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  #1  
Old 09-14-2002, 06:47 PM
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Question Valve Job and Valve Timing

So I finally took the replacement head I got to Metric motors for a valve job and I have been reading up on the proceedure for replacing the head and came across a couple of concerns.. mainly valve timing.


The thickness of the replacement head will be riding the minimum.. so is the head that is on the car now. The main reason I'm replacing it is because the water passages wre badly worn and the rockers and camshaft are worn. Anyway the current timings are retarded but I'm not sure by how much and I haven't looked at the woodruff key yet.

My question is how to test the timing without damaging the pistons. I don't want to crank the engine and have the valves get smashe dintot the pistons. I read a description of a technique in the Chitlon manual but it had much to be desired.

Any tips appreciated ;-)

Adrian
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Old 09-14-2002, 11:36 PM
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That's an easy one.

When you reassemble the motor, don't put the rockers back in until after you've checked the timing. That way, the valves don't move.

You are worried about chain stretch, or at least chain offset more than anything.

Once you've got it all together, get the cam marks lined up and read off the angle on the timing wheel.

-CTH
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  #3  
Old 09-15-2002, 04:31 PM
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Quote:
Originally posted by cth350
You are worried about chain stretch, or at least chain offset more than anything.
The chain that's in there only has a couple thousand miles on it so i was planning to reuse it.

Also because of the thickness there are shims under the cam towers to take up the slack ont he chain. I'm pretty much expecting to need those again as well.

This is an area where I have little experience so please set me straight ;-)

Adrian
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Last edited by gmask; 09-15-2002 at 06:27 PM.
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  #4  
Old 09-15-2002, 04:57 PM
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that's a different kind of timing. Let's go back to the basics....

An internal combustion engine has lots of moving parts that must stay in sync for it to work well, if at all. To name a few...

- The crank shaft
- Pistons
- Cam shaft
- Rockers
- Ignition Points
- Distributor advance hardware
- Valves
- Fuel source
- Timing chain

The crank, timing chain and cam shaft all work in concert. To keep them properly sych'd their relative geometry is fixed. The relative position of the cam shaft sprocket on the cam is the only human settable variable to ensure this.

The crank's geometry dictates piston position without any adjustable parameters. There are no offset pins for the crank pulley; piston rods are hardly adjustable.

Ignition point gap and the mechanisms for advance in/on the distributor control the relative position of the piston against the timing of the ingition spark (OK, it's measured using crank angle, which we said for any given engine is fixed to piston position).

The cam-shaft, rockers and valves move in a synchronized way. There is a fixed relationship between the cam shaft angle of rotation and the valve open/close times. There are several variables, but all of them are basically fixed for a given cam/rocker/disc combination. Changes to the cam lobes due to wear or regrinding can be exacerbated or corrected for by changing the rockers or the little disc inserts they sit on and clearances between them all.

Measurement of the valve opening angle (or valve timing) is made relative to the crank position, which is a combination of the above to make a crank/chain/cam/rocker/disc/valve relationship.

Like I said, "it's an easy one".

Ignition timing, with a timing light is the crank/distributor/points adjustment. Valve timing is as above. Since you didn't materially impact the cam/rocker/disc/valve relationship, first fix the cam timing, then measure the valve timing if you like.

-CTH
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Old 09-15-2002, 06:09 PM
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After doing a little bit more reading I realised my mixup there with ignition and valve timing.

So basically I just need to line up the markings so that everybody is TDC. In this postion though I would expect that there would not be enough clearance for the valve and the position so I need to retard using a woodruff key.

So once I matched the sprocket with TDC from the front of the engine then I can measure the valve timing with a dial indicator? I guess the issues here are that the because the head is at a minimum thickness that the chain will be loose.. this being corrected currently by both cam tower shims and a woodruff key. As I read you normally only use those to make up for a stretched chain. I think the thrust pieces were replaced with thicker ones as well to again make up for the minimum thickness of the head.

Maybe I'm worrying about this more than I need to? It may be more obvious to me once I am doing it.
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  #6  
Old 09-15-2002, 06:11 PM
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Gmsk,

Time the camshaft to the crankshaft. The only variable is an offset woodruff key in the cam sprocket. As you adjust the valves, turn the engine by hand via vibration damper/crankshaft nut (27mm). You will be able to feel if there is any interference between a valve and the pistons. There will be interference only if you have the cam timed improperly.

You cannot test valve timing with a strobe gun. That is for ignition spark timing.

P E H
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  #7  
Old 09-15-2002, 07:06 PM
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Quote:
Originally posted by P.E.Haiges
You will be able to feel if there is any interference between a valve and the pistons. There will be interference only if you have the cam timed improperly.
The normal clearance for valve head to piston is aprox .035 in. I think i understand how to measure this and then from there it's a matter of matching the Timing specs for this car using the right sized key.

Anyway the description for valve timing measurement in the Chiltons book is har dto understand entrierley because the pictures are so dark. They als mention a degree wheel that is not present in my copy of the book or was allready torn out and used.

The description in the mercedes manual is a little more concise.. now I just need to buy a dial indicator with a suitable holder.
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  #8  
Old 09-15-2002, 08:05 PM
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Tell us what vehicle you are working on - maybe I missed it somewhere in the thread.

If your vehicle is like most, there is a degree wheel on the crankshaft belt pully - actually just behind the belt pully. It's the same degree wheel that you might observe with a strobe light if you were timing spark ignition.

There is a TDC (zero degrees) mark on the crankshaft - and there is a mark on the camshaft sprocket that is also TDC. It might be on the outside of the sprocket or on the inside - but it has to be there.

Because on a four-stroke engine there are two TDCs of the crank for one TDC of the camshaft, you have to follow the instructions for determining which crank TDC you are on. Typically it is the compression stroke for cylinder one. That's why you see these things called "TDC Whistles" for sale that you bolt into a spark plug hole. On the compression stroke they will whistle and let you know you've arrived.

That's just valve timing - if you took off the distributor (or diesel injection pump?) there is a timing procedure for that too.

Ken300D
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  #9  
Old 09-15-2002, 08:10 PM
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Gmask:


Factory instructions to check valve timing. This assumes cam is in place and rocker arms are installed.

1. "Turn engine in direction of normal rotation until #1 cylinder intake valve is in the same postions as used to check valve clearance." (lobe of cam exactly opposite where the rocker arm will touch the cam when it rotates round.)

2. "Insert feeler guage (at arrow) of sufficient thickness to remove all clearance." ( Arrow points to top of valve, between valve are rocker arm). I assume this means a slight drag, maybe a bit more, just like checking clearance, no more.

3. "Install dial indicator to read the travel of #1 cylinder intake valve. Set scale to "0"." (You will need to "preload" indicator about half and inch so that it reads as the valve moves away from the indicator. Dial indicator must, of course, be exactly parallel to valve travel direction! Tip can sit on edge of valve cap or on feeler gauge if stiff enough).

4. "Turn engine direction of rotation until dial indicator reads 0.016" (0.04mm). The vibration damper should be a correct degrees.

Valve timing is 11 degrees before top dead center (almost one crank rotation).

To check valve saftey clearance:

1. Adjust #1 cylinder intake valve to correct valve clearance.

2. Set engine to 5 degrees After Top Dead Center on intake stroke on #1 cylinder.

3. Mount dial indicator as in valve timing check and preload and zero. Do not preload valve.

4. Depress rocker arm and until valve touches piston (it stops). Note travel on dial indicator.

Minimum safety clearance: 0.9mm (0.036").

If the clearance is below that, you will need to either have the valve seats ground lower in the head, or replace the head.

Hope this clears things up.

Peter
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  #10  
Old 09-15-2002, 09:28 PM
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I think the degree wheel they are talking about is a tool you attach to the camshaft to tell the exact position of the cam in degrees. It is more accurate than lining up the timimg mark on the camshaft.

When you time the camshaft for the powerstroke (TDC), both valves must be closed with the exhaust valve next to open. Otherwise you will besetting the cam timing to the intake stroke. In older gasoline engines, this was no problem, you just rotated the distributor shaft 180 degrees. But I don't know how it works on electronic ignition systems.

P E H
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  #11  
Old 09-15-2002, 10:37 PM
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I think you are worrying too much about this. You put the crank at TDC, then install the gear and chain with the marks aligned. If the chain is not stretched, you should be OK. Turn the crank a couple of revolutions by hand (with a socket). When you come to TDC after the second turn, re-check that the marks on the cam are still aligned.

Install the rocker arms, and adjust the valves. Turn the engine some more to make sure nothing hits, and that everything stays aligned at TDC.

If at TDC, the cam marks are off, then you need to check the timing as Fred described to determine if you need an offset woodruff key.
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  #12  
Old 09-15-2002, 11:08 PM
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Amen.

Adrian, I'll be in San Diego as of Thursday. Are you in the area ? I forget.

-CTH
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  #13  
Old 09-15-2002, 11:39 PM
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Quote:
Originally posted by cth350
Amen.

Adrian, I'll be in San Diego as of Thursday. Are you in the area ? I forget.

-CTH
I'm in Los Angeles which is about 2 hours from San Diego. I should have the head back by then if you feel like driving up.
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  #14  
Old 09-16-2002, 12:06 AM
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Gmask:

Don't feel bad about asking questions -- Chilton's manuals are VERY vague and contradictory when it comes to Benz, and they tend to oversimplify things, too, and leave steps out.

I sure would hate to bugger up something as difficult to replace as the engine in a 250S -- they are getting scarce!

I just happened to have borrowed the 1973 shop training manual for the 69-73 Benzes from my indy mechanic -- very nice diagrams and clear info. Let me know if you need any more data from it -- not a complete manual by any means, but has some troubleshooting and adjusting stuff in it.

As for setting the cam in initially, with the crank set on TDC (the zero mark on the balancer), pull chain up, set cam so that the mark (groove or ridge, could be either) on the cam and the front bearing housing align. Both lobes for #1 should point away from the head, one on each side. Place gear in chain so that it will slide onto the cam. You may have to wriggle it around a bit, or rotate the cam slightly. Once it is on, you can rotate engine, etc. I'd leave the rockers out until you are satisfied it's in the correct position -- then you can insert the rockers for #1 and check timing and clearance.

Good luck -- this is all somewhat easier than it sounds. If the engine feels too hard to turn, find out why before you crank on it, and don't use the starter until you are satified everything is right!

Peter

Peter
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1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Last edited by psfred; 09-16-2002 at 12:12 AM.
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  #15  
Old 09-16-2002, 07:20 AM
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The one thing to add to that task description is "lube", use lots of assembly lube. Basically, it's a tube of white grease. coat the cam bearing surfaces in the towers. Coat the cam running surfaces (all 12 lobes and 4 journals), coat the inside of the gear and the journal it rides on.

-CTH
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