Timing chain guides are often the topic of our conversations. Recently the thread 'Timing chain service, $9800?!?!?' (https://www.benzworld.org/forums/w126...ce-9800-a.html
) captured also important technical information that (I thought) should be captured in a separate thread, with an easier to search title.
The main issues that arose are:
1) Should the chain guides be changed due to mileage, age of the engine, or both?
2) When changing the guides should only the upper ones be changed (accessible after removing the head covers), or all (in this case the timing cover has to be removed to gain access to the lower ones)?
3) If the answer to 2) is 'all of them', can the timing cover be removed with the engine on the car, or does the engine have to be removed first?
My personal opinion on the above (I also welcome disagreements) are:
1) The mileage is the primary factor (as everybody knows that is 100,000 miles) if
they have not been in the engine for more than (approximately) 20 years (the same "age limit" was also brought up by Nitro88). It happens that our cars are all past the 20 year mark, so IMO they are due for a chain guide job if the original guides are still in, regardless of mileage
. The first two pictures show what I pulled out of my engine: 1989 car with 90,411 miles. Do I need to say more?
2)The timing chain has straight guides and round guides. The straight guides are only to prevent the chain wobble, so their contact with the chain is intermittent and with low chain pressure. The round guides have permanent contact and with high chain pressure because they "deflect" the trajectory of the chain, forcing it on a particular path. These guides have more wear. For that reason, is my opinion that when doing a chain job, all guides have to be changed: the upper AND the lower. The third and fourth pictures show by comparison the wear amount (from left: chain tensioner guide, lower timing chain guide, the oil pump chain guide). The last picture shows the wear on the lower timing chain (round) guide. The reason (IMO) the lower chain guides do not get much attention is simply because most people avoid changing them due to the fact that getting the timing cover off is not by any means an easy or pleasant job.
3) If all guides will have to be changed, and the lower will require removing the timing cover, the question becomes: can the timing cover be removed and correctly reinstalled with the engine on the car? According to Vaifanatic he has done it, and jackjtr is in the process and is trying to put together a writeup (https://www.benzworld.org/forums/w126...eassembly.html
). I cannot offer correct details in this matter (I first took the engine off the car for other reasons), but I don't see why the cover cannot be removed with the engine in the car (some small engine raising may be required) provided that everything is done with attention to details, you document every step (especially at disassembly) so you put things back together the way they were, read the FRM FIRST, think everything thoughtfully, and have plenty of time (this is no "Saturday morning job").