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1966 230S, 1991 560SEL
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51 Posts
Discussion Starter #1
My 91' 560SEL is burning about 2 quarts of oil every 300 miles, so I am preparing for a head rebuild. A very nice member of this forum @Stutz has already loaned me many tools that I need and gave me USED heads from an 87' 560SL w/ ~100k miles.

How do I best clean the gasket mating surfaces, and clean the intake and exhaust ports? How do I clean the oil/ water passages in the head? I plan on doing this during Spring Break, so I have a bit of time to prepare for this big job. I was wondering if I should polish the combustion chamber as AMG did, and maybe even do a small Port job. For the valves, can I just use valve grinding paste, or should I go to a machine shop and get the 3 angle valve job? If the valve guides need to be done, I have read that the guides need to be frozen with liquid nitrogen and the head heated, is this true for the M117? Any help/ input appreciated.
I am trying to do this on kind of a budget since I am still in High School and I am broke.

Thanks
 

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Registered
1988 560 sec
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685 Posts
I like the cut of your jib but broke and replacing the heads on a m117 does not compute. The oil burning is the valve seals which I believe can be replaced with the heads in place. This would be my course of action.

If you replace the heads you should helicoil all the threads of the head bolts which is a job in its own right. Done right by a mechanic this is quite an undertaking. Just don’t bite off more than you can chew. If you choose to swap the heads have a shop look them over better safe than doing it twice .

Best of luck .
 

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1966 230S, 1991 560SEL
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51 Posts
Discussion Starter #3
The valve stem seals were done by the PO maybe 20k miles ago. My car is at a 192k miles, and at 200k miles one is supposed to renew the valve guides. I already have the jig to helicoil the block.
 

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Moderator
1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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4,290 Posts
Hi S.C.,

That's a lot of oil usage, I agree.

Could it be that one or more of the valve stem seals were damaged, incorrectly installed? Have you looked at the plugs to see which cylinders are affected? [The signs would have to be patently obvious with that rate of oil loss.] I say this as I would not expect valve seals to wear out with but 20K miles of usage, you know?

If one/two plugs show oil contamination, why not address just those cylinders with new seals and see what happens? This theory, of course, rules out any problems with piston rings which a wet/dry compression test would diagnose accurately.

Should you go forward with this, please refer to Stutz's post on "valve stem height relative to head surface", making SURE the replacement heads are set up with exactly the same measurements as those on your car right now.
Even a simple 'lapping the valve surfaces' will change that distance appreciably.

I am sure he has stressed the need for this to you, but its importance deserves merit here.

MBL
 

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1988 560 sec
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685 Posts
A few helpful hints .
1 get a straight edge and feeler gauges to check the flatness of the heads and block . If they are out of spec they need to be made right .
2 its a good time to replace the timing chain and guides if they haven’t been done recently.
3 if you decide to rebuild the heads youself lapping valves is not difficult, when the valves are out you can use gauge pins to check the valve guides for tolerance. Make sure after lapping you remove all traces of the compound and check for a good seal.
4 if the budget allows you must replace all the intake manifold gaskets and fuel injector seals and o rings .
5 there is a great top end rebuild how to on the forum called 500e in the w126 section , Read it he goes over many of the problems and solutions you will run into.
6 pull the motor to do the job. I know that sounds extreme but everyone who does the job wished they did.
7 label all the parts and keep things organized.

Best of luck
 

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Premium Member
1999 500SL, 1988 SEC
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1,163 Posts
Well you asked about cleaning the head surfaces and chambers. Take out all the valves. Remove all old seals. Number the valves and the springs. Better yet put each valve in a bag and number the bag. Get a brass cup brush for your drill and a flat grinding wheel like you use on a grinder. First use a paint scraper with a sharp edge and remove all the old gasket material on all gasket sides of the head. Put rags into each cylinder so dirt does not go down it. Scrape all gasket meterial off the surface. On the now semi cleaned surfaces take the cup brush and clean thechamber, the gasket surfaces and the block. After you are satisfied they are clean take your grinding wheel (moderate grit) and remove the paper on the side. Slowly take the wheel which is on its side and in a circular motion clean upthe sealing surfaces. Move the wheelover a large section so you remain uniform. You can also take the heads to a machine and just have them surfaced. You then can just grind the valves by hand.
 

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1966 230S, 1991 560SEL
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51 Posts
Discussion Starter #8
The spark plugs of cylinders 1, 3 and 6 are wet with oil. I am broke, but this car is still my baby. I am getting all new seals, gaskets, etc. The Timing Chain and guides were done by the PO around 25k miles ago. I just did new fuel injectors, seals and vacuum tubes maybe 2k miles ago.
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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4,290 Posts
In addition to other's offerings above, Take LOTS of photos too. Document your process.. beginning so you can remember what things looked like, where they were in relation to each other, etc.

A notebook & pen/pencil.. Ziplock bags & a marker will be good friends to have along too.

M.
 

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1988 560 sec
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685 Posts
Note of caution!!

If you choose to use a steel scraper to remove gasket material on an aluminum head and block be extremely delicate. One wrong slip and you can ding a cylinder wall or gouge a mating surface in a split second and the parts may never seal properly.

A brass scraper and scotchbrite is a safer solution when working on aluminum engines.


This is a must read.

Best of luck
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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4,290 Posts
The spark plugs of cylinders 1, 3 and 6 are wet with oil.

The Timing Chain and guides were done by the PO around 25k miles ago.
I am suspicious of the previous work.

If you have mentioned previously, forgive me, but has a compression test been done to evaluate this condition?

Thanks.

MBL
 

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1966 230S, 1991 560SEL
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51 Posts
Discussion Starter #12
I disassembled and cleaned the carbon buildup from the passenger side cylinder head this morning. I will do the mating surfaces tomorrow. What scotch brite is recommended, the maroon or green? I did a compression test 2 weeks ago, and had consistent readings, with 140 psi being the lowest and 150 psi the highest. Also, I have receipts of the parts for the car from the PO, and the timing chain guide is looking nice. So I do trust him.
 
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