Cam Mark Confusion - PeachParts Mercedes-Benz Forum

mbboy · #1 12-22-2008, 04:05 PM

After replacing the chain, guides and tensioner on my 81 380sl, the notch on the right cam is about a 1/3 inch or so before the mark on the right cam tower when (a) the harmonic balancer is at zero, and (b) the notch on the left cam tower lines up perfectly with the mark on the left cam.

I was always under the impression that the cam notches should line up perfectly with the cam tower marks. But a friend, who's been working on MB's for quite a while, just gave me a copy of a page from what he thinks is the Haynes Mercedes manual stating: "Due to the design of the chain tensioner on V8 engines the right side of the chain travels further than the left side. Thus the right-side camshaft must be seven degrees retarded compared to the left side and both timing marks CANNOT be simultaneously aligned."

Now, if a chain link is roughly an inch, and that corresponds to app. 20 degrees on the balancer, seven degrees would be about 1/3 in. on the cam, and my cams are timed correctly. OTOH, if the Haynes manual is correct, 107 engines where both cam notches aligned perfectly with the cam tower marks when the balancer is at zero are actually off? I dunno ??

Gurunutkins · #2 12-22-2008, 05:30 PM

Hi mbboy
I am not 100% certain on the 380sl but assume its the same as all of the others and the timing marks have to line up. I am pretty sure your friend is wrong. I have replace the double chains on all of the other types of engines but never a single chain on the 380. I have never specifically looked at the RHS cam but generally they always seem to line up after replacing a chain. maybe i am wrong

here is a great article by our moderator on changing the chain (not specifically for a 380 though) in the diy section of this forum. hope it helps

http://www.peachparts.com/Wikka/M117TimingChain

cheers
barri

Strife · #3 12-22-2008, 07:55 PM

The bottom line is, with the chain BETWEEN the cams taut, with the crank pulley AND the distributor at the zero degree notch, you should be very, very close to those marks. 1/3 of an inch (not sure what this translates to in degrees, but...) is a lot, and retarded, too, which makes it seem like the chain SHRUNK(!).

Did someone put in an offset Woodruff key before you got this car? This would be the sort of thing to do to compensate for a worn chain. With a new chain, it would seem that the chain would come up short!

But I defer to others more expert...my double-chain 380 was right on the marks when I did this. I know the Woodruff keys were not offset because I looked at them when I changed the gears.

Another idea, those marks are known to be "not entirely" accurate...

mbboy · #4 12-23-2008, 01:48 AM

Quote:

Originally Posted by Strife

The bottom line is, with the chain BETWEEN the cams taut, with the crank pulley AND the distributor at the zero degree notch, you should be very, very close to those marks. 1/3 of an inch (not sure what this translates to in degrees, but...) is a lot, and retarded, too, which makes it seem like the chain SHRUNK(!).

Did someone put in an offset Woodruff key before you got this car? This would be the sort of thing to do to compensate for a worn chain. With a new chain, it would seem that the chain would come up short!

No, the Woodruff keys are definitely original, and the cam marks lined up perfectly when I replaced the chain once before. However, I just read somewhere that shaving the heads can throw the marks off, and I think the machine shop I used to replace my valve guides may also have shaved the heads. So I'll check with them in the AM. However, it would seem that shaving the heads would make the chain appear longer, not shorter, because the heads aren't as high?

I'm still wondering, though, why the Haynes manual says the right cam should be 7 degrees retarded, which my right cam is?

Gurunutkins · #5 12-23-2008, 10:17 AM

if the heads were skimmed then the chain would be "too long" and the timing mark would be to the right of the right cam (looking from the front) with everything lined up on the other pointers. I have added a small picture of the chain rotation, remember the cams rotate left to right or clockwise and the first "pull" is on the right cam (not the left) from the crank. I don't know where yours is as you say before and I don't know if you remembered that the right cam rotates clockwise too

ctaylor738 · #6 12-23-2008, 10:22 AM

I agree, something is wrong. They have to line up. I have no idea why Haynes would say that. These are assembly marks, designed to keep the line moving, so it has to be very simple. You can't have someone on the line eyeballing seven degrees.

I think you may need to go a tooth forward on the right, then put the tensioner in to pull it back.

mbboy · #7 12-23-2008, 10:51 AM

Quote:

Originally Posted by ctaylor738

I agree, something is wrong. They have to line up. I have no idea why Haynes would say that. These are assembly marks, designed to keep the line moving, so it has to be very simple. You can't have someone on the line eyeballing seven degrees.

I think you may need to go a tooth forward on the right, then put the tensioner in to pull it back.

Thanks for the replies, guys. I can't figure out why the Haynes manual says that either, but I've been thinking along the same lines as you Chuck: I need to move the right side cam one link to the right, since the difference between the right cam notch and the tower mark seems to equal that of the two studs separating one link from another.

OTOH, an archive search turned up the following picture of a right cam that strikingly resembles mine: http://i19.photobucket.com/albums/b177/wb300se/MB_420SEL/DSC_0132.jpg

However, the poster seems to have fixed it by rotating the left cam back a tooth to match the right cam, though he didn't explicitly say that. Anyway, I'll try moving the right sprocket first and see where it gets me before I try moving the left side. Thanks again for the replies.

Strife · #8 12-23-2008, 08:37 PM

I hope you plan on gingerly spinning the engine by hand (twice) before completing reassembly and starting it!

This is why I like to remove the RH lifters - removes 1/2 of the things that can go terribly wrong...

mbboy · #9 12-25-2008, 10:09 AM

Actually, it's a good thing the marks were off, because closer inspection revealed that I had apparently reversed the notched washer on the left cam in a way that caused that cam to be 180 degrees off; i.e., when the cam lobes were up at TDC for piston 1, the lobes over piston 6 were also up when they apparently should have been down.

I managed to fix both problems by removing the tensioner, both sprockets, flipping the washer, and lining the cam marks up with the crankshaft zeroed. I then pulled the upper portion of the chain tightly over the sprockets, and reinstalled the tensioner --which quickly took up any slack that was left in the ascending section of chain on the right side. So the marks remained aligned after repeatedly turning the crank by hand.

As for removing the right side rockers first, Strife, you apparently didn't realize that i'm the same guy who posted the question in the tech section on Rocker Removal and Repositioning, because I let a seemingly experienced MB mechanic-friend convince me that it didn't make any difference where they went.

So, any advice anybody in this forum can offer on solving this potential problem also be appreciated.

I would also like to say thanks again to everybody in this forum for all the help you've given everyone over the past year, and offer my best wishes for a joyous holiday.

Strife · #10 12-25-2008, 11:43 AM

Because I have been known to be careless and forgetful, I take pictures of stuff before I take it apart. On my last chain/gear change, I took pictures of where the cam lobes were pointing at the zero marks before I started work. When I finished, I compared my pictures to what I was seeing, and I was OK. Gave me a lot of confidence. I think a "junky", earlier-generation digital camera belongs in every toolkit.

mbboy · #11 12-26-2008, 07:37 AM

Quote:

Originally Posted by Strife

Because I have been known to be careless and forgetful, I take pictures of stuff before I take it apart. On my last chain/gear change, I took pictures of where the cam lobes were pointing at the zero marks before I started work. When I finished, I compared my pictures to what I was seeing, and I was OK. Gave me a lot of confidence. I think a "junky", earlier-generation digital camera belongs in every toolkit.

I agree. In fact, I took a number of pictures, just none of the cam lobes. And the guy I was working with was so confident of his ability to put things back together correctly, he was taking things -- including the valve train -- apart so fast I couldn't take pictures of them. So I suspect I may be back here again asking for help. As most of us have already realized, this forum is an invaluable resource.

Until then, best wishes to everyone for a Happy, Healthy and Prosperous New Year