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  #1  
Old 09-01-2001, 11:56 AM
LarryBible
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Matching pistons with prechambers

I got my block back from the machine shop with new pistons fitted. I started putting the engine together and found that the prechamber extends further into the combustion chamber than the piston relief accepts. Turns out the pistons are wrong for this engine.

My question is, are there prechambers that I can use that do not protruded into the combustion chamber as far and work with parallel glow plugs etc.?

It would be easier to change prechambers than to change pistons. If I change pistons, I will have to strip the block and return it to the machine shop, and then rescrub the block when they're done to get the honing grit out. Also it could be that the block is already honed to large for the other set of pistons.

Has anyone ever run into this before?

I almost forgot. This is an '81 nonturbo 300D engine number 125XXX.

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  #2  
Old 09-01-2001, 12:31 PM
engatwork's Avatar
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durn Larry

I'm sure you probably had some additional comments. The first thought that came to my mind was would it be possible to machine the prechamber down some to fit? I am not quite sure exactly how to figure out how much you would have to take off.
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  #3  
Old 09-01-2001, 01:16 PM
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Larry:

The block doesn't have to be re-honed since it hasn't been run. A "simple" matter of removing the pistons and replacing them with the correct

I didn't know there there were "wrong" pistons for these engines -- did they put turbo ones in instead? If indeed the wrong pistons were put into the block by the shop, I'd just insist they change them. If you did it, you will probably have to talk them into exchanging them after they have been installed.

Also, if you've taken the prechambers out they must be set to proper height with shims -- check how far they protrude from the head before getting the pistons changed.

Peter
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1972 220D ?? miles
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!
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  #4  
Old 09-01-2001, 06:05 PM
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The prechambers were redesigned and the pistons matched to them sometime around when your engine was built. I presume you have the prechambers with the flat bottom and a sharp edge around it below the fireholes. The later prechamber is rounded with no distinct edge. The holes in the pistons match the profile of the chamber.

I think you could match the chambers to the piston. And, as Peter said, you need to set the protrusion depth. You also need to be sure that the pistons are not too high out of the block.
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  #5  
Old 09-01-2001, 08:26 PM
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Larry, I have had the same experience you are having.The slanted tip prechambers are for the 1st version engine and the round tipped prechambers are for the later uprated version. It looks like your supplier did not send you the right pistons for your engine number. I am assuming that your pistons have the Star shaped combustion recess instead of the saucer shape with the recess for the slanted tip precombustion chamber.
I think your best fix is going to be to change the precombustion chambers although this will set you back another $ 200.00 on the rebuild you are doing.

Good Luck

MBJOE

Shadetree Mercedes Mechanic

83 300TD
81 240D
75 300D
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  #6  
Old 09-01-2001, 10:11 PM
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Larry

Your engine should have the star shapped recessed areas and round tipped pre-chambers.

You need to check the pistion projection and the distance the prechamber necks protrude past the head Piston projection should be .50 to .90 mm above the deck and the prechambers should protrude 7.6 to 8.3mm.

If the prechambers protrude too far you can reset the depth with a thicker prechamber sealing ring, if the pistons projection is too far I'm not sure what you can do.

Good luck

Tim
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  #7  
Old 09-01-2001, 10:21 PM
LarryBible
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Thanks for the replies. The prechamber relief in the pistons that are now in the engine are shaped like a short paper clip. They look as if they were simply cut with an end mill that was move about 3/8" or so. They have sharp corners, not a smooth transition down into the recess.

The prechambers are rounded on the bottom. From what Steve said it sounds like the pistons are the early type and the prechambers are the late type.

The bores were honed to fit this set of pistons. I then brought the bare block home and scrubbed away all the honing grit, and began building a short block. My parts store can get the correct pistons on Tuesday. If the new pistons are exactly the same OD then it will be relatively easy for me to change the pistons. But I have always seen the machine shops bore the cylinders a thou or two small, then hone the rest of the way to fit the pistons that will go in the bore. If I have to take the block back to the machine shop, rehone to fit the new pistons, then clean the block all over again, then it will be easier but more expensive to replace the prechambers.

Can I use the early OR late prechambers in this head with no trouble. This is a pin type glow plug engine, will there be any difference when using early prechambers which were meant for loop type glow plugs?

Thanks for all the information,
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  #8  
Old 09-02-2001, 09:57 AM
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Larry

Your description of the piston recess area does not seem to correspond to the descriptions/illustration in the OM616/617 engine manual......

The illustrations show two versions of pistons used on these engines. The early version has a round tapered recess area that tapers to a small dented area that provides clearence for the prechamber neck. According to the book these pistons should use the flat tipped prechambers.

The later pistons have the star shapped recess area. These pistons should be used with the round tipped prechambers. The book is quite explicit that the prechambers must be matched to the correct piston type.

Both piston versions also have relief areas cut to provide valve clearance.

Where did you get your pistons? Did the pistons you removed have the star recess in the pistons?

Tim
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  #9  
Old 09-02-2001, 10:32 AM
LarryBible
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I paid no attention to the pistons I took out except to inspect for condition. I took them to the machine shop with the block. They removed the pistons to recondition the rods, and I assume that they threw them away.

I got the new pistons, put them on the rods and began assembling the engine.

Again, is the tolerance of pistons OD close enough that I can change the pistons with the same oversize, but different head recess style and find that they fit with the same clearance as the set used to hone fit the block?

Thanks for the information,
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  #10  
Old 09-02-2001, 12:50 PM
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Larry , Again I would recommend that you change the precombustion chambers. Your machine shop has already gone to the trouble of honing each cylinder to the correct tolerance for each of the new pistons, if you change the pistons out they will have to go threw this process again. The round tip prechambers with work with these pistons and the pin type glow plugs are the correct type.
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  #11  
Old 09-02-2001, 05:15 PM
LarryBible
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Okay,

I have measured the depth of relief and protrusion of prechambers. If I move all the prechambers up 2.5 millimeters they will all clear by at least the thickness of the head gasket. Since these are not moving parts like valves, it would seem that .030" would be plenty of cleaance.

Here's the question. Assuming that there is already a standard 2mm seal ring in place, will have plenty of room to add a 2mm and a 1mm or even a couple of 2mm seal rings, without running out of threads on the prechamber nut? I am assuming there is plenty of room there, but I'm hoping someone call answer this before I order parts.

Thanks for your help,
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  #12  
Old 09-02-2001, 06:41 PM
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Larry, The prechambers have holes in them to direct the jet of fuel sprayed into the combustion chamber. The Star shaped
( Milled) pistons are supposed to match up with the spray pattern of the precombustion chambers. The round tip ones have three holes in them. The slant top precombustion chambers have four holes in them in different locations from the round tip combustion chambers which have three. If you try to use the slant top with the star shaped combustion chambers they will not direct the flow fuel in the right direction. The MB manual says you cannot use the slant top precombustion chambers with the star shaped pistons.


I hope this helps

MBJOE
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  #13  
Old 09-02-2001, 07:58 PM
LarryBible
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MBJoe,

Thanks for sharing your experience with me. I guess I will order the other prechambers. I assume that they are shorter so that I won't have to use an excess of seal rings to space them up for clearance.

As often happens, these kinds of things are busting my budget for this rebuild. The prechambers plus the tools to change them is going to be in the neighborhood of $400.

Thanks again,
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  #14  
Old 09-02-2001, 08:53 PM
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Larry,

I probably have a few of whichever prechamber laying around. If you don't mind used ones.

My go-fer would probably round some up for a contribution to our beer fund.
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  #15  
Old 09-03-2001, 09:48 AM
LarryBible
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Steve,

Thanks a bunch. I need the ones that everyone has been calling "slanted" as opposed to the ones now installed which are the rounded ones. Just tell me how much beer these guys need, and of course, how much shipping cost I need to send you. These are for the 300D so I need, of course, five of them.

Thanks and have a great day,


Last edited by LarryBible; 09-03-2001 at 09:54 AM.
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