M116 Alloy Block Bore Condition

[TJ 450]

I thought this might be relevant to anyone with a 380/420 500/560 engine in their 107.

It may be a long shot, but in particular, those who have delved into the alloy engines this far.

I have torn down the cylinder heads for evaluation, off my W126 380SE's M116.963 Aussie spec low compression motor. Of course, now is the time to investigate whether the bottom end needs attention.

I got this car for nothing and seeing as the body is practically rust free, I would like to use this car as a daily driver.

On an alloy block, this invokes visions of large wads of cash if anything relating to the bores needs attention, due to the special etching process involved.

So, I have noticed for a start, that the all the exhaust valves have previously made contact with the pistons on the LH bank. This wasn't too much of a surprise, but fortunately those valves were replaced (with Italian OSVAT valves) circa 1996, along with the timing chain and rails. This wasn't done very well, as none of them [the valves] were seating properly allowing compression leaks, but nonetheless, those valves are still good.

At the moment, the verdict with the heads is that I need to have the guides renewed and the the valves lapped properly.

Now, this all depends on whether the bottom end is serviceable.

I have noticed a slight lip on the bores and that is my concern. If I run my finger over it, I can just feel it. Is this a concern?

Here are some pics...

By tim_w116, shot with DMC-FX01 at 2009-07-20

By tim_w116 at 2009-07-20

Tim

 

[Styria]

Hi TJ450, I have had no experience with any of the 116 nor 117 engines, whether alloy or cast iron, so any comments would be purely on a 'what I think I know basis'. In my opinion, obtained from examination and working on the M100 (6.3 and 6.9), as well as Rover 3500 engines (alloy block - steel liner), ridges in bores are, as a rule, non existent. Gleaming Beauty's engine, with probably 370k. kilometers on he clock, was ridge free. More so, rather than anything else, piston ring wear is a more likely shortcoming - I guess nothing lasts forever.

Therefore, I'd be vary of the 380 block you have on hand - either the mileage is phenomenal. or the engine has possibly suffered from overheating problems, all round neglect (lack of oil changes), and it would be chancy to close up the engine and hope that it is going to run satisfactorily. Regards Styria

 

[TJ 450]

It is possible (even likely) that the engine was overheated. Two of the head bolts were snapped (before I applied any force). Neglect is also an issue... the valvetrain was quite sludgey, plus the oil was like tar, possibly burnt. There was coke like deposits around the camshaft area on the LH head and in the return oilways.

I will not rule out this block as scrap... a good used engine should be easy to obtain. It all depends on economics.

However, I don't think the bore wear is much of an issue... the ridge can only just be felt and there is no scoring of the bores suggesting severe overheating.

I'll see if I can get some "expert" opinions before tearing down the bottom end.

It has done an indicated 230,000km, although that is no indication of true mileage... odometer failure is not uncommon.

Tim

 

[TJ 450]

I decided that I was not going to take a chance with the bottom end, so I've stripped it down. I didn't find anything too unexpected, although both compression rings on no.2 were seized and emulsion was present when I broke them free.
All the main and big end bearing shells are a little worn and it is obvious that some foreign particles have gouged a few of the big ends. The crankshaft appears excellent. The bores are a bit worn, with some corrosion from where the rings were sitting... obvious depressions on a couple of cylinders. One cylinder has a couple of small nicks out of the bore about mid way down.

I will clean everything up and go from there. If I so wish, rings and bearing shells are available at Autohaus.

If I can get the block 100%, I should be in business. If not, this engine will probably become spare parts.

By tim_w116, shot with DMC-FX01 at 2009-07-21

By tim_w116, shot with DMC-FX01 at 2009-07-21

By tim_w116, shot with DMC-FX01 at 2009-07-21

By tim_w116, shot with DMC-FX01 at 2009-07-21

By tim_w116, shot with DMC-FX01 at 2009-07-21

By tim_w116, shot with DMC-FX01 at 2009-07-21

By tim_w116, shot with DMC-FX01 at 2009-07-21

Tim

 

[John S]

In the dim dark ages when bore wear was normal, before synthetic oil was even thought of, water injection and bug deflectors were the latest inventions, a stepped top piston ring was available to ensure the top edge of new rings stopped short of the ridge.

I don't know if these rings are still available, but they may be worthwhile using if you decide to stay with that block (assuming the bore marks you mention are very minor). The rest looks like it will clean up OK, so if the block gets thoroughly cleaned out and you use a high quality oil there should be a lot of miles left in the engine.

 

[TJ 450]

The stepped rings are available for the cast iron block engines... I'm not sure about the alloy blocks.

There is an ex- M-B bloke here in Perth who is very knowledgeable on these engines. I took the block to him for inspection and he tells me all it needs is a light hone using the approved method.

The honing precess basically involves cutting back the surface, polishing and then applying a special silicon lapping paste to the surface. This allows for the aluminium to receed, leaving the silicon crystals sitting proud of the surface on a microscopic level. This leaves the original dull grey surface appearance and texture, which allows the new rings to bed in properly. There will be no need for stepped rings, because the very slight ridge will be removed in the honing process. The wear limit is 0.2mm.

From the factory, the block allows for one 0.5mm overbore, otherwise sleeves will have to be installed. The reason for this is that the density of silicon crystal in the structure is greater at the bore walls than elsewhere in the casting.

Contrary to popular belief, a surface hardening "coating" such as Nikasil is not used in these engines. The alloy is cast with a high silicon content, thus the hardening technique is integral to the block itself, although the highest concentration of silicon is at the bore walls, as stated before.

Tim

 

[John S]

Thank goodness for the lapping paste technique, in the original manufacturing process a chemical etching process is used to leave the silicon proud.

 

[Styria]

You guys are just too brilliant for me - however, looking at those bearing journals, that crank looks okay. Probably new bearings would not go astray (presumably standard), but the cost of those alone may make one think twice before choosing the most economical option. Regards Styria

 

[TJ 450]

The bores were indeed originally etched. I'm not quite sure how the lapping paste actually works, perhaps it is acidic.

I'm going with your observations too, Styria. I think that the bearing shells suffered from the lack of oil changes in that last decade, but the journals show little, if any damage.

The cost of the bearing shells and rings is a good part of 1k.

It's still early stages, but I think I will go ahead with the rebuild. The chassis is very solid and straight, with many years of use remaining, as long as I prevent the ingress of water.

Tim

 

[Styria]

Hi TJ450, there is no doubt that I have to admit, and appreciate, that you are imparting some very sound and informative facts that hitherto I had never really appreciated, nor understood, fully. I have subconsciously always felt that I needed to steer clear of the 380 and perhaps even the 500/560 engines, especially when knowledgeable people start bandying about figures in the thousands of dollars as far as rebuilding costs are concerned. I dips my hat to you, and at the cost of parts quoted, whilst not particularly cheap, you seem to have a feasible project in the making. Good luck with it, and keep us posted. Regards Styria