Connecting rod bolt stretch question

[WConley]

I have seen a few cases where a rod bolt backed out, causing a cap failure.  Mostly stock stuff, but one notable rebuilt SCJ engine.  I'm with you Doug that it's pretty unusual for a properly torqued rod bolt to break.  Now if you're re-using bolts it's another ballgame entirely!

[cammerfe]

I once started to put a short-block together using a set of rods Carl Holbrook had done-up for me. Forging lines removed, polished, and shot-peened. Quality bolts and nuts. The one rod bolt seemed soft, and when I stopped and looked, the rod was crushing. Goodness knows what had happened, but the parting line was beginning to mushroom around that bolt. I took it back to Carl and he had another ready for me by the next day.

You never know what might be 'off'.

KS

[Hipopinto]

Ok

I will call dyers again today

I just ordered a long 7/16 boxed end wrench

Will try this again and see my results

Thanks guys
Dave

[blykins]

What's the wrench for?

[Hipopinto]

I’m thinking to use it to keep a steady pull on it while leaving the stretch gauge in place

Bad idea?

Just thinking it would be easier for one person to do it

Dave

[blykins]

I’m thinking to use it to keep a steady pull on it while leaving the stretch gauge in place

Bad idea?

Just thinking it would be easier for one person to do it

Dave

I don't think you're gonna be able to pull hard enough on a little wrench to get to 70 lb-ft.   You also can't stop and start and stop and start.  It takes more torque to break-away than on a steady pull. 

It's just trial and error.  Easiest with the rod out on the bench in a rod vise, loosen the bolt, zero your stretch gauge, then torque and recheck it. 

[Barry_R]

Pretty fond of the torque/angle concept. 

An initial modest torque value gets things squared away and takes the "give" out of gaskets and assembly.  Then the angle part of the process gives you an accurate stretch number without any fancy gauge or need for access to the other end of the fastener.  It becomes a straightforward math problem - number of threads per inch and degrees of fastener rotation are always going to give you a fixed value.

Accurate statement about running a few tightening cycles to burnish the threads and underhead areas.  If you pay attention you'll find that torque values and tightening feel gets smoother and more consistent after a few "go arounds".  Anything that feels wrong or looks wrong needs to just get tossed.  A hard lesson learned a long time ago...

[Falcon67]

Another way that has worked with success, is to measure bolt stretch, and see what the torque was required to get there, and then torque all the bolts to that torque.  We survived two world wars, multiple regional wars, built cars for over a hundred years, and now all of a sudden, we need to stretch our bolts for them to survive?  Joe-JDC

This is what I do - cycle new bolts, check a set for torque to stretch, then run the all to that torque.  IIRC, the H-beams in the 393 came around at 82 ft/lbs (vs 70 called) Ain't nut'in flown out the motor yet. 

[70tp]

I’ve done the wrench and stretch gauge on the bolt at the same time.   It takes a GOOD 7/16” wrench with a cheater on it to make the roughly 80ft lb it takes.    I got to where I could make the torque in one smooth pull.     It also makes some nicely bent wrenches that work really good for big block header bolts    Sucks that my collection got stolen and have to go back to regular straight wrenches. It would have been easier to use a torque wrench with the available bolt lube offered now as opposed to what I had on hand at the time

[CV355]

I've had bolts (not rod bolts in my cases) that suddenly went 'soft' when tightening. At that point you have to stop and carefully check the threads for pulling out, or toss the bolt if the threads are ok.

Makes me think of when the impact's rpm suddenly goes through the roof and a nut turned into a fat washer...  (no, I don't use an impact on rod bolts or anything else critical for that matter)

I'm not denying that stretch is the best method, but has anyone experienced, or even seen a failed engine due to a failed rod bolt? Even the stock stuff?

I haven't, personally.  In fact I've never actually broken any engine part, and I've pushed several engines beyond their stated limits.  Almost every catastrophic failure that I have witnessed was when a rod wanted to see daylight, but the failure point wasn't the bolts.  One drag racing buddy I had managed to twist several rods in a 460 but they didn't snap, and the engine was still running.  That one was bizarre.  I can't think of what could ever twist a rod unless something got into the combustion chamber and magically got back out on its own.

Torque and angle is the main method we use on industrial automation projects, whether it is for trim panels or engine assembly.