Monday

Don't Blame the Rod Bolt



Don't Blame the Rod Bolt
By Don Terrill (c)2005

I feel sorry for the poor little rod bolt - it's been wrongly accused of failure more than any other part in the engine.

My eyes roll back into my head every time I hear; "Blew up my engine last weekend - broke a rod bolt." Then they go on about rumors of a bad batch of rod bolts and how they hope to get the rod manufacture to pay for their engine.

Forget about bolts breaking for no reason - it doesn't happen.

Here are the reasons:
  1. Spun bearings - which starts a chain of events that lead to bolt breakage.
  2. Improperly torqued rod bolts
No one tip can be given to stop spun bearings, but to stop improperly torqued rod bolts is easy and requires only one thing - A rod bolt stretch gauge. Here's how to use it.
Stretch method:
  1. Install the rod cap and snug both bolts/nuts
  2. Completely loosen one bolt/nut - must spin freely
  3. Check length of bolt with gauge
  4. Torque to manufacturer's spec
  5. Recheck length of bolt
  6. Tighten bolt/nut until desired stretch is reached
You may ask, why use a torque wrench for step number 4? One; it's a great little test to see how short on stretch you'd be had you used the torque wrench only, and two; it can be used to see if bolts have been stretched too many times - If torquing the bolts to the manufacturer's spec yields a stretch greater than specified, you may have a problem. I usually see bolts 30-40 percent short on stretch when torqued to the manufacturer's spec. Typically bolts require .006" of stretch, but only obtain .004" when fastened with a torque wrench - no matter what lube is used.

Do you have any idea how much clamping force is lost with just .002" less stretch? A ton.

So, the next time you see a broken rod bolt, don't call the rod manufacturer, ask who torqued the bolt.

Engine assembly tips:
http://racingsecrets.com/racing_engines.shtml
http://speedtalk.com/engine_building_dvd.html

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