We got a little sidetracked with seats, but now we're back to the engine overhaul. And we have a question for you knowledgable gentlemen.

First of all, yes we did measure with a micrometer (supposedly more accurate, but the digital caliper appears to be right bang on), but for the photo we used the caliper. I thought you'd be able to read the display, but it's hard to see.

Anyway, the question....

The instructions say to measure 'the skirt' of the piston. Where exactly is that? Are we measuring in the right place? We just keep taking measurements till we get a reading we like. :p We can get 90.85 if we try hard enough. :p

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Natalie

Are you going to get new pistons? This is a 617 outa your wagon, right?

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

The skirt of the piston is what you are measuring in the picture.

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1985 300SD
1983 300D
1984 190D
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Steve, that is probably the oil hole leading to the annular oil cooling ring in a turbo 300 engine ..

I would say that you want to find the largest diameter 90 degrees to the piston pin along the skirt to use as the basis for matching cylinder size to ...

Use a micrometer, dial calipers aren't anywhere near accurate enough for this.

A micrometer with flat faces will only measure the minimum diameter if used correctly (not forced). With dial calipers the minimum diameter will be correct, but the chances you will find it AND be accurate are small -- dial calipers (electronic or not) aren't meant for that use -- too flexible, jaw faces MUCH to narrow.

Remember, piston to bore clearance is quite small on these engines.

The best way to check pistons is to

1): Reject ANY piston with visible signs of wear. If you can see it, it's undersized already.

2): Check the ring lands -- any palapable burr means toss it. New rings must fit in the grooves to factory specs. Any more clearance, toss the piston.

3): If a piston passes those tests, measure very carefully and NUMBER it for a cylinder -- the shop will have to hone the appropriate bore for that piston to get proper clearance.

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!

Stevo, to answer your question, this was the motor that was in a parts car that we bought. It only had about 140k miles on it. It started smoking badly one day and failed emission testing. The PO had compression testing done, which was terrible, so he decided to sell the car, and we bought it. For a car with a 'shot' motor, it had a surprising amount of power. Given the low mileage, we thought it might be worth rebuilding and putting in to Buttercup.
That's good news! That is the spot where the readings were exactly where we wanted them to be. I guess there's a reason for that.

psfred, we do have a micrometer accurate to 4 decimal places, and are measuring it the way it is described in the manual. We both took measurements (several times!) and the readings are consistant. So I think we're measuring right, we just need to know the right SPOT to take the measurements.

So far we have discovered:
-the pistons all measure exactly what they should (90.85mm).
-the crankshaft looks in great shape
-the bearings look brand new
-cylinder 5 has some scoring
-cylinder 1 has crosshatch marks (the rest look like factory)
As far as the measurement of the cylinders, that brings me to another question. We measured in the middle, top and bottom. All the readings are within limits (between 90.92 and 90.94mm) EXCEPT for a 1/4 inch ridge at the top, immediately below where the carbon buildup starts. In that ridge the readings are 91.1. If you go one quarter inch below the carbon buildup, the readings are acceptable. All the cylinders are the same. What do you make of that? Is it significant? The manual says to read 'one inch down from the top'... do they mean EXACTLY one inch down? I'm not sure if one inch is in the ridge or not, I haven't measured it's exact distance from the top.

Thanks again!

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Natalie

The "ridge" at the top is a combination of where the rings don't travel and accumulation of carbon. The carbon will come off with carb cleaner or a light honing, and can be ignored.

If you go the pistons out without damage, that ring rigde can't be very large (the rings will stick otherwise, and break going past it). I've seen then 0.035" deep!

Exact distance down isn't critical, just measure about an inch down. The critical measurement here is the difference between the upper bore (one inch down) and the bottom area (no wear) -- if more than the factory limit, you need to decide on oversize pistons and rebore, or keeping yours and putting new sleeves in, plus the rebore (new sleeves have to be bored to fit the pistons).

If the pistons are the correct diameter, check the ring lands (grooves) for wear after you clean them. Rings must fit within the factory clearance spec (don't remember it right now) with NEW rings. Put the new ring into the groove (don't install it, just insert it) and check the "top to bottom" clearance with a feeler guage set. Check at least 4 places around the piston. If within the wear limit, you can re-use them.

Chances are the rings failed -- sounds like the cylinders are OK. If you can still see the original scratches at about 30 degrees in the bore, it's OK. A very light honing to "rough" it up a bit will work fine. If you can still see the original honing marks on the cylinder with vertical scoring in the areas scored, it's still OK, too.

However, if any of the bores are smooth and shiny, no honing marks, or the vertical scoring removed the honing marks, it's rebore time.

Stuck oil rings (or overheating the engine and annealing the springs behind the oil ring) can cause large amounts of oil to get "pumped" into the combustion chamber.

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!

I think I shouldn't have said 'ridge', I should have said 'trough'. There is a ridge of carbon, yes, but immediately below that, one quarter inch all the way around, is a trough where the reading is beyond limits. The rest of the cylinder is acceptable.

Ping! That might explain a few things. Thanks for the other tips too Peter. I'll let you know what happens.

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Natalie

"cylinder 1 has crosshatch marks (the rest look like factory)"

I don't know if you picked up on what Peter said about this.... due to the fact that this is a situation OPPOSITE to regular cars... what you said would be how ANY other cars all my life would be judged.... however, MB expects the hatch marks to remain....and uses the lack of hatch marks as indicating that the cylinder needs help....

I know you are using the shop manual ... and should continue since plenty of correct info about measureing the rest of the piston is there ( the ring lands particularly ) ....

Sleeves are cheap compared to pistons.... if your pistons are good then pulling the sleeves and boring to fit ( each ) piston would be the proper way to get you back to STOCK out of the factory settings....Notice in the Manual that a simple homemade tool is shown for pulling the sleeves....and that a hammer is used to install the new ones....

Yes, sleeves are cheap, but have you ever paid a machine shop to re-sleeve a Mercedes diesel? Trust me -- it isn't cheap at all!

Yes, I did catch that, and didn't realize that before. I'm not quite sure what to make of it... there are signs that the motor has been worked on before (gaskets and stuff). We figured it was cylinder one they worked on, and that they crosshatched it. But maybe it's the other way around? Someone resleeved them and didn't crosshatch? The measurements look okay. We had figured to re-sleeve cyl 5 and 1, and the jury was still out on the others.

I am stopping at a machine shop that supposedly works on Mercedes today to check out prices. Our mechanic (the woman we have used before) told us it would be about $100/cylinder. I didn't expect it to be cheap GermanStar, but we are machining whether we buy pistons or sleeves, and sleeves are cheaper.

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Natalie

Last time we had this done it was $35 to resleeve and $35 per cylinder to bore and hone to size (I think). We drove out the old sleeves ourselves and saved some money since the shop only had to do the installation.

New sleeves must be bored.

The "trough" at the top of the cylinder is normally called a ring ridge, since it feels like one. Sometimes this has to be cut out with "ridge reamer" to get the pistons out intact since the rings won't go past it -- the ones on the used block we got for my brother's 300D were at least 0.030"! Terrible rebuild.

That deep trough is why you have too much blowby -- the rings cannot seal at the top of the compression stroke anymore.

Check your pistons for ring clearance -- if that is OK, just put new sleeves in for all cyinders and get new rings (the old ones are worn out -- never put used rings back in!). I'd not try to save money by only doing one or two cylinders -- the others are marginal, and you are very likely to end up with too much blowby and rough running, when for the small amount of extra cash you can have a new engine instead.

The "factory" look is a fairly coarse set of crosshatching from top to bottom at about 30 degrees. If the bore is shiny, its' worn. The scratches are what allows oil to stay in the bore to lubricate the rings as they slide up and down -- if the bore gets too smooth, the rings get oil starved and both ring and bore wear much faster.

Peter

__________________
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!

Since I know they have the actual factory shop manual... I was thinking they might do it themselves....

Mr. psfred:
Did you drive them out with a punch, or a chatter gun?
Charlie

I actually phoned the dealership and asked who they used.

I then phoned this machine shop, and the price to remove, install and machine the sleeves is $88/sleeve. If hubby does the removal and installation, the cost is only $22/sleeve for the machining and honing. We have decided to replace all five sleeves.

So what do you think? Is removal and installation a DIY job?

Hubby has gotten a little sidetracked, and thinks he has found a low mileage car for sale for about the same price as this rebuild is going to cost. After all this work (feels like I've been cleaning waffle irons forever :p ), I would like to just complete it. And we have to think of what the neighbours will say. :p

I'll let you know what happens.

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Natalie