617 cracked crankshaft - PeachParts Mercedes-Benz Forum

jobah · #1 03-13-2002, 04:45 PM

My father in law had some car trouble......

It appears that his chain broke and the result was that the as the pistons slammed into the valves, the valves were pushed into the rockers and eventually the rockers were slammed into the camshaft snapping the camshaft at two places (piece 1: cylinder and 2; piece 2: cylinders 3 and 4; piece 3: cylinder 5). Also, the middle bearing support (i believe between cylinders 2 and 3 is cracked.

My question is, what is the chance that there is no valve damage and no piston/crank/rod damage.

I have had the head off on my 601 a couple of times, along with the front cover, etc. , and I have no problem taking on a 61x engine.

However, before I dig in, I would like to know if the answer may be as simple as a new camshaft, new camshaft bearing supports and a new chain?????

Also, is it possible that the valves were out of adjustment that the valves did not properly close which led to the eventual destruction???? The car had some out of the blue starting problems two weeks ago, but these were cured with new glow plugs (i think the problem is unrelated, but I'm not sure).

Thank you

#2 03-13-2002, 05:17 PM

About 20 years ago a friend of mine had a timing chain break on a '69 220D, and upon really careful examination, we found no damage below the piston tops, because those cam bearing towers had all snapped before the parts in the cylinder block could be damaged, other than some small marks in the piston tops. I tend to hypothesize that when they designed the cam bearings, D-B engineers were applying the engineering practice of the "sacrificial part" -- you deliberately make one part of a chain of parts weaker than the rest, choosing one that would be cheaper and easier to replace than the others, so that if something goes wrong, the damage is thus minimized.

There are those who say that for years Ferraris had such wimpy clutches just so that clutzy millionaires wouldn't keep tearing up the transaxles. Of course, on American "super cars," the sacrificial parts were just the tire treads.

mcaro · #3 03-14-2002, 09:33 AM

jobah,

I can empathize with your FIL. I just had something similar happen to me this past Saturday. My situation was a little different. The chain rail guide?? on the oil pump let go 1st causing the oil pump chain to break, then went the timing chain, I imagine do to some debris being tossed up top or getting caught in a gear. My camshaft was broken in 3 places as well, however I did not have any camshaft tower damage. I drive my car hard and count my blessings that I was at a stop light in town and when I hit the go pedal and the engine died. If it had gone when I was on my way to work 80 mile commute at 65-75 mph, I'm sure I would have distroyed the engine.

My wrench has the care apart now. There was no damage to the crank as he pulled a bearing and everything looked OK. He was doing a leak down test to see if any valves are bent and I'm hoping to escape this disaster with timing chain, oil pump chain, camshaft. I will pass on the results of my leak down test when I hear from him and any other news when I get it.

Mark

mplafleur · #4 03-14-2002, 10:10 AM

I'm suprised to see so many cams breaking. When my chain broke at 75 mph, I only got bent valves. All but 2, I think. Just dents in the piston tops.

If the cam breaks because the lobe can't turn, I would expect that the pistons are gonig to slam up against whatever valves are still down and you are still going to end up with some bent valves.

#5 03-14-2002, 12:21 PM

I too have seen an 83 300D that snapped a chain, cracked the camshaft in two places. Must be a diesel thing.

mplafleur · #6 03-14-2002, 03:52 PM

I'm wondering if it could be a high compression thing...

It's been 10+ years since I rebuilt my 300D, and I don't remember what everything looked like. Is there clearance enough for an open valve not to get hit by the piston? I'm thinking that there isn't and that all MB engines are interference type.

psfred · #7 03-15-2002, 07:20 PM

Chances are that the crank is fine. The worst that would happen is a bent rod.

There is only 0.050" clearance between the piston crown and exhaust valve on the diesel -- doesn't take much of a lag in the valve train to cause them to hit.

The cam will break, or the cam towers will break, when the chain lets go -- the diesel valves go straight up and down, they aren't at an angle like the gasoline engines.

Replace all the cam towers, they may all be cracked but not showing it, they are cheap compared to doing in a new cam!

Peter

HGV · #8 03-19-2002, 04:04 PM

I had a similiar occurance on my 1963 190D in a blizzard on the way to tahoe. I was in stop and go traffic and the chain tensioner failed creating slop and it stripped the crankshaft chain gear and tore the chain to shreads. No vavle, piston or cam damage. I think alot has to do with what RPM the engine is when it happens and what kind of load you have. I was at idle when it happened. For me the difficult part was removing the crank gear without taking the engine apart. Inpsect the chain tensioner for sure and good luck.

Henry

mplafleur · #9 03-20-2002, 10:56 AM

I didn't realize that the valves are perpendicular to the piston face. That makes more sense now. Now the force of the piston against the valve is transmitted straight up the valve stem to the cam towers and cam.

Indirectly it is a high compression thing. If the valves were at an angle then the piston travel would have to be shorter or would hit the valve. Thus resulting in larger volume and lower compression. The valves have to be straight so the piston can get close enough for high compression.

mcaro · #10 03-25-2002, 08:31 AM

jobah,

I got my car back this past Saturday and I'm pleased with the results, although quite a bit lighter in the wallet. As it turns out there was no damage to any of the valves/pistons/rods. Total damages were $1800.

$1500 - Mechanic Labor: (I thought this high but he had to pull the crank pully which had rounded off hex bolts to replace the oil pump chain)

$200 - Fastlane Parts: (timing chain, oil pump chain, chain tensioner, front crank seal, injector seals, oil pump chain guide, oil pan gasket)

$90 - Potomac German Auto: (camshaft and towers)

Although expensive, I fell this was money was well spent. Most of the systems on the car are in good condition with nearly new parts (trans, suspension, brakes, etc). I would have been hard pressed to find another for that money not needing work already completed.

Mark

lrg · #11 03-25-2002, 02:37 PM

Is there any conventional wisdom on when to replace the timing chain as a service item? It seems there are a bunch of guys with 250K+ miles on their engines with no problems. I'd certainly replace it with a rebuild but what about before then? Any advice?

psfred · #12 03-25-2002, 10:58 PM

Irg:

Check the stretch, replace if more than 8 degrees, certainly if 12 degrees or more. Otherwise, leave it alone.

On 380 and 420 models, check often, at least every 30,000 miles, better every 15,000.

Peter

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