Finding/Confirming TDC: Inquiring Minds Want to Know

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SP3
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Finding/Confirming TDC: Inquiring Minds Want to Know

Post by SP3 »

Wanted to ask the council about this to insure method I use is fool proof.

I was taught to find Top Dead Center on 4-stroke motors you can pull the plugs for easier crank pulley rotation and to place your thumb/finger over the spark plug hole of cylinder you are seeking. Rotate motor in correct rotation. Clockwise for our motors?

Rotate until feel air push finger off plug hole, that is Compression stroke. You can then confirm with screwdriver that piston top is up near plug opening in cylinder. Then on to the next cylinder in the Firing Order.

Is this correct for R16/U20’s or is there another stroke that will have similar characteristic of air pushing out of cylinder and piston being at top of travel, creating incorrect interpretation?

I notice on my R16 some cylinders have less pronounced air pulse out of engine, so l rely on piston detection in cylinder and if valve cover off both the Intake and Exhaust Rocker Arms have play.

Seeking confirmation on method and engine rotation direction if working from passenger-side fender.

Also would distributor Rotor point to cylinder at TDC as 3rd validation?
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by JT68 »

Crank pulley on the last (big) notch, both number 1 rockers off the valves, distributor rotor points to number1 wire.

#1 is at the front, some folk miss that detail..
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by SP3 »

Thanks JT,
For #’s 3 then 4 and 2, would that same timing mark on crank pulley be on the last big notch as well once at TDC?

From leaning on passenger fender, is one pulling on rachet (clockwise rotation) or pushing away (counter-clockwise rotation of crank bolt)?

Is it faster to find TDC for each cylinder going in order of 1342 or 1234? Assume former. Faster meaning less turns on crank bolt procedure.
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by JT68 »

The engine rotates CW (looking from the front), so just rotate it that way. Keep it simple.

Only pistons 1&4 are at the top with the crank pulley on the TDC mark.

2&3 are 180 degrees out, so they are at BDC when 1 and 4 are at TDC.

If the plugs are out its exceptionally easy to rotate the crank.

Assuming the engine runs and the distributor is in correctly, just rotate the engine until the rotor points directly to number 1.
The timing pointer should be at TDC and the both the #1 cylinder rockers should not be compressing any valves. Its that easy.

If the distributor is out, put the crank pulley on TDC, verify both the #1 rockers are not compressing the valves (rockers for cylinder #4 will be)
and that is TDC.
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by redroadster »

With the compression guage on 1 the needle rises on compression stroke watch for the mark
Some treks put talcum powder on the threads wait for it to poof
With a snap on type guage guys would put a balloon over the nipple it inflates on compr.
I did one oncethe plugs were out we put #1 in and put it in 4th pushed it gets hard er to push on compr.
These are ways to do it .....ya know time is $ in auto work right
& No on the exhaust stroke ,the valves both intake and exhaust are open at the TDC ,so no compressing of air
Last edited by redroadster on Sun Mar 21, 2021 9:39 am, edited 2 times in total.
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by bobd »

To identify TDC on the compression stroke with the valve cover off, turn engine in direction of rotation, watch the intake valve go down, and as it comes up to the mark you will be there.

A degree wheel and a dial indicator are needed to verify accurate DTC. With indicator in plug hole and degree wheel on the crank, rotate clockwise until maximum reading on indicator, note reading on degree wheel then rotate in opposite direction and note reading on degree wheel.

True TDC will be half way between the two degree wheel readings - assuming they are different.
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by redroadster »

What is Top dead center referring to ?
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by cbez »

piston at the highest point in its travel
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by redroadster »

cbez wrote: Sun Mar 21, 2021 10:55 pm piston at the highest point in its travel
No it's not .....how is that dead center ?
I've known since 75
many are confused to what it refers to
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by spl310 »

Oh boy, we are arguing semantics today
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by redroadster »

spl310 wrote: Mon Mar 22, 2021 1:32 pm Oh boy, we are arguing semantics today
Training is important ,
TDC top dead center ....refers to the crank throw of that cylinder's journal being at the top ....and dead center in the cylinder....it can be found with a dial guage on a fixture to check if the normal marks are suspect
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by spl310 »

When the crank journal is at TDC, the piston is where? At the top of it's travel. Like I said, arguing semantics.
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by unklpat »

I would also add that everything about the importance of TDC involves the piston position, like ignition timing, valve opening and closing, etc. If you are concerned, rotate the crank either way, until the piston starts to drop, then split the difference. I believe red asked a trick question to be self answered...
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by cbez »

that depends on what the meaning of 'is' is
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Re: Finding/Confirming TDC: Inquiring Minds Want to Know

Post by redroadster »

It has to do with the crank ...say the mark on the balancer is off of lost ...the way to find it again is with micrometer accessories off the crank throws
They're pretty strick in manuf classes, you score bad enough the dealer will drop you