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  #1  
Old 03-30-2003, 03:58 PM
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Top Dead Center

Hello All,
I have a 1980 300SD. I am going to do a leakdown test, but i dont understand how to put each individual piston at TDC.
Thank you for your help.

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Old 03-30-2003, 06:54 PM
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MJ,

Zero degrees on the vibration damper is #1 piston at TDC. Every 72 degrees from there the next piston in the firing order would be at TDC.

You could also get each piston close TDC by observing where the cam lobes are on #1 cylinder at 0 degrees and turn the engine so the the cam lobes are approximately the same for the other cylinders.

I'm not sure why you need the pistons at TDC to do the test.

P E H
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Old 03-30-2003, 11:52 PM
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PEH,
Thanks for the help. The instructions that came with my leak down tester said to posistion the cylinder to approximate TDC. Thats why i wanted to know how to locate TDC.
-Matt
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Old 03-31-2003, 01:06 AM
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MJ,

Observing the cam lobes should get you close enough to TDC. The 72 degrees are not marked on the vib damper. There is a degree wheel that goes on the cam for exact measurements but you probably don't have one and don't need that accuracy. The camshaft alignment marks must line up so #1 is at end of compression stroke ( beginning of power stroke).

Possibly they want you to have piston at the top because that is usually where the wear in the cylinder is the greatest.

Start with #1 cylinder and follow the firing order and you will have to turn engine only 1 revolution. If you want to do the test a second time you will have to turn engine 1 full revolution before starting again. Always turn engine in clockwise direction facing engine from front. Best way is by using a 27mm or 1 1/16" socket wrench on crankshaft bolt.
P E H
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Old 03-31-2003, 09:13 AM
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Leathermang,

In your post about Force on the piston when applying air pressure you first say there is 100 PSI on the piston then 1200 pounds of pressure on the piston. SO how can it be both?

You were right about PI R squared to get the area of the piston (12 square inches) but when you multiply that area times PSI (100) and do the dimensional analysis,you get pounds force.

So there is 100 PSI on the piston and 1200 pounds of force on the piston.

I would have caught this earlier but I was away on vacation.

P E H
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Old 03-31-2003, 09:46 AM
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I suggest you read this thread very carefully first....

Leakdown Tester and ?'s

Remember ... Safety First !
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Old 03-31-2003, 01:43 PM
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PEH, I do not understand your question....maybe you needed more vacation time....

The piston ( and for that matter the head, what ever surface of the bore ) has what ever number of pounds of air pressure you put into it with your air compressor....

That is referring to EACH SQUARE INCH... thus... Pounds PER Square INCH....

But the force on the piston pushing towards the crank.... which we are concerned with possibly turning the crank when we are not expecting it... is the number of square inches on the top of the piston times the Pounds per square inch...

Thus the force pushing down on the top of the piston is 1200 pounds...

One is referring to each square inch.. one is the total pushing on the piston...

I expect when you think about this you will wonder why you posed the question....

Was this all about you challenging my use of the word pressure for downward force instead of 'force'?
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Old 03-31-2003, 04:57 PM
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Leathermang,

I was refering to your post that was displayed when I went to the link about safety first.

What you wrote in your last post was correct: 100 PSI, 1200 pounds (force).

Yes, it was about using the term pressure for force.

P E H
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Old 03-31-2003, 06:09 PM
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GEEZ...

In the first post on that other thread.. I used

"One cylinder of a 300 , when 100 psi is put on top of it is 1200 pounds of force... THE ENGINE WILL TURN IF NOT PERFECTLY CENTERED AT TDC...."

In a later post in that same thread I said
"thus at 100 pounds per square inch makes 1200 pounds of pressure on top of the piston." (which I still say is true)

And this latter is what you are (trying) to bust my chops on ?
This is what Dictionary.com uses for these two words...

Force:
A vector quantity that tends to produce an acceleration of a body in the direction of its application.

pressure:
Force applied uniformly over a surface, measured as force per unit of area.

Just consider my "unit of area" as Piston Top.... and I know you knew that with the piston in a bore and connected to the crank that it was going to be a directed (vectored) application of power...
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Old 03-31-2003, 07:31 PM
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Leather,

This is all in fun now but:

"thus at 100 pounds per square inch makes 1200 pounds of pressure on top of the piston" IS NOT TRUE. The pressure on the piston is still 100 PSI but the force is 1200 pounds.

Pressure is measured in force (pounds or newtons) and area (square inch, foot or meter).

Force is measured in pounds or newtons only.

When you multiply pressure times area, the area dimensions cancel out so you are left with pounds or newtons only.

P E H

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