Torquing Rod Bolts

I was torquing my new rod bolts while fitting the rods to the crank last night on my 3.2 rebuild and one of the new bolts yielded and snapped. It makes me very suspect of all the others. My torque wrench was set to 39 ft.lb and most of them seemed to hit that torque pretty evenly apart from one or two which maybe took 1/2 a turn more than the others. I could tell by how much thread was exposed above the nut. Has anyone ever head of rod bolts failing during assembly. It would seem as if they are taken very close to yeilding on assembly. I've ordered one more bolt but have now got this uneasyness about the rest of the bolts. The last thing I want is to be scared to rev my motor once its rebuilt from fear of throwing a rod.

What kind of lubricant were you using on the threads of the rod bolt? Dit you 'sneak' up on the final torque value 5 lb-ft at a time?

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Warren Hall
1973 911S Targa

I understand your anxiety. My expectation is that they are strong enough taht the torque spec gets nowhere near snapping them.

Some time ago I read a thread where someone warned about lubricating, even with anti-sieze or thread lock. This, sayeth the poster, reduces friction in the threads and results in too much bolt stretching at given torque settings. All bolts, by the way, stretch. Not just 'stretch' bolts.

I think this comment was interesting because it seems quite plausible. I had a bolt snap during my clutch experience, at a torque setting and (subjective) amount of pressure that shocked me.

I'd like to hear more about this. And so would beetos since rod bolts are, shall we say, fairly busy and critical fasteners.

Aw, you beat me to it, Warren.

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'83 SC



[This message has been edited by Superman (edited 08-16-2000).]

I used engine assembly lube, very slippy stuff, as per Bruce Andersons recommendations.

My inital thoughts were that of supermans, that the lube caused too much stretch. I'm sure that when the factory determine a pre-load for the fastener, they take into account thread friction when coming up with the torque value to achieve the bolt preload. 'Over' lubrication of the threads means that more of the torque is translated into bolt tension, so for a given torque with lubricated threads there will be more (probably too much) bolt tension, hence the yeilding.

I set the torque at 39 ft.lbs and after lightly preloading the bolts I went straight for the full torque with a slow application of pressure. I found that the more gradual I put the pressure on, the more the bolt would turn until the torque wrench clicked. I think maybe the lube allows the torque to creep up too much. Warren, whats the relavance of 5 ft.lb increaments?? Anyway, I cannot bring myself to put the cases back together with this in the back of my mind so I've ordered a complete new set of bolts. Should get them tommorrow. Any advice on torquing them down second time round?
Cheers

I hate to be the bearer of bad news, but any of the rod bolts you torqued to the 'indicated' 39 lb-ft level lubricated with a moly-based assembly lube were ruined before the first one broke ... consider yourself 'lucky' that it gave up, because your engine would indeed probably not stayed together very long ... probably only long enough to make one sad 'WHUMP' when you tried to rev it up!!!

If you know which bolts were not yet torqued when you got the warning signal, they are OK, and can still be used. But, there are lessons here to be learned, and remembered!!! First of all, there are charts ... usually included with most torque wrenches, that give the percentages that 'standard' torque values intended to be used with dry, clean fasteners, must be reduced by, when various lubricants are used, and, to my knowledge, moly has the greatest reduction value, in the range of 40-50%! Another important point is that assembly lube is intended ONLY for moving parts, such as bearings and cams, that need extra protection upon start-up, not fasteners!!! The only way rod bolts should be torqued is dry, or with a couple of drops of Loctite!

Does it surprise you that the bolt broke when the stretch value was equivalent to the amount that would have been applied if the bolt was dry and you had the wrench set for 75-80 lb-ft? I'm not!

As far as the matter of using small increments to approach a final value goes, that is the way critical fasteners are always torqued, whether it be your alloy lug nuts, manifolds, or block halves ... or rod bolts. Think about it for a minute ... if you don't evenly apply torque in increments, one side is essentially loose, and you suddenly apply FULL torque to the other side ... is it any wonder why the rod cap might bind the bearing because it shifted 0.001" or 0.002", and binding is always something you want to check for when installing rods, because bearings that are binding when you start up an engine ... DON'T LAST VERY LONG!!!

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Warren Hall
1973 911S Targa

thank you for the knowledge of torque values
when diiferent substances are put on threads
i agree that clean threads and loc tite and
shouldn't have a problem.. any knowledge passed along about the differences with locgtite and anti-seize would be appreciated
knowledge is power!!

Good call Warren,
I expected this would be the case. My next set will go on dry. Thanks for clearing this up for me. Andersons book is confusing here as it calls for these fasteners to be lubed. Factory book makes no such instruction.

I get what you are saying about 5 ft.lb increments and my previous message may have implied that I went to full torque on each side. Not so, I did each side progressively by feel, although I must admit, It would be better going up in prescribed torque increments and not feel.

What you say sounds like perfect sense to me and its advice that saved a potential disaster. I will be carefull with my next set due to arrive tommorow. Guys like you are a real asset to this board....Yes, knowledge is power!!

You might try emailing Bruce Anderson at Bruce1485@aol.com to let him know what happened. I would be interested in his reaction. He is rarely wrong, and obviously knowledgable, but in this case Warren's got it. In all the engines I've rebuilt, (no porsches though... yet). I have never lubed those or any other similar fasteners (main, head, etc.).

[This message has been edited by leon (edited 08-16-2000).]

Dito Leon,
I have rebuild a 944 motor...no lube, Adui 5 cyl Turbo.....no lube....a number of other motors including a Ford Cosworth 4 cyl....again no Lube.


I was wrong with the 911 though!. I'm extemely glad I posted this question or it could have turned out to be one hell of an expensive mistake if I just changed the one bolt. The others held the torque but on closer inspection it seems that some had started to yield. I can tell by measuring the installed length ofthe bolt with a vernier, some are way longer than others. I think the biggest problem is using factory figures that assume a dry thread, and then being advised to lube bolts without being warned of correcting the torque (ultimately, no ones fault but mine!). I think if you've never done a 911 engine before and tend to over research before and while doing it, obvious mistakes like mine are all too easy to happen without applying some common sense. I wonder if anyone has challenged Anderson on his advise in his book, advise that is not clear, or maybe just plain wrong. Hey Warren, if your ever in Houston, I owe you a couple of beers.

is dry the way to go?? i have always put a bit of loctite on bolts/nuts that dont have
any locking tabs... i am sure a fastener
coming loose/apart would be ugly... i know
i have seen rod bolts, chevrolet comes to
mind that are several 100's a set.. my experience is limited to just a few auto and
more than a few 2 stroke engines..
you have rebuilt a 944? how was that, my
usual driver is a 83 944, the thing has been
real dependable and gets better mpg than my
other driver, 63 bug if i could only keep the
body straight!!!

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Bruce 1970 911E

If i remember correctly, the 944 bolts were about 75 lb.ft dry. I think I'll use loctite on them next time.

So, Loctite does nothing to "lube" the fastener? Or perhaps there is and adjustment, or conversion, to make.

My torque wrenches did not come with conversion charts.

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'83 SC

You might try contacting loctite's applications engineering dept. and see what they say. I'd be interested, or maybe I'll do it and post their response here.

THANKS! Once again to the people on this board.

I'm slightly behind beetos since I'm about to UNDO my rod bolts (crank is currently sitting on the kitchen table looking like some exotic modern sculpture).

But I now won't be making a mistake when I torque up the new bolts in a day or two (I too had read Bruce Anderson and was planning on spraying the threads with WD40).

This board saves me $100s every year, I'm sure of it...

- roGER

Very interesting discussion, gents... I had heard of the concept of inaccurate torque pressures when using lubricant before but I had no idea the actual torque would be so far off.
When I change my plugs & put anti-seize on the threads, what would then be the recommended torque upon installation? 5lbs less??
regards,
jlex.

Now you have another issue - what is the proper method to clean the female threads before reinstalling the bolts?

Just a quick update guys. My new set of rod bolts went on cleaned with a blast of break parts cleaner (nut and bolt). I used just a drop of loctite and did them up progressively to 38ft.lb. This time, all hit torque in a much regular way, non crept upon re-application of torque. Much happier now! Cases went back together, pistons and cylinders on, flywheel bolts went on dry with loctite (66lb.ft) and everything looks swell. Cam housings and chain covers are in the dishwasher......thats right, in the dishwasher! (no, the wife does not know...yet). Took the heads in to get guides, the guy said the guides I got (Vertex) did not have the stop collar on them so he would just have to use check the each one did not go in too far. Should get my 964 cam grinds and heads next week for the rest of the rebuild.

I've been following this topic for a little while and finally had to go look in my service manual concerning this. The factory service manual say to coat the threads and mating surfaces with oil and torque to 36.2 ft-lbs. This is for the older pre 3.2L connecting rod bolts. Aren't the 3.2L bolts smaller diameter (9mm vs 10mm)and if they are shouldn't the corresponding torque to tighten them be less? I hope I'm right about this because the bolts in my 3.0L have about 800 miles on them at 36 ft-lbs coated with oil.

Oh by the way I'm new to this BB. Nice to meet you guys.

regards, Pat

Pat,

The use of 10 mm rod bolts goes all the way back to 2.2 'reinforced' rods in the factory 'spec books,' and continued with the 3.2 engine, according to the latest Bentley manual ... all use the 5-5.5 m-kg value that calculates to 39 lb-ft on top, and 36 on the lower end. My assumption is that your 36 lb-ft value is reduced by 10% to compensate for the use of motor oil on the threads.

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Warren Hall
1973 911S Targa

Changing rod bolts is a wise decision to make.Connecting rod bolts stretch over time do to Fatigue Cycle,the constant hammering the bolts take.After teardown,race engines are checked for rod bolt stretch ,the bolts exceeding maximum amount of stretch are discarded.If your old bolts were longer than your new bolts,I'm wondering if your bolts were maxed out and ready for the bin!ARP automotive fastners makes aircraft quality rod bolts,I wonder if they make them for Porsche!