Could someone explain to me how the hydraulic lift compensators work (HLC)? I'm considering replacing a few or all of mine on a 1989 420 SEL. I have a few really slow moving HLCs and I think they maybe filled with sludge.

As I remove a HLC, I find sludge in the bottom of the main HLC hole. Is there a small oil inlet hole somewhere in the bottom of the main cavity where the HLC is screwed into? I see a small oil inlet hole at the top of the main HLC cavity, but is there one at the bottom too?

Also, what happens if the thrust washer is too big? Does it prevent the valve from closing completely?

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1989 420 SEL

At least on the M116 that I have apart, there is only the one hole near the top of the hole where you screw the HLC into the head.

The HLC is pressurized by an oil flow through that hole and the one in the HLC itself. And this pressure holds the rocker against the cam and provides a bit of cushion as the valve opens, keeping things quiet.

The thrust washers come in seven thicknesses starting at 3.7 mm and going up to 6.15 mm in .35 mm increments. So that is a difference of 2.45 mm. The measuring device has a 1.2 mm groove and you are supposed to be in the top .6 mm of it. All of this to say that IMHO if you are more than two sizes off on the thick side you would probably be keeping the valve open.

BTW, measured the cam lift while I was looking at the hole. About 6.5 mm, so the adjustment potential is about 40%, surely enough to keep the valve open if it's really screwed up.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Thanks Chuck, I sincerely appreciate your reply.

So now I understand that if the thrust washer is too thick, then the valve will be open slightly. So if a compression test is done, on that cylinder, and the compression is lower than the other cylinders, it could well be caused by the washer being too thick, right?

Also if a leakdown test is done, one may hear air escaping through the exhaust, if the exhaust valve has a washer that is too thick? Is this correct?

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1989 420 SEL

Not sure if you would hear it unless you had the manifolds off, would depend on the volume I guess.

If the washer is too thin the valve will open slightly less and will be noisy.

I had seen someone posting that they had built their own tool, can't find that thread though.

Matt.

I do have a go-no-go guage, from Sir Tools, that I am using to determine if the washer is the appropriate thickness. It is showing that all of the existing washers are too thick. If I put in a new HLC, the correct thickness washer is 4.05 mm, according to the guage.

I'm still wondering what a leak down test and a compression test would show if the washers were too thick and the valves did not fully close?

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1989 420 SEL

If either valve is not closing completely, you will get zero compression, and there won't be anything to leak down.

Recommend that you rotate the engine 5-6 times and let it sit overnight, then re-measure.

Herr Fuchs likes 'em in the top half of the groove.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe