Jetpilot
DTR Advertiser Join Date: Apr 2002
Location: Hummelstown, PA
Posts: 888


I would not retorque head bolts, we have seen too many failures from doing this. If you need more clamp load go with studs...... Studs are an inexpensive insurance for blown headgaskets.

Doug
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As I said, John, you need to look at a service manual. He is correct on the type of sequence for tightening the bolts. I did not check his torque values. The same type of sequence is used on the mains and rods also. That is a change in the last few years, the engineers found that a more precise pressure was developed using the quarter turn after a precise torque at a lower pressure.

xlr8r gave the poop on how to check the bolts for reuse. If they are properly cleaned, torqued, and lubed on the install, it is rare to find one streched and unusable. I could not see clearly but the photo that you showed of the broken bolt looked to have rust damage to the threads and should not have been reused.

I have built a few of these engines, have never had to purchase a new bolt, and have never had a problem with them. But having much machiine experience I do know what I am looking for as signs of fatigue. What has not been mentioned here is the necessity to clean the tapped hole in the block very well, making sure that there is not hardened crud in the hole preventing the bolt from going all the way down. That is a sure way to twist one off. A good cleaner for the internal threads is a 38 calibre bore brush. A drill of the approprate size run down the hole till it hits bottom will break loose any rust, crud from antisieze or other hardened lubericants.


When assembling any engine, I use the methods and specs that the manufacturer engineers have provided for their engines. That requires a manual provided by the manufacturer.


After torquing the bolts to the last torque value and going over them in sequence at least two times, clean off the head of the bolt, use a short rule or straitedge of some sort, use majic marker to draw a line on the head of the bolt, I do it front to back. Then go over them tightening the quarter turn, in sequence of course.

 

Yes, that's exactly what the CD says, Mark - I just didn't type in every word for brevity's sake.

 

Good point, Dixie - few people realize that the difference in thread lubrication alone can result in up to 25% variance in bolt stretch when tightened by a torque wrench.

 

Headbolts that require turning after torquing, ie 45 degrees, 90 degrees, etc, are torque to yield, stretch, and should not be used unless you're a certified cheap-o.

The purpose of the 90 degree turn at the end is also to ensure you hitter a higher torque number BEFORE you go the full 90 degrees, otherwise the bolt is stretched.

IE: sequence being 45-65-65 1/4.. On some cases, you may need to set the tq wrench for 90 ft lbs and within that last 1/4 turn the tq wrench needs to hit 90ft lb. If it doesnt hit 90ft' before you get to 1/4, then you need to back out the bolt and either check the threads or replace the bolt.

Im not too certain on these headbolts as i haven't had to do them yet... but alot of headbolts torque specs are stated to be using oiled threads too.

 

On a lot of OTR diesel engines, the torque-turn method has been used for years, Caterpillar and Cummins are two major companies who use it.

The main idea behind the torque-turn method is a more consistent clamping force across the board. In caterpillar applications, I have seen several instances of 5 consecutive 15,000 hour rebuilds, using the same bolts. If these are truly torque to yield bolts, should they not have failed long before that?

On the other hand, I have run into many import engines that indeed use torque to yield bolts for the heads. This is just a fact of life, and premium rebuild kits for those engines come complete with new head bolts.

I do strongly agree with cleaning / tapping EVERY threaded hole in a block and head when rebuilding, cleanliness makes all the difference in the world.

The best solution to determining whether a bolt is torque to yield, or simply torque-turn, is use the Factory Service Manual. I realize that there are times the manual is not available, but what you need can always be found, you just have to look. And asking nicely helps too

Good luck

 

what happend to the original poster ?did we scare him away?
any way he never did say what vehicle he was having problems with