Trying to find the max measurement for the 12mm triple square head bolts and where to measure from to, on a 95 C280. Looks virgin after head removal, but want to order parts, and am planning on replacing only those bolts that have streched past their limit. Any help greatly appreciated.

My understanding is that stretch bolts are only good for one usage. Why take a chance?

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1991 300 SEL
218,000 Km

"Xsbank's rules of mechanical intervention: Always go for the easiest solution first; 90% of what ails a gasoline engine is ignition; After that its all a WAG."

apprpox 60.00 for new bolts and washer set .. subscription to alldata would tell you but i dont have it .. id buy new if in doubt

Torque-to-yield bolts are good for one use. These are designed to reach a plastic state while tightening, ensuring even clamping force without retorquing.

Stretch bolts are good until they hit their length limit. These work in the elastic range of the metal. Many engines using these require retorquing after an initial cold-hot-cold cycle.

163.5 mm max

Attached Files

104 bolts.pdf (17.3 KB, 240 views)

Exactly what I needed. It seems there is confusion why they are stretchy bolts and how they work.

Lets give it up for the person who tighted these puppies on the line. All fell between 162.0 and 162.5, I would say an almost perfect 1st use. But this means this is the 2nd and probably last time for these bolts. 150000 on clock now, if it gets half that again, won't mind at all. Thanks again.

When I did my head gasket, the bolts were within spec so I reused them. No problems 25K later. I'd reuse them again in a heartbeat if they stayed within reuse limits.

People have a lot of fear and misapprehension over reusing torque-to-yield bolts. If you understand how torque-to-yield works, and have intimately studied stress-strain diagrams as much as I have, and know when necking occurs in a stretched bolt, this removes a lot of angst over reusing these rather tall bolts. Bolts have a lot of usable stretch. Only a small part of this stretch is used each time.

Some people can lose track of the bolts they tighten or get easily distracted, and can end up unnecessarily torqueing a bolt twice. In part, these specs are written to protect the next person who needs to use the bolts.

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95 E320 Cabriolet, 159K