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  1. #1
    Neal27's Avatar
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    Thumbs up OT-How to find TDC on a gas engine?

    I need to do some engine diagnosing on my van (4.9L Straight 6). I'm going to do a cylinder leakage test on it, and the book called for it to be at TDC at testing. I thought about taking off the Distributor cap and locating the position of the rotor but i'm not sure how close or far the rotor would have to be to the terminal spot. I'm also contemplating shelling out the $30.00 To buy a TDC indicator. Any Recommendations?

  2. #2
    skmod is offline Plastic
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    Default how to find tdc on a gas engine

    pull out the plug on the cylinder you want to check tap the starter first pull the coil wire and or the power to the dist. with something in the plug hole till you here or feel air then with a wrench turn the motor over with a stiff wire in the plug hole till you find tdc look at the balancer and make a mark you should be able to turn the motor every 90 degrees to get tdc for the other cylinders just take the wrench off the bolt before you do the test in case the motor moves
    Ernie

  3. #3
    Neal27's Avatar
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    HEY Skmod, it would be 60 Degrees for a 6 right? Im not really sure i understand what you're telling me about putting a wire in the hole.

  4. #4
    Norman Atkinson is offline Titanium
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    Default Tdc??????

    If it has a flywheel, the TDC will be there engraved together with the advance mark

    You can crack a spare spark plug and replace the ceramic with sleeve and a rod to the piston top.

    Again, it may have a pulley which may have dots drilled in it.

    Then there is rocking the valves but we are getting technical.

    Ok. Then you mark both with white marker and set your running timing with the strobe.

    Have fun

    Norm

  5. #5
    metlmunchr is offline Diamond
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    Attach a degree wheel to the crankshaft, and set a dial indicator such that its stem can extend thru a spark plug hole to contact the piston.

    Rotate the engine forward by hand until the indicator passes thru TDC and shows the piston moving downward by some set amount, say .030". Make a note of the reading on the degree wheel when you reach .030 on the indicator.

    Now rotate it backward until it passes thru TDC and down to .030 in the opposite direction, and record thhe degree wheel position at this point.

    TDC is halfway between the two readings you recorded from the degree wheel.

    Because of the amount of angular movement of the crank when the piston is near TDC but the piston movement itself is near imperceptible, any other method is just a guess.

    The above method is used by engine builders for determination of TDC for cam timing purposes. For a leakdown test, using the zero mark on the balancer should be close enough. On a 4 stroke inline 6, you'd have a compression event each 120* of crank rotation because it takes 2 revolutions for all cylinders to fire. To determine where 120* and 240* is with sufficient accuracy for leakdown, you can take a strip of paper(not string or anything stretchy) and wrap it around the balancer, and mark it to get the balancer circumference. Then, stretch it out and divide it into 3 equal segments. Put the same dividing marks on an equal length of masking tape, and stick that to the balancer starting with one end of the tape at the zero mark, and you oughta be good to go for what you want to do. If there's some error between the balancer zero mark and actual TDC, you'll still be checking each cylinder at the same relative position and you'll still be well within the range of cam rotation where both valves are fully closed.

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    jim rozen is offline Diamond
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    If you are going to jog it back and forth to find TDC then you can always put a breaker
    bar on the pulley nut. Remove all plugs to make it easier to turn over. Put a twist of
    paper towel over the plug holes you are not working on to keep things from dropping
    in.

    Jim

  7. #7
    piniongear's Avatar
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    Quote Originally Posted by Neal27 View Post
    I need to do some engine diagnosing on my van (4.9L Straight 6). I'm going to do a cylinder leakage test on it, and the book called for it to be at TDC at testing. I thought about taking off the Distributor cap and locating the position of the rotor but i'm not sure how close or far the rotor would have to be to the terminal spot. I'm also contemplating shelling out the $30.00 To buy a TDC indicator. Any Recommendations?
    I use to have to find absolute exact TDC on my BSA Gold Star dirt tracker all the time. I needed precision because I had to adjust the valves and magneto timing exact.

    The only way I know to do this is with a degree wheel. Attach the degree wheel to the engine. Attach a piece of fine wire to act as an indicator over the face of the wheel.
    Take an old spark plug and weld or attach a bolt or stud to it. The stud must be long enough to touch the piston and stop it before it reaches TDC.
    Now, the exercise goes like this:
    Insert the spark plug with the stud on it.
    Rotate the engine until it stops with the piston against the stud.
    Look at the degree wheel and note the reading. Let's say it stops on 25 degrees.
    Rotate the engine the other direction, again stopping when the piston reaches the stud. What does the degree read now? Let's say it reads 352 degrees.
    The difference between 352 and 25 degrees is 33 degrees. Divide that by 2 and the result is 16.5 degrees.
    Without moving the engine from where it currently sits, move the degree wheel so that the pointer is at 16.5 degrees on the wheel. Zero will then be your top dead center. This method eliminates the rock back and forth at top dead center, which can be a measurable amount.
    Easier to do on a single cylinder motorcycle engine than a car, but it is the accurate way to do it.......pg

  8. #8
    Norman Atkinson is offline Titanium
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    Default Tdc??????

    Correctly, TDC is found using a suitable length rod through the No 1 cylinder and a clock gauge fitted to it and one of the cylinder head bolts.

    I didn't expect the guy to want to spend a lot on a 'one off' task.Strictly, the head should have come off. Again, the valves should have been set up after grinding in and the capacities balanced after being measured with an oil and paraffin/kerosene mixture with a pipette- and a glass plate with holes to get the exact capacities. The bores should have been checked for wear with not more than 0.003 wear. Finally, the tappets should have been set with dial mikes and not feeler gauges.

    err, yes?

  9. #9
    MechWerks is offline Cast Iron
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    Being a 4-cycle you will have two top positions for the piston but only the true TDC for using a leak tester. New engines often do not have indicator marks since they have computer sensors. The degree wheels work great but you also have to know where the valves are at. Having a distributor rotor is a help as it should be pointing at the right spark wire at the top position you want.

    With a leakdown tester, you can crank the engine while the tool is attached with a bump start switch or bar and observe valve position with intake or exhaust air escape noise. Often tells you a lot right there. But getting the piston to TDC, with valves closed and determine if the results are position or the issue you are looking for puts you right back to figuring things out and setting the piston where you expect to start the test.
    Once you have the piston and tool set right, you get to look for where the air is leaking out (crankcase, through oil filler, for ring/piston issues), radiator (bubbles in coolant for leaking gaskets or cracks), intake or exhaust leaks for bent/burnt valves or a general test of wear balance between cylinders for comparison. Very useful tool but takes a lot of thought and engine knowledge.

  10. #10
    Norman Atkinson is offline Titanium
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    Default Tdc??????

    With the valves 'rocking' on the Number 1 'pot', just before TDC is the point where the firing takes place- on the compression stroke in an Otto Cycle.

    As Mr Punch said in the Punch and Judy Show- 'That's the way to do it'

  11. #11
    MechWerks is offline Cast Iron
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    Or another way to look at is that the cylinder only makes compression right before the firing stroke. Put a finger on the hole, run the piston up making compression. Then locate TDC for that cylinder (degree wheel being the exact metod). Except for engines with high cam overlaps, you can often feel a piston to a basic TDC after it is on compression and start using the leak down tester to figure out enough without the piston at an exact position. What you want to know is when the cylinder has the valves closed so you can check air leakage. Sometimes going up on compression but not all the way to the top is desireable to look at cylinder wall condition (scoring, wear).

  12. #12
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    Excellent info. I'm definitely going to buy a degree wheel. I'll also try using the Dial Indicator and compared the two processes thanks for info. If any one knows of any other methods please leave them here, "There's more than one way to skin a cat."

  13. #13
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    I think you guys are making it more complicated than it has to be for a leak-down test.

    As MechWerks pointed out, the point is to find where the valves are closed so all of the air from the leak-down test doesn't end up flowing through the exhaust or intake and thus telling you absolutely nothing.

    When you work at a mechanics shop and you want to do a leak down test, you pull the plug and crank the engine with your thumb over the hole. Depending on which cylinder your looking at you might need a buddy to crank the engine for you. Once its clear that your on the compression and not the exhaust stroke, you stick a wood dowel down the hole. Crank until the dowel stops moving. The crank will continue to rotate a very small amount and then the rod starts to go back down. Back up approximately half of the amount you turned from the time that the wooden dowel stopped moving.

    This is more than accurate enough for a LEAK-DOWN test. This is not suitable for cam timing, sure - but for a leak-down test this is the way to do it.

    If you have the gaskets or need to pull a valve cover anyway, than it can be done by watching the movement of the push rods. As long as the valves are closed and the piston is close to TDC your fine.

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    On my old 750 (really 736) cc Royal Enfield Interceptor, I used a DTI and measured the true TDC of the piston by introducing the DTI through the open (14mm) spark plug hole.

    Worked every time to set the timing of the dreaded "Prince of Darkness" (e.g., Lucas Electric) magneto.

  15. #15
    speedsport is offline Cast Iron
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    I have a problem accepting advice from anybody who describes a 4 stroke otto cycle internal combustion engine as a "4 cycle" engine.

  16. #16
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    One thing nobody here mentioned is, if you are doing a leak test at TDC you will need to have somebody with a long ratchet/socket on the crank hub to keep the engine from rotating when the air enters the cylinder. Another point, if the engine is an overhead cam you should NOT rotate the engine backwards, IE counter clockwise. The timing tensioner is designed to take up slack in one direction only. The engine should be rotated like the ratchet is tightening a bolt, it may be a bit more work but this will ensure you do no damage to the valve train. Also; to do a leak test the engine should be warm,run the engine till it is at operating temperature loosen the spark plugs and let it cool enough to work on comfortably.

    Steve

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    He mentions in his post that it is a 4.9L straight 6 this is a pushrod engine .Easiest thing to do is to remove the valvecover and remove the rockers and pushrods . Then test away it will not make any difference where the crank is and you can check all cylinders without having to find TDC for each one .Bill

  18. #18
    Neal27's Avatar
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    It will be impossoble for me to get the engine warm as it does not start,my plan is to pull the engine due to it making an ungodly screaming noise on the highway, when i got out to check i opened the radiator cap and it had oil coming out of it. I also noticed the other day i went out to crank it and there was oil leaking to the ground. I wanted to do a vacuum test and a compression test as well but read that you will get unreliable results if the engine is not at operating temperature.

    So where i'm at is to 1.pull the engine 2. perform a leak down test. 3. with that result i still want to disassemble it and inspect everything, check for cracks in the block to see where the oil is coming from hopefully a seal or the pan
    inspect everything thoroughly take the head to be tested
    take the the block to be tested and HOPEfully i can save this engine. My other option is to get one from the junkyard for $275.00 they have one still in the truck, I could put this one in if the other one is not salvageable, and probably end up rebuilding that one in 1-2 months.

    These would be my choices due to a limited budget. what do you all think?
    Last edited by Neal27; 03-03-2008 at 06:01 AM.

  19. #19
    piniongear's Avatar
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    Quote Originally Posted by Neal27 View Post
    It will be impossoble for me to get the engine warm as it does not start,my plan is to pull the engine due to it making an ungodly screaming noise on the highway, when i got out to check i opened the radiator cap and it had oil coming out of it. I also noticed the other day i went out to crank it and there was oil leaking to the ground. I wanted to do a vacuum test and a compression test as well but read that you will get unreliable results if the engine is not at operating temperature.

    So where i'm at is to 1.pull the engine 2. perform a leak down test. 3. with those results i still want to disassemble it and inspect everything, check for cracks in the block to see where the oil is coming from hopefully a seal or the pan
    inspect everything thoroughly take the head to be tested
    take the the block to be tested and HOPEfully i can save this engine. My other option is to get one from the junkyard for $275.00 they have one still in the truck, I could put this one in if the other one is not salvageable, and probably end up rebuilding that one in 1-2 months.

    These would be my choices due to a limited budget. what do you all think?
    You are correct. You need to pull the engine and tear it down. Don't even be concerned with a compression test or a degree wheel right now.
    You have bigger fish to fry. Find out what is allowing oil to enter the radiator. Do you have water in the oil pan? Water will be a milky mess and the oil will float on water, so the water will be at the bottom. Remove the drain plug to check this. If oil can enter the radiator then water may enter the engine. Water does not compress, so do not try to crank it up before you discover what is wrong. Yes, take it from me, water does not compress, as I found out 2 years ago when my Chevy 305 got a leaking head gasket and locked itself up tight. I removed the plugs and as I cranked it over, watching green fluid (antifreeze) shoot out of 6 of the 8 spark plug holes. Water pump had failed and the engine overheated. I just pulled the 305 out and set it by the curb on heavy trash day. I bought a 350 to replace it........pg

  20. #20
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    It's a pushrod motor.

    Pull the valve cover and crank it until you see that both rockers have gone slack. When you can spin both pushrods with your fingers, you're close enough.

    Don't matter exactly where the crank is, you just need both valves closed.

    If you can get a guaranteed runner that's not clapped out for less than $300, that's what I'd do. You'll spend that easy with even a minor rebuild (assuming that motor could even be rebuilt). If you've got oil in the water system, that means you've got a crack somewhere close to high pressure oil. Chances are good your block is done. If it ain't the block, you might save it, but the odds ain't real good.

    If it were mine, I'd buy the runner, jam it in there, and hope she sticks around long enough to clap out the rest of the drivetrain. Sometime there's just no point in trying to save the original.

    Eventually, you've gotta ask yourself how much money you're willing to put into an old motor (or an old van for that matter). At some point you gotta cut bait and run for shore.

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