Torqueing Technique. Snapped a main bearing bolt.

[fordman]

there are differences when torquing down bolts with and without oil. with oil they are more apt to seat faster and change the torque values. so the values you were using might have been too tight for that bolt.
http://www.imperialinc.com/pdf/A_Fasten ... Charts.pdf

[bjde0b]

The values I was using are out of two different books. One is a classic by Tom Monroe and both call to oil the bolts. Either way, the torque at the bolt head will be the same whether you are reaching the correct true bolt stress.

I think it is a problem with a $20 free torque clicking torque wrench. I may invest in a torsion bar Craftsman or clicker. This is the only non craftsman tool in my toolbox and it bit me in the butt today.

[lee_ford]

Glad to hear you got it out.

I vote number 1. It was a bad bolt.

Lee

[lee_ford]

I reread your post and had a comment or two.

I got to 75 and then I slowly worked it up a couple pounds at a time.
…Is there something wrong with going a couple pounds at a time,…?

Yes and No.

Tightening a couple of pounds at a time is not a problem, as long as you wait a few before doing the next level. If you had been going around and doing this on all the mains at the same time, you may not have snapped that bolt.

If you want a scientific explanation, keep reading. If not, just remember to torque in the correct stages.
-------------
Scientific Explanation:

Short Version;
If you are only working on two bolts at a time and only jump a pound or two each time, there is little time for the bolts to cool down. Too much heat and snap.

Long Version;
As you tighten a bolt it will have force working on it in a pulling and a twisting direction. This twisting pressure results in a very very very slight twist at the molecular level. A brief pause on one bolt as you tighten another bolt is used by that first bolt.
There are two reasons a bolt needs this pause.
One is that the molecular twist pressure, unlike the pulling pressure, will cause heat and a pause gives the molecules time to settle in or rearrange themselves better against the pulling pressure and stop getting hotter. This rearranging will straighten out the heat causing twist pressure at the molecular level.
The second reason is that all steel has flaws and those flaws in the metal will give way to this twisting pressure more than the steel molecules around them will. Each steel molecule around one of these flaw molecules has to hold against the twist pressure that the flaw cannot handle. This extra twist pressure on those steel molecules will cause a hot spot in the steel. The pauses between stages of torque give the bolt time to disperse this extra heat away from those internal flaws. If the pause is too short, the heat builds up around a flaw and weakens the nearby steel molecules’ bonding ability. When that bond becomes weaker than the pulling pressure, the bolt snaps.

And that is why they say to do it in stages.

Lee

[bjde0b]

Now I am scared of torqueing. I twisted it off pretty easy, but any weiner should be able to twist it off if it yields at 100 ft-lbs.

Does anybody recommend the $30 torsion beam craftsman?

Also, what do you think the ultimate torsion load of the bolt would be?

By my calculations using a hardened carbon steel with a uti of 110,000 psi the ultimate shear would be 63,470 psi. If the bolt thread is 5/16 (I don't have it in front of me. Just a guess. ) Then the minor I'd would be 0.2523 in. The ultimate load is 133 ft-lbs which doesn't leave a large safety factor. I could pull 130 ft-lbs pretty easy.

BTW I liked the detailed explaination. The bolt was hot when snapped so the yield strength was reduced.

The numbers above were calced quickly so they may not be correct.

[george worley]

I have craftsman beam torque wrench, when I bought it click torque wrenches weren't as readily available as they are now and were much more expensive. The disadvantages of my torsion beam one is hard to read the individual foot lbs. marks are hard to see. Some times where I use it I wouldn't even be able to see the scale. I bought a craftaman click half inch and I love it ,bo matter where I have to use it I just listen for the click. A torsion beam has to be handled carefully or the beam will not be on 0.

[bjde0b]

George, do you have the microtork? I read bad reviews but those people might not take good care of their tools. Also, the microtork only has a one year warrenty I thought.

[fordman]

the torsion bar type isnt as good as the clicker type. the little bar on top can becaome distorted after it is used.

[bjde0b]

My above calculations don't include tensile stress values so I will rethink all this when I have more time to evaluate it.

[oldschoolrods]

The craftsman torque wrench is the biggest POS I have ever owned in my life, I threw it in the dumpster. it sat in my tool box unused for months with the handle wound down, went to use it and the handle pulled right off it. of course while replacing a headgasket on a genset 3.5 miles off a main road in the middle of nowhere. I went and bought myself a snap on. For a hobby guy I'd recommend atleast an S&K for a torque wrench.

[averagef250]

A free HF torque wrench or a $500 CDI do the same job used appropriately.

NUMBER ONE THING!!!!! If you would like an accurate torque reading your tools absolutely must be calibrated. For a torque wrench this is as a simple as finding someone with a calibration machine and using your wrench multiple times at short intervals and writing down the average actual torque for each spot on the scale.

I have a dozen or so torque wrenches from 0-30 in/lbs through 400 ft/lbs mostly clickers, but a few dials. I have a crappy 20-150 craftsman that's 40 years old that I use everyday, I know it is 100% repeatable, but the actual torque is around 15% low at most on the scale. I have to do some basic math to figure the actual torque for exact readings, but usually use an annually calibrated CDI for anything important.

Use the correct lube. If it says motor oil, use motor oil. If it doesn't specify- use motor oil. If it says motor oil and you use moly lube then you're going over torqueing the bolt.

For torqueing bolts a clicker torque wrench is the appropriate tool. Beam types are hard to read and dials are generally used for checking preload, not torqueing bolts.

If it was a bad bolt I would consider myself quite lucky if the bolt snapped while torqueing rather than driving down the road.

[bjde0b]

I did buy the Craftsman Microtork last night. I had some giftcards and a coupon so it only cost me $10.71. I then went to a pawn shop and bought a 1/2 drive socket.

When I got home I put the socket in the vice and used different weights from my workouyt equipment to verify since I had a bad ezperience. I found that the harbor freight wrench was 1 lb more accurate at 100 ft-lbs. I more confidence in my craftsman wrench which I used to torque the main bearings in place last night. The wrench that actually snapped the bolt was a no name that was in the bottom of my trash can so I didn't test it.

For the record I did not use moly on the bolts' it was engine assembly lube (Lubreiplate 105).

[bjde0b]

Yours truly screwed up. I didn't finish reading Monroe's sentence to where it said the 95-105 ft-lbs is for the 351. The next sentence says all others 60-70 lbs. Mine is a 302.

Therefore, I overtorqued my main bearing bolts by 25 ft-lbs.

This is one of those, if you have to ask, then you probably already know the answer.

Do Ineed to replace all bolts? The strain hardening could potentially make them more brittle. I am glad I didn't brake any other bolts than the one.

EDIT:

Maximum load for these bolts is 150,000 psi and my load results in 140,000 psi. I bought new bolts. They will be on my step Friday for $32. Tough way to learn, but it could be worse.

[Levis]

I had a very similar experience, with a set of 302 rod bolts. The manual read:
rod bolts: 75 ft.-lb. High output only
50 ft.-lb. all others

of course I was in way too much of a hurry to read the whole page and snap went the first rod bolt. Then of course I had time to read while the new set arrived in the mail

[DuckRyder]

If a micro tork is the one witn the rubber handle and the little window in it, do yourself a favor and take it back and get one with the scale on the outside of the handle.

And, use motor oil, not assembly lube or lubriplate or grease or anything but motor oil (if thats what it calls for).