W108 Engine Rebuild - Mercedes-Benz Forum

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post #1 of 6 (permalink) Old 06-29-2014, 03:17 PM Thread Starter
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Date registered: Jul 2012
Vehicle: 1967 250S, twin Weber carbs, automatic trans, power windows, A/C, R134a conversion
Location: Texas
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Angry W108 Engine Rebuild

Well I thought I could make it until this winter to begin an engine rebuild but I just ran a compression check due to excessive black exhaust smoke and found the wet compression on the best cylinder to be no more than 114 psi. The worst cyl (#2) was only 96 psi wet and 72 psi dry. Curiously the spark-plug for this cyl had the tip bent to the point of contacting the electrode. I replaced all spark plugs and ran the engine during the re-tune and carb balancing. Pulled the plug on the low compression cyl and again found that the tip had closed up a bit. My best guess is that the piston/rod assembly is moving excesssively and about to let go. An independent MBZ repair shop reviewed the operation prior and felt strongly that the noise the engine produced was not related to a rod/bearing problem. I thought it was a main bearing/rod issue and the current inspection/symptoms seems to support this direction of diagnosis. The carbs are producing a dry black soot on all of the plugs but adjusting the mixture and balancing the carbs did not correct this condition. With over 300k on the engine I'm thankful the old girl runs as well as she does. She's been my reliable daily driver since 1982.

So the 1967 250S (108.920.12.01.5279) engine rebuild begins. I'll keep everyone posted as I progress along. The part of this I dread is lying on the concrete floor, but I'm excited about the opportunity to drive another 250K miles in a classic that constantly gets public attention. I'm also glad my wife is undestanding about my love for this vehicle, and the fact she won't be using her garage parking spot for a long time to come.
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post #2 of 6 (permalink) Old 06-29-2014, 03:36 PM
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piston cannot hit plug . secure oversize pistons before , you may be surprised .also bearings , if crank is fine use standard .
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post #3 of 6 (permalink) Old 06-29-2014, 03:53 PM
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Vehicle: '71 250, '78 450SLC 5.0, '78 450SL, '81 380SLC 5.6, '89 260E, '15 Kia Sorento
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I'm looking forward to following your progress. Hope everything goes smoothly.
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post #4 of 6 (permalink) Old 06-30-2014, 05:53 AM
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Vehicle: 1968 250S, 1991 300se, 1998 SL500, 2x 1977 450SEL 6.9. Past MB's; 300SD, 300E, 300TE, 190E, ML420
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Sounds like it is time. Keep us posted.
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post #5 of 6 (permalink) Old 06-30-2014, 06:01 AM
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Date registered: Oct 2002
Vehicle: 1967 250 SL, 1963 220Sb
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Lying on the concrete floor shouldn't be that much time... I found that the removal of the engine/trans from my 280S and install into 250S wasn't that horrible and mostly above the car. Nice that you have webers. I had those on my 250S. Are you going to increase the displacement? Or is that taboo on MB engines...

Having the engine / trans out for a period of time makes it easy to access other things you might want to replace. Be careful not to leave the car suspended in the air for a long time with front wheels unsupported - not good for the subframe mounts.

Currently: 1967 250 SL, 1963 220Sb, 1965 300SE Lang, 1971 280SE parts car, 1972 Alfa GTV, 1965 Alfa Duetto, 1993 BMW R100R
Past: 1971 250 C, 1985 300 TD, 1967 250 S, 1968 280 S,1981 300 D, 1982 280CE, 3 Facel Vegas, Borgward Isabella Coupe, Alfa 2600 Sprint, Volvo P1900 (yes), numerous less interesting Volvos, ...
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post #6 of 6 (permalink) Old 07-04-2014, 09:14 AM Thread Starter
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Date registered: Jul 2012
Vehicle: 1967 250S, twin Weber carbs, automatic trans, power windows, A/C, R134a conversion
Location: Texas
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Not planning an increase in displacment. The factory rates the engine at 147 hp @5200 rpm. Not much to expect with added cc's and stock I/E. I do enjoy the twin Webers though. Such ease in tuning compared to my old Zenith's.

Anyone out there having trouble with the stock aluminum torque converter? Mine split in half many years ago and I replaced it with a domestic built one that saved me big $$$. It has performed well since then but I was thinking I would go back to stock since I'll have the drivetrain out for the engine rebuild.
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