Does anyone know of a good repair shop capable of honing a 116 V-8 aluminum block the MB recommended way, in the Cleveland, Ohio area? I was refered to a local shop by the MB dealer and am not satisfied with the results ( still some slight pitting). I think instead of honing first the shop just used the silicone paste and felt wipers. I am reluctant to reassemble the engine and not have the rings seat properly.
Thanks, Bob

There is no honing in the aluminum block in the traditional sense. The bore is polished and coated with silicone, final finish looks like a really bad case of orangepeel in paint.

Under no circumstances should you apply a standard cylinder bore hone, it will remove the coating and you will end up with a ruined block in no time.

Serious pitting will probably require boring to an oversize and re-coating.

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!

Thanks for the reply. I read the procedure in my MB engine manual and it shows a stepped honing procedure with the last step using felt wipers and silicone paste. I think the shop only did the last step because the pitting is very slight and it is still there. I will ask the tech what procedure he used next week.
Bob

Wel the story goes like this. You start with molten Aluminum and disolve all the silicon dioxide in the melt as can happen. This is about 16-17% by weight I presume.

The block is cast using hypo eutectic cooling. This is done by slow cooling to a temp where just the Aluminum with the highest concention of silicon solidifies first. This causes Aluminum with about 35% SiO2 to form. The queching is done on the cylinder walls first. In the end the block has 35% SiO2 in the surface of the cylinder walls and probably less than ten percent through the rest.

The holes are bored with a Sunnen CK10 cylinder hone to approximately the right bore and then to the right bore with increasingly fine grit with the end stage almost polishing grit. There are no visable hone marks.

Now for that last stage called the siliconizing procedure, It is the exact opposite of what people think. Silicon is not added. it is already there. The siliconizing process is the leather hone soaked in an acid etch. The process is done for a certain time during which the aluminum is etched from the surface leaving the SiO2 matrix.

The casting process is done to leave a depth of high silicon large enough for one 0.5mm resizing. (Boring)

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Ah, that makes sense. I'll bet the silicon dioxide surface is porous, too, so that bore wear is insignificant so long as you don't seize a piston due to overheating, and the rings should likewise last forever.

Certainly Benz does a much better job of this than GM did with the Vega engine -- imagine, an aluminum block (high silicon dioxide, to be sure) with a cast iron head. Usually blew out about 30,000 miles, I think they eventually reverted to cast iron liners to fix the problem.

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!

FYI the metalurgy is the same as the Vega, but Mercedes used a slow low pressure casting process rather than the high pressure die casting process used by GM.

Bore finishing initially consisted of boring and honing in the conventional sense. The OEMs then used an alkyline solution with electrical potential and current applied to etch away about .0005" of surface aluminum leaving the pure silicon wear surface.

In the field this etching process is replaced by the "silicon lap" to remove surface aluminum. You must also use pistons with the proper plating to be compabible with the bore surface, which are available off-the-shelf for Mercs. I've found shops that work on Merc aluminum blocks, and they can do proper bore refinishing for Cosworth Vega blocks, but there are no off-the-shelf pistons with the proper plating, so the most expedicious and least risky path for CV owners is to install dry iron sleeves in the bores and install semi-custom forged aluminum pistons that are made in batches from time to time.

I had the head of my Cosworth Vega off for a refresh last year at 70K miles and could not measure ANY bore wear/taper using both dial bore gages and inside gages and micrometers. I was quite impressed.

You mention silicon dioxide as opposed to pure elemental silicon, which is my understanding of what the wear surface is. In any event, inspecting these bores with a borescope is interesting. The surface is a mottled gray color with no evidence of the final hone, and you can see small darker gray splotches homogeneously distributed in the field of view. These are the silicon particles that form the actual wear surface.

Duke

Duke

Duke's right - it's pure Silicon metal, not SiO2 that stands proud from the etched bore surface. At higher mag Silicon has a coca-cola color to it. This is a great wear surface with the relief areas acting as mini-reservoirs to hold the necessary oil for lubrication.

The drawback is that compared with cast iron, this alloy is a ***** to machine. It dulls tools in no time. For this and other reasons, I'm personally not a big fan of aluminum blocks and prefer cast iron for mass-produced vehicles.

Peter, the surface is not necessarily porous, except for some microporosity.

Vega's demise of the aluminum block back in 1975 was warpage. The block was otherwise a fine piece of metallurgy.

Steve - it's hypereutectic, not hypoeutectic.

__________________
95 E320 Cabriolet, 159K

Gosh, you could be right about the hyper thing I debated using the term as I wasn't sure which expressed the concept right, sort of like trying to remember that .9v means rich and .1v lean which gives 100% duty cycle or 0%.

Up or down, eutectic fractional cooling is the point after thirty years I forget the non necessary directions.

I don't think I have forgot the important things so I will re affirm my choice of alloying material SiO2. tHe point of having glass cylinder walls (SiO2) was they were hard. Do you suppose elemental Si to have those properties.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

It sure does! Have you ever seen a piece of zone-refined Silicon from the electronics industry? It looks and behaves like glass.

You're probably remembering the fact that pure metals are typically very soft. If you look on the periodic table, Silicon is a nonmetal. The only way you could get SiO2 in the casting is from burned-on sand or entrained inclusions from the melt, either of which is highly undesireable.

By the way, I've actually poured metal to produce Al-Si alloy castings in our lab for prototype parts and I've studied these alloys using various microscopy techniques. I am quite intimately acquainted with these alloys and their problems.

__________________
95 E320 Cabriolet, 159K

Ah, Kestas, sounds like you've got some real expertise here. The last year for the linerless Vega aluminum engine was 1977, but the last year for the Cosworth Vega was '76.

The GM etching process was flawed in that it was not always consistent. Some areas might be heavily etched, but others might have no etch at all or very little. Then the rings would tear off bits of aluminum an score the cylinders, and this was accelerated/exacerbated by overheating due to loss of coolant.

As is so typical of GM, they gave up about the time the process was perfected due to the bad publicity from the earlier models that had problems, but Porsche licensed the process from GM/Reyonlds for the 928 that went into production in '78. Mercedes and BMW followed suit. They all improved the etching process, which virtually eliminated the scoring problem and continue to improve it to this day, as the current generation of engines with integral aluminum bore wear surfaces or aluminum liners are developments of the original GM/Reynolds technology.
Interestingly GM has stayed away from it ever since, but the upcoming LS2 and LS7 engines in the next generation C6 Corvette have larger bores than the current generation LS1/LS6, and my hunch is that they may finally re-embrace the linerless technology that they pioneered.

If the cylinders don't score these aluminum/silicon blocks can last virtually forever as the wear rate is about one-fifth of cast iron. As I previously stated, there was no detectable bore wear on my Cosworth Vega at 70K miles and that includes about 4000-5000 miles of race track hot laps - the equivalent of the "48 Hours of LeMans.

Duke

Very interesting reading,
I passed by the local MBenz dealer here to ask about Alusil boring/honing techniques, and I was told that they don't do that, they install a brand new engine instead To further compound the problem, there aren't any "reputable machine shops" out here... So I'd REALLY REALLY appreciate it if someone where to outline the proper procedure gived in the MB engine manual so I can have someone do it with me standing over their shoulder! Thank you.

Gosh. It looks like we have come along way from when someone making iron accidentally got some coal in the molten iron and made steel. The story goes something like that.

Hydra,
Why do you want to spend all the bucks to rebuild one of these engines? I'm betting your car isn't worth the cost of a rebuild since you didn't say what it was. Considering the testimony in this thread as to how long these blocks last, I'd opt for a used engine.

Peter

__________________
Auto Zentral Ltd.

Autozen,
Why jump to such drastic conclusions? For the record, I'm installing a Euro 560 into a freshly restored '77 450SLC, so I wouldn't go that far. Besides, my reasons for asking are threefold

a) curiousity, I'm a Mechanical Engineer with a keen interest in metallurgy and surface engineering
b) the local supply of 560s were in all likelyhood poorly maintained (I live in Lebanon where operating conditions are pretty severe and maintenence standards aren't exactly up to scratch)
c) above all I'm a car nut, and would like to see how much potential this engine has for prospective mods... I'm already fabbing a custom IR intake manifold along with custom EFI...

But I digress,
can anybody tell me how to bore/hone this damn thing?

__________________
"There are a number of mechanical devices which increase sexual arousal, particularly in women.
Chief among these is the Mercedes-Benz 450SL convertible." --P.J. O'Rourke

Cosworth Vega is a neat engine in a disposable body. I went through a couple of them. Rust got one, a drunk old lady got the other. Both '75s. I had one spin a rod bearing and got to take it down to the bare block. The factory had forgotten to drill a hole that let oil go from the filter outlet to the main gallery. All of my oil was going through a bypass valve for a clogged filter. No wonder the filter always looked so clean after changing it. It's a miracle that it lasted 30,000 miles. Amazing how much better the pressure was after I drilled the hole. That lasted another 65,000 until the body fell apart. Sold it to a guy for the engine & trans.

Anyway, I had all the factory manuals for the beasties. The material in the aluminum block is definitley silicon, not any form of silica or silicon dioxide. The recommended hone after re-boring was abrasive on felt pads, but that was in 1975. The neatest application of that technology IMHO was the 427 "Chevy" race engines in the McLaren Can-Am cars of the late Bruce & Denny show. They make a nice rumble.

I autocrossed the CosVeg a bit. Took it to a Corvette club autocross at a go kart track by mistake but they let me run anyway. Sympathy for the "little four cylinder", I guess. I came in second overall but no trophy because it wasn't a Corvette. Buncha waxers & polishers.

__________________
Norm in NJ
Next oil change at 230,000miles

Hydra,
Now that I understand your situation and frame of mind I will have to sign off, because you are in never never land, and I never go there.

Good luck,
Peter

__________________
Auto Zentral Ltd.