanks Bill
 
8ba should have a dot on the crank pulley. The dot/bump is 2 degrees BTDC.
 
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Not if you truly want TDC. If you are just wanting a guess it would work. For a true TDC you need something to work as a hard stop as the piston comes up in the cylinder. When it is against the stop mark the position on the crank pully. Then reverse the rotation of the crank until it touches the stop again and mark that position on the crank. TDC is half way between the two marks. To be accurate repeat several times to get constant results. This is much easier with the head off!

Now, what are you needing TDC for? If you are just wanting the engine in position to mount a distributor your method should work fine. If you need true TDC, there are no short cuts that I know of.
 
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at this point in time just wanting dizzy in proper place so I can run wires. using the water and hose. I rotate til bubbles stop and then go til it starts to suck water then back to bubbles.
 
That should work for what you are doing. Are your stock crank pully and timing cover marked?
 
The crankshaft goes through a fair amount of degrees from the time the piston tops out until it starts going back down. You can use a finger over the spark plug bore to tell that then just a bit more to the timing dot. You might end up one tooth off if you do it too early.
 
rotorwrench said:
The crankshaft goes through a fair amount of degrees from the time the piston tops out until it starts going back down. You can use a finger over the spark plug bore to tell that then just a bit more to the timing dot. You might end up one tooth off if you do it too early.
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Yup, that is how Walt Dupont taught me. Have the wife hold her thumb tightly over #1 hole. Then turn the crank over quickly by hand with a bar. When it blows her finger off you will know you are on the compression stroke. Then bring it up to line up the pimple and pointer. Insert the distributor with the rotor on and keep inserting it until the rotor is pointing at the #1 wire in the cap. Close enough to get it running. Use a timing light to set advance on the pulley mark. John
 
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one more question and I am off to the races. This is one of Bubba's Chebbie conversions. Is this the number one wire??? Thanks Guys
 
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Someone likely knows, but normally you take the cap off and see where the rotor is pointing. The edge of the rotor should be just touching the position of the number one position on the cap. I normally mark the rotor position of the edge of the distributor then you can see the position with the cap on.
 
Well lets see. That timing cover has 2 places for the pointer, the top one RT is for the big trucks, they usually have a bigger diom pulley, you can use any hole in the cap for no. one but where you are is all right or the other one up, I use the one close to no. one plug, just make sure the rotor is pointed to that hole. You ought to find TDC so you can actually see where that pointer is pointed on the pulley. Walt
 
The number 1 wire location is correct. Assuming the rotor is correct as Jerry states, you should be ready to go.
 
The rotor is pointing to that hole. Mac Van Pelt has a drawing for stock cap and it is close to there. Thanks Bill
 
Getting back to TDC, when there are no marks... Bruce (I think) and Ron, have posted several times, using a big zip tie. Search. Pretty accurate.
 
Technically, you can put #1 at any point you want as long as you follow the firing order after that and in the correct rotation. The problem is remembering where you put it. What ever reference you use will work and you will generally remember the reference you used or just follow the #1 back to the distributor to find it.

The zip tie head will stop the piston near top out then mark the crank pulley. You then back off and remove the zip tie. Rotate in proper direction past top center then reinstall the zip tie and back it off till it stops against the tie head again and mark the crank. Find the center between the two marks and you have TDC. It should be very close to the little dot on the crank pulley.
 
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On a block we had with the head off we clamped a dial indicator to the deck and went that route. Amazing the amount of float or crank rotation between the up swing and the down swing of the piston.

With that said is the tbc the top of that float... at the beginning or end of the stroke. I would assume it's the difference.

Zip tie will get ya there.
 
With the head off your best approach for TDC is a strap across the cylinder with an adjustable bolt in the center to use as a dead stop. Again rotate the crank to the stop, mark the pulley at a pointer on the front cover. Rotate the opposite direction to the dead stop and mark the pulley again. TDC is half way between the two marks. TDC is just what it says Top Dead Center, the point in the cycle when the piston is at it's highest point in the cylinder. It is just difficult to determine that point without using a point equal distance on ether side of it where the piston is moving a greater distance per degree of crank rotation.

With the head on, anything will work as long as it provides a consistent result and is non-damaging to the engine.
 
Ya'll making it way to difficult for the man...mercy/

If you're close to your mark, fire it and adjust the timing...man, never mind.

Hey...why didn't anyone bring up the use of a degree wheel?

I'm sure that all do that EVERY time they pull out a distributor to get it "right"....

Pizzing and moaning about TDC dwell on the piston, isn't helping the guy out...must be some just like to let everyone know how much they know?

Stab it, stick it and adjust it.

Sorry all...

Oh, and to answer the question posted in the topic...

It's not critical at all to fire the danged thing off.
 
Mike B said:
...must be some just like to let everyone know how much they know?

Stab it, stick it and adjust it.

Sorry all...

Oh, and to answer the question posted in the topic...

It's not critical at all to fire the danged thing off.
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Might want to re-read the thread! Think the "it not critical, just stab it and adjust it" was covered. There was additional questions about TDC. Two different subjects. TDC is not needed to "stab it and adjust it".:)
 
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