Author Topic: 6.9 engine seized while cranking  (Read 2511 times)

raueda1

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Re: 6.9 engine seized while cranking
« Reply #60 on: 27 September 2019, 06:36 PM »
Update:

The engine is now pretty well stripped apart from removing the crank.  Toady I made an interesting discovery after pulling the pistons for carbon removal and inspection.  The engine has indeed been rebuilt.  Maybe that's why it was so clean, though clearly it wasn't all that recent given the carbon on the pistons. 

The pistons are +0.5 oversize, code 75.  This corresponds to 8.4:1 compression instead of 8.8:1.  That makes it a demi-Euro engine in my mind, so I'm annoyed.  The main difference between the versions is mostly the depth of the cavity on top.  From what I make of the specs it looks like MB progressively reduced the euro engine's compression over time.  I wonder why.  Too many knocking issues?  Declining fuel quality at the time?  Who knows.  Maybe somebody can cheer me up with news that 8.4 is actually pretty close to 8.8, so no big deal.  ::) 

Haven't checked bearing journals to see if they messed with that (need a suitable micrometer).  There was an old mouse nest in the V between the heads and under the intake plenum.  Slowly progressing.......              Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Randys01

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Re: 6.9 engine seized while cranking
« Reply #61 on: 28 September 2019, 01:22 AM »
...not surprised....I thought the minimal scuff on the piston skirts suggested low mileage= rebuild.
When you pop the big end shells out you will see on the back if they are oversized or stds.
The 6.9 variant of the M100 saw dramatic modifications compared with the 6.3....As I recall there are 4 compress ratios and some of these also saw a change in gudgeon offset.
In Aust we got the so called 8 :1.  Introduct Down-under was towards the end of production. A mate of mine has a lively Euro import aka 9:1.no smog etc. Seat of the pants, there's nothing between them but I reckon I reckon mine goes very well for its spec. [ I have spent hours finessing it ]
So 8.4 or 8.8...if there ever was any discernible difference, the gap narrows as the old dears start to wear.

raueda1

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Re: 6.9 engine seized while cranking
« Reply #62 on: 28 September 2019, 09:01 AM »
...not surprised....I thought the minimal scuff on the piston skirts suggested low mileage= rebuild.
When you pop the big end shells out you will see on the back if they are oversized or stds.
The 6.9 variant of the M100 saw dramatic modifications compared with the 6.3....As I recall there are 4 compress ratios and some of these also saw a change in gudgeon offset.
In Aust we got the so called 8 :1.  Introduct Down-under was towards the end of production. A mate of mine has a lively Euro import aka 9:1.no smog etc. Seat of the pants, there's nothing between them but I reckon I reckon mine goes very well for its spec. [ I have spent hours finessing it ]
So 8.4 or 8.8...if there ever was any discernible difference, the gap narrows as the old dears start to wear.
Hmmm.  I can't make heads nor tails of the bearing designation "100 3110 KZ".  However, they're also stamped 11-75 which, per Mahle's system, suggests that they were manufactured in Nov 1975.  If so then perhaps they're original.  The car is #923 and was built in April 1976 but first licensed in the USA in 1977.  Engine is also original, matches the build card.  So, this chronology seems to make sense.   And if so, that suggests that somebody did a piston job at some point but left the big ends alone. 

Or maybe not?  I guess there's the possibility that they are oversized and were on the shelf for some time before the piston job.  The bearings are all in good shape.  The shop guy actually commented on how good they looked.  The other pistons all look good as well.  Obviously the piston job was done when pistons were still available - when was that?  Given low wear I'm guessing that the car got little use after that job.  Now I wonder what necessitated piston replacement.  I can't wait to see what surprises are lurking with the heads!  ::)

Well, all this stuff is fun I guess, but doesn't really impact the task at hand.  I'll measure the crank soon enough.  In the meantime there's a lot to inspect, clean, carbon to remove, stuff to repaint, plate, reassemble etc. 
Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0