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  #1  
Old 11-19-2000, 12:48 PM
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Join Date: Jan 2000
Location: PA
Posts: 5,440
This question is for Larry Bible:

You said I could use the piston at TDC to hold the valves from falling into the cylinder when replacing the valve seals.
Here's how I plan to proceed.
The #1 cylinder is easy, just go to 0 degrees on the engine balancer. The next cylinder in the firing order should be 72 degrees from 0 so I measure the distance on the balancer from 0 to 24 degrees (because the marks don't go to 72 degrees) and multiply be 3 then turn the balancer this far. I then repeat for the other 3 cylinders.
I got the 72 degrees by dividing 360 degrees by 5 which is the number of cylinders in engine.

Is this a practical and correct solution to putting the piston at TDC of the cylinder I want to replace the valve seals?

P E H
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  #2  
Old 11-19-2000, 04:04 PM
LarryBible
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P.E.,

You are failing to take into account that this is a four cycle engine, the next cylinder in the firing order would be 144 degrees, not 72. It takes 720 degrees of crankshaft rotation for all five cylinders to fire, not 360.

Is there any way you can get a degree wheel on the crankshaft or some other accurate way of locating TDC of your other four cylinders. If it were a gas engine, you could simply determine by probing or looking through the spark plug hole that the piston is in deed at the top before proceeding with the associated valves. With one of these engines you can't because of the prechamber.

Because of the consequences of disconnecting a valve wihout the piston at the top, you need to find a way that you can ENSURE that the piston is at TDC.

I don't know if you would want to try this, but I wonder if you could move the piston to what you believe is TDC for that hole, then GENTLY lever on the valve in such a way that will protect the valve stem and then ensure that it hits the piston top.

I know that your valve seals can be replaced by using the piston at the top of its stroke to hold the valve, but I have not personally done this on one of these engines. I know someone who has done it successfully, so I know it can be done.

Proceed carefully and best of luck,

------------------
Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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  #3  
Old 11-19-2000, 09:29 PM
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Join Date: Jan 2000
Location: PA
Posts: 5,440
Larry,

My 300SD is a 5 cylinder engine. I realize it takes 2 revolutions of the engine to fire all cylinders however each piston in the firing order should come to TDC every 72 degrees but they would not all be at the beginning of the power stroke. Every other cylinder would be at the beginning of the power stroke (taking into account the firing order) but each piston should come to TDC every 72 degrees.
Do you agree? I just want to make sure I don't drop a valve into the cylinder. I did that once, only problem was the engine was running about 3000 RPM at the time.
P E H
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  #4  
Old 11-19-2000, 10:06 PM
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Join Date: Aug 2000
Location: Houston, Texas
Posts: 325
Larry and P.E.

Is it possible to determine that you are at TDC by the following method?

First, remove all the glow plugs. Then place a finger at the opening where the glow plug was located on cylinder #1 and then rotate the engine by hand. YOu should be able to feel pressure from the compression stroke of the piston as the valves are closed. Once the pressure stops as you are turning the engine by hand, then you should have reached TDC for cylinder #1. The valves should then be able to be held up by the piston and the replacement of the seals can accomplished. But to make sure the piston is as close to the top as possible, couldn't you losen one of the valves from the spring and slowly bring the valve down onto the piston and gently turn the engine slowing at either direction to assure the highest setting of the piston?

I would then continue to the next cylinder as per the firing order of the 5 cylinder engine (i'm not sure what that would be because I don't have a 5 cylinder) and perform the same exercise by placing my finger on the hole where the glow plug was and check the air pressure building up and squeeze past the finger as you rotate the engine by hand. As soon as the pressure stops as you are rotating the engine by hand, you've reached TDC again.

I would continue in the same fashion on all the other cylinders. Larry, do you think this would work? I've never actually tried it but I just thought about it in my head because I will have replace my valve seals pretty soon also.

Herb
82 240D
107,000
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  #5  
Old 11-20-2000, 08:41 AM
LarryBible
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P.E.,

No, the cylinders fire 144 degrees apart, thus, moving the crankshaft 72 degrees does not get the next cylinder in the firing order to TDC. You are correct that each piston will be at TDC somewhere during that 360 degree rotation, but they will not be in the firing order.

Removing the glow plug to see if there is compression only tells you that the piston in that cylinder is on the way up on the compression stroke, it does not determine that the piston is at TDC.

For your purposes, it does not matter if the piston is on it's compression stroke or exhaust stroke, it only matters that the piston in that hole is at TDC to keep the valve from dropping. That's why I suggested GENTLY prying on a valve in that cylinder to ensure that the piston is at the top.

Remember, you will have the camshaft out to obtain access to the valves, so it doesn't matter if it's compression or exhaust stroke, just that the piston is at TDC in that cylinder. But, they will not come up in firing order every 72 degrees.

Good luck,

------------------
Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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  #6  
Old 11-20-2000, 11:36 AM
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Join Date: Jan 2000
Location: PA
Posts: 5,440
Larry,

You might be right about the 144 degrees. I will have to do some serious thinking about it.
Are you sure I have to remove the camshaft? I did a valve seal replacement on a 190D many years ago and I don't think I removed the camshaft. Its been a long time ago so I can't remember for sure.
Any way, I thank you for your replies and with your help I will eventually get this figured out. When I get the valve cover off maybe I can figure it out by the valve action.

P E H
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  #7  
Old 11-20-2000, 01:38 PM
Geezer
 
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
Let's see, if I turn a crankshaft through 360 degrees, each piston comes up to TDC once and we're back where we started. So, for a 5-cylinder engine, TDC arrives for a piston every 72 degrees.

In a four-stroke, each cylinder "fires" on one TDC and exhausts on the other. A complete cycle thus takes 720 degrees for each cylinder.

If the firing spacing was 72 degrees, each cylinder would fire once per revolution. This won't work; once per 142 degrees does.

Some piston will be at TDC every 72 degrees, but not the next one in firing order.

OK, now I've got it. :

PS. How do you cure MB withdrawal?

BCingU, Jim


------------------
'96 E300D 61k mi (wife's daily ride)
'95 Audi 90 120k mi (my daily ride...)
'92 GMC Suburban 140k mi
'85 300SD 235k mi KIA/RIP
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  #8  
Old 11-20-2000, 03:20 PM
LarryBible
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Posts: n/a
Jim,

If you're talking about mercedesshop.com withdrawal, there's no known cure.

Bill Woods would be the one to contact for the address of the detoxification center.

Good luck with the cold sweats,

------------------
Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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  #9  
Old 11-21-2000, 08:26 AM
Geezer
 
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
Sorry Larry, it's not as simple as MercedesShop withdrawal. As you can see from my signature, the 300SD has been declared Killed In Action.

I'm waiting for the insurance company to offer the proper amount of $$.

There is no MB car for me in the near future, as we still have plenty of wheels. We WERE going to sell the Audi, but...

Sometimes I get to drive the wife's car, but not often enough.

My consolation is that the Audi is also a fine German car, but has been a bit on the expensive side to maintain. Someday I'll post my maintenance costs for the 4 vehicles; it's interesting...

When I'm not in the Audi I drive the Suburban, a.k.a. "The Box It Came In." My, that Suburban is big!

Enough of this off-topic post.

BCingU, Jim

------------------
'96 E300D 61k mi (wife's daily ride)
'95 Audi 90 120k mi (my daily ride...)
'92 GMC Suburban 140k mi
'85 300SD 235k mi KIA/RIP
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