Timing chain guide rails - WHY replace these?
 

I consider myself a logical person.
I know the plastic timing chain guide rails can snap. Before examining, touching, molesting, or otherwise disturbing my current rails, I ordered new rails figuring mine were probably about to snap.

BUT... WHY would I want to replace my metal-backed timing chain guide rails, which look pretty damn strong and reinforced (and don't show too much wear for 140K miles) with a 100% plastic rail?

Seriously - I don't see the logic in this! The new plastic guide rails are about 0.5cm longer, but I think the original ones are about 1mm thicker.

Can anyone give me a VALID reason to replace my original, metal guide rails with plastic ones that are likely to snap? I'm not really LOOKING for an excuse to put a 5.6 in my Benz quite so soon!

:confused:

Tomguy, the only thing I can think of is that the plastic replacements are not real M-B parts?

If they came from the dealer or similar, then another explanation is necessary- maybe the length difference you describe is more important than the material the whole thing is made of- mine were done just before I acquired my car, so I have no idea what they put in there.

I like your tag line, hooked on 108's- I'd have to say I am right there with you-

JAS

I got the new rails from mbz.org - they're Swag, supposedly OEM. The rails that were on it appear original as far as I can tell. The plastic is very, very dark - but the metal backing appears fine. The metal has "4,5" marked on the back of it. It also has a "Wire" going through the center of both holes to secure it to the stud/pin (the stud has a groove in it). The wire is riveted in on either end about 1cm past the mounting holes.

I haven't owned a V8 Mercedes but from what I've gathered from following this Forum the past 3 years, the later V8s (Late '70s-early '80s?) came with the all-plastic chain rails and it's those engines that have problems with them. A few years ago, I did a single to double chain conversion on an '82 380SL and that one had the plastic rails, which were replaced with wider plastic rails as part of the conversion. I've never had any of the aluminum backed hard rubber rails break on my 4 or 6 cyl Benzes, though they may have been very worn. V8 owners here may have had other experiences.

Happy Motoring, Mark

Here is the reason I change them:). Took me quite a few shots to get one that was good enough to post. This is the oil pump chain tensionser from a '89 300SE - showing 155k miles.

Well, the rail I pulled shows nowhere near that much wear. My tensioner rail is pretty bad (looks about like that) and I do plan on replacing it, but as for the other guide rails - why replace a metal-backed piece with a solid plastic one?

Seems like the newer design is worse than the original (At least to me)!

Re: Timing chain guide rails - WHY replace these?
 

The metal chain guides will wear, not the metal itself but the stuff bonded to it, the chain will stretch and the engine will become out of tune and the engine will self destruct. The plastic replacements will work and are, no doubt, cheaper. Can you even purchase the original metal ones?

Check your chain stretch by aligning the timing marks on the crank and cams.

Yeah, I do plan on measuring my stretch - AFTER I get the tensioner rail and install it (and reinstall my metal rail with plastic top). I don't see metal rails on FastLane unfortunatley :( I would replace the rails ONLY with metal ones at this point... I have yet to check the left side but have no reason to believe they'd be plastic (but if they are, I will replace them).

Well, the rails on the left are also metal. I'm only going to replace the tensioner rail.

But the tensioner rail visibly pulsed in and out while turning the engine over. I only have 5deg of stretch, though. Still, should I at least consider a new chain? It looks like it has quite a bit of slack.

Put a new chain in it.

The rubber on the rails will get deep grooves with wear, and allow the chain to slap more than it should. This accelerates wear on the chain, etc.

I don't think the rubber faced metal guides have been available for many years. The 72's appear to have all had the metal ones originally, but the plastic was substituted in the aluminum blocks (if not in the W116 chassis!), so if they have been replaced, they will be plastic.

Leave the metal one in there if it doesn't have deep grooves in it. The new tensioner will pull the slack out of the chain. However, I'd be thinking about a new chain, too -- on the V8, the right cam is always later than the left anyway, so it may be as much as 8 degrees off already.

Peter

Peter: I measured the right - it was the one which was 5° off (I just installed a German keyboard layout to do the ° symbol - I got tired of using * or deg). I didnt do jack on the LH side once I saw the rails were metal.
The rails didn't have much wear at all on them. I know they get weak when they change color, but I don't care about it because they are metal-backed, after all. Why replace metal with plastic? Seems like it should go the other way around!

At any rate, my tensioner rail didn't arrive Friday (understandably - I did place the order late Tues and with the holiday on Thurs, if it was shipped out late Wed it wouldn't have come with the 2-day since Fri would technically be day 1). It should be there tomorrow but I don't need to rush to put it in right now. This is the next-to-last week of the semester for me ("dead week" - finals start next week) so I'll be out soon and have plenty of time. If I see that, with the new rail in there, the tensioner still "Pumps" in and out and oil visibly bubbles out as it goes WAY out towards the opposite chain end, I WILL put a new chain in it (even though cranking is probably when it puts the most stress on it, I don't want a late timing at any point). Besides, the chain is 32 years old. That alone should be justification for a new one! ;)

Judging by how far out the tensioner has to go to keep it at the 5° mark, it's due for a chain. By reading old posts on the subject, it doesnt SOUND too hard. I assume I have to take the tensioner out though (meh, minor issue ;)). I keep saying though, she's been around for 32 years and 140K miles, I want her around for another 32 and 140K at LEAST! I suppose you could honestly say it's love of these wonderful vehicles that keeps them going! I don't care how she looks, she's still my baby!

A little off-subject, but is it just me or does a spark plug gap of 0.023" (.6mm) seem really tiny to anyone else? It's just odd that I never heard of a gap that small before! Oh well, she does run even better now with the gap that small :) (it said that in the emissions manual, in case anyone is wondering)

If you are in college, you are way too young to remember 12,000 volt coils! Plug gap is that small as the spark has to jump it under wide open throttle, and the low voltage coil in there isn't able to get the spark across any more compressed air than that!

A new chain will be a big PITA -- I just did the one in the 220D last week -- unless you have a helper. I had to use the visegrip method since Hans didn't have the nice Klann tool to hold the chain on the sprocket for the 615. For the 117, he has one -- bolts down over the sproket so the chain can't come off, has an opening for the swaging tool, etc. Just grind off the swage, unlink the old chain, link the new one on, and crank the engine around till the ends meet. Way too easy, destroys any vestige of moral fibers, etc.

Without it, you must have either two people, one to hold the old chain tight and guide the new one on, or use vise grips (three, please!) to hold both ends of the chain on the sproket. I got lucky, when I dropped the slack side, it didn't drop into the pan and I could fish it back up. I need to check the injection timing yet, but I think I managed to keep the timing correct even with the dropped chain -- cam and crank agree now instead of bein 12 degrees off.

Peter

Peter: I was thinking of using those reuseable zip ties to hold the chain on the sprocket - sounds a ton easier!

But after mentioning how the slack side can fall in I think I'll tie it to the hood with a long fishing line ;)

Jeez, after thinking about this for a bit, it REALLY makes me want to put my Bosch "Super Blue" coil on (bypassing the resistors) and opening up the point gap a bit (0.030", I'm thinking) in order to get more power! I really wonder if it would work? :D