Date registered: Feb 2010
Vehicle: 1972 280SEL daily driver. 904 Dark Blue with cognac interior. euro 4.5 engine
Location: Williamsburg, VA
Mentioned: 3 Post(s)
Quoted: 34 Post(s)
My two cents...
I think there are a few reasons people do not change the lower guides. One is, the upper guides (in the heads) have long ends which tend to get brittle, break off, and cause the chain to jump a tooth or break.....all bad things.
The ones in the lower block to not have these long skinny ends so not as much of a worry about causing the chain to jump.
Secondly, the lower ones require the removal of the front timing cover...which requires the removal of pretty much everything (alternator, PS pump, water pump, harmonic balancer, oil pan). The head gaskets can also be damaged when removing the front timing cover so there is also that risk.
On my old engine which had 400,000+ miles on it, I replaced upper guides and chain. The timing marks were within a degree or so of each other so I don't think the lower guides are that big of an issue. Also, when I scrapped that engine, the lower guides didn't really show any wear.