TECH  Q&A

Richard Simonds

Tires: Tubed & tubeless

Q: Last year I purchased a 1958 220S that had a tire with a slow leak. The tire-repair shop dismounted it only to discover that it was tubed. The repairman removed the old tube and remounted the tire on the wheel with a new stem, but without the tube. For more than a year, it has held its bead and not leaked at all, but another mechanic recently said that the wheels on that year car required tubes. Is that true? And if so, what is the risk of operating tubeless?

A: The rims on wheels designed for tubeless tires have a slightly different shape to better grip the bead of the tire to retain air, especially while cornering. Pre-1964 cars all had tube-type tires, but by 1964 virtually all cars had converted to tubeless tires, frequently radial-ply design, and used wheels designed for them. All bias-ply tires required tubes, so if you have replaced a bias-ply tire with a radial, there should be no problem. Mercedes-Benz has always had very high engineering standards; if you are using a tubeless tire with no issues, there are advantages to keeping them.

Tubes in tires generate more heat because of friction between tube and the tire. Tube-type tires were bias-ply (pre-radials), and they, too, generated more heat (affecting tread life and risking blowout – especially on long trips). The biggest concern that I know of is that a tubeless tire on an older rim should be kept at recommended pressure to avoid the tire rolling off the rim while cornering. Early radials used a lower pressure than was typical for bias-ply tires to give a more comfortable ride, but that could cause the outer edges of the tread to wear rapidly. So, if you are using radial tires, be sure to keep the pressure up to a modern level (e.g., 32-36 psi rather than 24-28 psi); You can use the "Maximum Tire Pressure" text molded into the sidewall and drop down a few pounds to assure a good bead and good wear.

For more information, the Coker Tire website has good information about using tubes – or not – with your classic vehicle. The site also has a very useful radial cross-reference guide to tire sizes and their equivalents over the years. The links are www.cokertire.com/blog/tube-tech and www.cokertire.com/blog/tire-size-cross-reference.

Finally, both tires on the same axle (front left/front right and rear left/rear right) should have the same brand and model of tire to avoid control problems due to different tire diameter, traction, rolling resistance and other driving characteristics. Ideally, all four tires should be identical except when staggered sizes are called for on newer, higher-performance vehicles.

A key issue for R129s

Q: I recently purchased a 2000 500SL in pristine condition from a local Mercedes-Benz dealer. It had only one smart key; the dealer ordered a second key from Mercedes-Benz at my request. Several weeks later, Mercedes-Benz advised the dealer that it no longer could supply replacement keys and offered no help in finding an alternative. Several subsequent attempts by the dealer to get a new key from other sources have ended in failure, as have my independent attempts.

Because I cannot be the only owner and club member with this issue, I am hoping that someone has had more success than I have. Obviously, losing the key – or its failure – would be problematic, to say the least. Second, the lack of information at the dealer level about this issue makes me wonder about Mercedes-Benz support and commitment to owners of older cars.

A: According to our sources at MBUSA and the Classic Center (which now supplies parts for the R129 models) replacement keys for the R129s, W140s and W163 MLs are not presently available “due to current constraints in our global supply chain.” However, they promise they are making every effort to locate a new supplier and hope to have the problem solved in the near future. They encourage customers to “call the Customer Assistance Center at 800.367.6372 to report this and other vehicle concerns. The CAC’s role is to address and assist customer concerns and utilize all available resources to help meet those objectives.” 

Timing chain and guide wear

Q: I’d like guidance on evaluating timing chain stretch and chain-guides wear. I have seen horror videos on YouTube of M119 and other Mercedes-Benz vehicles with badly worn, timing chains. I have also read stories of bad M119 experiences on marque forums.  Since my 1999 CL500 has reached 165K miles, I am concerned about the condition of the timing chain and its guides, especially because there are reported issues once a vehicle passes 125K miles (200,000 kilometers).

A: The inevitable stretching of the timing chain over time is not in itself bad; the timing chain tensioner is designed to compensate for chain stretch. Rather, timing chain failure starts when the tensioner does not maintain tension when the engine is shut off: The piston of the tensioner backs off and the chain can then jump off due to lack of sufficient oil pressure in the piston. The chain almost always breaks during a cold start: The slack in the chain prior to the oil pressure pushing the piston back to tighten the tensioner allows the chain to slap the guide and break off a portion of the brittle chain guide rail. The broken piece can then move upward and jam between the cam sprocket and the chain, helping the chain jump off the gear. The delay is the point of destruction.

Then it’s over for the engine.

The wear doesn't just occur with the chain. Over the years, the five sprockets on a V-8 all open up/spread open to match the chain’s wearing dimensions. When you install a new chain, the conflict between the new chain and the worn sprockets is significant. This means that when the timing chain is replaced, the gears and guides should also be replaced and the tensioner should be updated to the ratcheting model that maintains pressure on the chain even when the engine is not running.

The nylon guides and clamping rail can become deeply grooved, lose flexibility, and the chain can jam in the wear groove. To access the chain guides, you need to take off the front cover, which is not a DIY job for sure. When inspecting V-8s or during valve-cover replacement, it's not unusual to see the top half of the upper chain guides gone/missing and it usually breaks at the steel pin that holds the guide in place. It ends up in the pan if you are lucky; if not, it ends up between the cam sprocket and chain. The broken pieces can also pierce the steel oil pump pickup screen and then create other damage related to blocked oil passages.

To correct this, Mercedes-Benz slowly integrated ratcheting tensioner pistons that stopped the chain backing off on M110, M102, M103, M104 and most diesel engines. However, M119, M116 and M117 engines do not have them. (Thanks to Fred Hanson at Hanson Mercedes Technology in San Mateo, California, for this information.)

Most mechanics find that catastrophic engine damage from a timing chain jumping its gear sprockets doesn’t occur until after the vehicle has traveled 200,000 miles. But this varies depending upon maintenance over the years. If you are not sure about how your vehicle was maintained before its purchase, our specialists advise having the full chain, gears and guides replaced.