Tech Tip 9 Correct Cam Bolt Tightening

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Tech Tip #9

Correct Cam Bolt Tightening
Unskilled “shade-tree” mechanics universally have a fear of the cam sprocket bolts backing out of the camshaft in a SOHC (single overhead cam) motor and destroying the head. So, in response to this fear, they over tighten the bolts. Since most of these bolts are only 6mm (That’s less than 1/4in.) the most torque you can safely apply is around 140-150 cm-kg. They will break at about 180cm-kg. Note that the recommended torque is way down at 100cm-kg. The bolts shown here were, by my guess, torqued by “feel and fear” to around 165-175cm-kg , or “Plumbers Torque: As tight as you can get it then another 3/4 turn”. In this case that was almost to the breaking point but not quite.

Now, as a bolt is tightened it actually tends to stretch. A mild steel bolt tends to be softer and being more “elastic”, can actually stretch and contract quite a few times before metal fatigue sets in. These bolts however, are special “cam sprocket” bolts. That is, they are most likely harder than an off the shelf bolt. This means they can withstand the stress of their important job better than a “common” bolt can. The problem is being harder and a better grade of steel makes the bolts less malleable, that is they can tolerate over torquing even less than a softer mild steel bolt. When approaching their limit they will not stretch as much and just suddenly break!

These bolts broke inside the motor after some use, they simplyundefined were over-stressed from over-tightening and could only withstand the stress of running for so long. The interesting thing was the motor continued to run with the bolts in the condition shown. Note the polished shanks, they beat themselves against the interior bolt hole walls of the cam sprocket until they were the polished condition shown. Also note the cone shape of the ends, these were held down inside the cam just far enough to get a “purchase” on the cam and rotate it because there was a boss on the cam cover that held the bolts from backing out too far. So they beat and polished themselves by spinning and shucking in and out.

Since the average person can’t “judge” torque without a torque wrench, with a critical application like this, do yourself a favor, use a torque wrench and by all means find out the correct torque specification. One without the other “ain’t gonna cut it”! If you don’t believe me just wait until your motor tells you, “Hey Boy, you assembled me wrong!”, with that characteristic metal grinding sound!! 

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