450SL bent valves - PeachParts Mercedes-Benz Forum

#1 10-20-2002, 09:41 PM

At unknown mileage I rebuilt the engine of my '76 450SL. It was running fine before but smoking a little. Rebuild included new timing chain, rails, timing gears, hydraulic compensators & one new cam. After rebuild, with about 5000 miles the valves on the left head bentand woodruff key sheared off on cam. I removed head and corrected the problem. After another 1000 miles the same thing happened, more bent valves on left head only with woodruff key sheared off again. Timing chain looks a little loose only at top idler gear between heads. Any ideas?? I'm getting more experienced at head removal than I ever wanted to be.

NormanB · #2 10-21-2002, 07:50 AM

Hi Radser

I know nothing about your engine so this carries health warning.

Assuming you installed correctly with correct valve timing etc- something has slipped timing chain wise - not tensioned correctly I guess.

Good Luck

NormanB

#3 10-21-2002, 08:55 AM

Something may be blocking one of the passages by which oil gets up to the cam bearings; the result is that the camshaft seizes in its bearings and shears the Woodruff key. On Volvo B21 motors it's almost inevitable that the oil passage in the head will clog eventually if someone has not changed oil frequently enough or used low quality oil, but it's possible that, in your case, an incorrect or incorrectly installed gasket is blocking the oil flow. On M-B inline engines of that era (I've never worked on 4.5 V8), a misaligned cam bearing/stand (they must be replaced in matched sets) could also cause cam seizure, but that would surely have been detected on assembly by any competent mechanic. A warped head will cause misaligned cam bearings on many OHC engines -- if the head is planed back to flat, the cam bores must be align-bored oversize and an oversize cam fitted.

The Volvoes of that model, of course, have a non-interference motor with a timing belt, so when the cam seizes it just tears a few teeth off the belt at the crankshaft end and the car coasts to a stop with no further damage, but people often just replace and ruin a number of new belts before they finally realize that something else must be wrong.

autozen · #4 10-21-2002, 11:17 AM

Rich,

Since you only have the problem with the left bank, I would rule out machining errors like valve face to parting surface depth. The only other possibilities would be incorrect VALVE timing, malfonctioning chain tensioner, or stretched chain. When a stretched chain on an MB V8 starts whipping, it breaks the plastic guide on the left bank which climbs up the chain and causes the cam gear to slip on the broken piece of plastic. It's a classic problem. It happens all the time. Every 47 minutes this happens some where in the world. Even though you replaced the chain, what is the quality of the part you bought? If the chain is ok, I'd look at the tensioner next.

Good luck,
Peter

#5 10-21-2002, 11:30 AM

Even though the chain is new I suppose it could have been stretched during the first cam siezure episode. In this repair I am changing the tensioner as it clearly is not doing the job. If chain appears loose with new tensioner I can assume a stretched chain?? None of the guide rails were damaged either time. Perhaps that points to some other fault. Is it possible for the cam to sieze at one of the bearings and not show any visiable sign of that during dissassembly and inspection? During original rebuild the oil duct in head was cleaned, rodded out and washed with solvents.

autozen · #6 10-21-2002, 11:54 AM

Rich,

VEEERY interesting. The fact that the rails were noy damaged is a very important clue. The chain would not have been stretched the first time. Based on the new info plus the fact that the offset keys are shearing off points me in another direction. In the scenario I played for you, the keys are never sheered off. I suspect the cam is binding. If the head was overheated and warped on the top side, it could cause the problem. You could also have a bent cam. Seen it on a 560SEL. A professional mechanic constantly checkes fit and ease of movement during assembly jobs. With the head torqued and the cam bolted down, you should be able to rotate the cam easily with your hand. No pliers should be needed.

Peter

Behind the Star · #7 10-21-2002, 03:47 PM

You did reinstall the oiler tube across the top of the cab towers right? Just a thought but i'm sure you did.
Brian

#8 10-21-2002, 06:23 PM

Yes, I did reinstall the oiler tube across the top. I also cleaned out the oil tube in the head and checked for flow from the oiler tube. On the first rebuild several rocker arms were quickly worn down due to lack of oil. Cleaning head oil ducts corrected that problem. I just called the machine shop that installed the new valves this second time and they did check the head for warpage and there was none. I tried turning the left cam by hand and cannot turn it. I can turn with BIG pliers. Right cam turns much more easily. I was assuming problem was the valve springs I was compressing. Head and cam towers are all bolted down now. So, this could be a bent cam??? Can they be straightened??

autozen · #9 10-21-2002, 11:25 PM

Rich,

You have to check the cam ease of rotation without chain or cam followers installed. I don't think you can straighten a cam,but I am not a machinist.

Peter

psfred · #10 10-23-2002, 08:31 PM

I would suggest that the cam towers are damaged from lack of oil, or less likely the cam is damaged. This is the classic failure to the MB V8 cam from lack of oil in the tower bearings -- they are just aluminum, no "bearing" like an American cam, and the aluminum will gall when not lubed, reducing clearance until the key shears or the cam breaks.

Have a machine shop check the cam for straightness, and replace the cam towers. You may want to replace the cam, too, just to be on the safe side. Once one goes, the other isn't far behind, usually.

Peter

#11 10-25-2002, 12:08 PM

Thanks to the info supplied by several members I believe I know the solution to this problem and there is a beautiful symmetry to it. It explains everything. The two mechanics who rebuilt the engine have rebuilt hundreds of engines, but never a MB V8. They did not know and I did not know that the camshaft bearing towers are a matched set. They almost certainly mixed the towers. This would explain the sheared cam key. I had been thinking that the timing chain would explain the sheared cam key. when the valves bent, it would shear the cam key, but that is unlikely to be true. More likely, when the valves bend they stop the engine and with the automatic xmsn I never knew the engine had stopped. With the engine stopped it could not shear the cam key. Soon as I get back to the car I will check it out. Maybe there is a mark that shows which set the towers belong with.

psfred · #12 10-25-2002, 08:21 PM

Match the oil holes in the head to the cam tower -- the right and left rear towers MUST NOT be swapped -- result is no oil to one of the cams, left I think, but???

That a breaking the plastic connectors for the drip tube are the leading causes of short-lived rebuilds!

Peter