Internal corrosion 250SL engine??

[anders]

Dear Friends,
Being a new member, first of all thanks for this great forum with lots of useful information to dig in.

Bought recently a 250 SL Pagoda imported from the US to Denmark.
Car in pretty good shape with interior showing expected signs of a life under the Californian sun having lived in the surroundings of San Diego. Car is almost 100% rust free being the reason I chose this one.

Decided for a restoration based on the real good shape of the body and undercarrige. Before dismantling anything, engine was started up showing quite a lot of smoke, which did not look like the oil burning blue smoke, rather black indicating rich mixture. Several adjustments was done improving the result, but still smoke over acceptable level.

Dismantling of engine without major surprises. Noticed that pistons were standard size and that the head had never been machined, crankchaft with main bearings as well as big ends looked ok...and at that point I considered myself as lucky guy...until I examined the cylinder bores where I found exessive wear both in walls and pistons.
Clearance app. 20 mm down being some 0,5 mm!!
I wondered why "the rest of the engine" did not show this kind of wear and gave corrosion a thought here. Decided for machining bores to first oversize (82,5) including new pistons.

Now to my question:
During the final cleaning of the block and head, I found something like cavities on the block surface solely on cylinder 5 and 6.


 

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Also for cyl.5 and 6 cavities were found in the cyl head



Download Attachment: Cavity cyl head.jpg
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From earlier experiencies I know that this is the picture you can find when an alian like a fragment of a worn piston ring or a valve has done it's job....but all rings and valves are in place.
These findings scares me a bit. I have read Joe's mails from 2004 with excellent illustrations of corrosion in cylinder heads, but this head is not at all in such a bad shape.
But worst of all, the cavities on the block. Anyone seen it before?
Can anything be done, and/ or does it influence to the final result?

Really appreciate any input or comments to this. Thanks in advance.

Best regards
Anders Busch

[Benz Dr.]

I've seen this sort of damage to cylinder heads before but not on a block. Usually it's caused by a leaking head gasket that lets coolant into the cumbustion chamber or moisture getting into the cylinder while sitting for long periods of storage.
Both the block and the head can be fixed if the damage isn't too severe. These repairs are done by welding on new material where the corrosion has taken place. The block would be a fairly easy repair but the head would be much harder to repair. Only a specialty shop can handle this sort of work as it's hard to do and costly.
Check to see how thick the head is before you even look at repairs. The minimum thickness is 84 mm - anything under that amount would be throwing a lot of money on a worn out and pretty much useless cylinder head.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[anders]

Hey Dan,
Looking at the history of the car, it left California in 2001 headed for Ohio where it stayed until shipped to Denmark in 2007. I believe it sat there unsused for these 6 years, but still, if corrosion, it's a lot!
I take it that you dont really like the idea of applying material, artificial or welded, in the combustion chamber. If so, I see only the following alternatives,

Machine the block to app. 0,5 below standard
(I guess pistons will protrude to the same amount...is that     possible without conflicting with the valve timing?)

Find an alternative block
Are any 250 SE/SEL/Coupes interchangeable?

Now for the head, it is untouched. The full height 85 mm is available.
My idea here was to straighten out the texture with a hand held ball cutter or similar ending up with polishing of the infected area(hope you understand what I mean here)
I understand that doing this would lower compression for these particular cylinders, but without having done the proper calculation, I believe this ought to be of minor influence.
(believe it or not, with the described clearance in the bores as well as the other here described problems, the compression test taken before dismantling showed excellent results)

Dan, am I getting too concerned here, and should forget about the injuries in the block, repair or change the head and go ahead assemple the engine with the new bores and pistons??

On the other hand, it not funny to do it all over again having neglected what was to be dealt with.

Looking foreward to advices of how to proceed!

Thanks
Anders

[Benz Dr.]

This isn't the end of the world - just the begining of a new one for you. ( lol )
The welding repairs are good fix if done propely. Aluminium can be fixed and repaired easily enough and is why racers prefer to use them in high preformance aplications.


Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[ja17]

Hello Anders,

I have seen this a few times before. As long as the damage to the block is not under the gasket area, this does not seem to cause any real problem. "Decking" the block should also help this issue. Be sure to follow factory specs.

This problem is usually caused by moisture allowed to set in cylinders during long periods of storage. The two different metals eat away at each other.  Normally most of the damage occurs to the cylinder head.

The cylinder head may also clean up enough after "cutting" to have a good gasket surface. You should have it pressure tested. In some cases the old acidic coolant will actually eat its way through the aluminum cylinder head and end up in the cylinders. The cylinder head will have pin holes in the combustion chambers and other areas will be thin and may soon fail. I had one of these "perforated" cylinder heads on and off a car three times after being welded each time and finally had to give up on it. It was a 250 series also. The head had become too weak from the corrosive action of the very old coolant. A pressure test should tell you if the head is perforated. If the head is not perforated then welding the gasket area or combustion chamber will often work.

The good news is, I do not think the block is a big concern. You can  measure the volume of  the cylinders if you are worried about it. This will let you know exactly what the difference in compression will be in the damaged cylinders. I suspect it to be within specs.

The 250 SL head is not special. Find a good used one from a 250SE sedan, they are exactly the same. Hopefully your's can be saved.

There is a new process in which a perforated cylinder head is placed in a vacuum and impregnated internally with epoxy resin. I have no experience here, just heard about it.

It sounds like your engine was a victim of bad storage and poor maintenance. Finding a good rust free chassis and body is a real plus!

Keep us up to date.



Joe Alexander
Blacklick, Ohio