Torque Precision

[artbuc]

In my current project, the Toyota FSM calls for 11 ft-lbs for intake manifold and 12 ft-lbs for camshaft bearing cap. Really now, does Toyota expect their techs to have ultra precision torque wrenches? If I had this job done by a first rate skilled mechanic, would there actually be a 1 ft-lb difference between intake manifold and camshaft bearing cap fasteners? I guess these torque values are the result of computer calculations but do we have to be that precise in practice? My guess is that uniformity might be more important than absolute value. Eg, would it be ok if all intake manifold bolts were 15 ft-lbs instead of 11? If some were 15 and some were 13 and some were 11, then we might have a problem.

 

[Tegger]

Those figures are calculated numbers based on bolt-size, thread-pitch, and grade of fastener. Your average consumer-grade torque wrench will have an accuracy of about 4% plus-or-minus, so that 11-lb could end up being 11.44, and that 12-lb could be 11.53. 0.09-lb is meaningless, of course.

There's generally a fair safety-factor built into each fastener for that very reason, so a few percent one way or the other won't make a difference. This means that a setting of 12# would do for both sets of fasteners.

BUT... The foregoing does NOT mean you can just tighten fasteners by-grunt-and-by-golly. If at all possible, you MUST still use a proper torque wrench, and try to come as close to the proper torque as you can, especially with such low-torque, high-precision fasteners as the ones you're discussing.

There are exceptions and additions to my statements above, but I let them go for now.

Evenness IS more important than absolute torque, but your example of 11 vs 15 lbs is a poor one. 15 is about 37% overtorque from 11. 10 or 12 vs 11 would be acceptable.

 

[Roadkingnc]

I have a torque wrench that is adjustable in inch pounds, 1 inch pound at a time. You can easily remove a fastener touqued to 30 inch pounds with a nut driver.

 

[rpn453]

You'd be using a torque wrench with about a 5 to 25 lb-ft torque range for that sort of thing. It would be measured in inch-pounds, and should be pretty accurate at that range if it's calibrated properly.

It doesn't have to be dead-on though. Many manufacturers provide a torque range rather than a single value in all their specs. The IM probably needs about 10 to 12 lb-ft, and 11 to 13 for the bearing cap.

 

[fsskier]

I would expect different bolt sizes, different hardness grades of bolts do have different specs, even if sometimes close. Gaskets, and assemblies may also have specific torque requirements.


Now, if you want to see an unexpected torque number......I purchased a 1967 BMW new, and shortly ordered the repair manual. All the torques specs were converted from metric measurements in ft/lbs and apparently nobody saw fit to round off any numbers.

The torque spec for the rear wheel bearing nut.....157.31 ft lbs!!! Imagine, torquing it to 157.29, then 157.30 then finally with arms trembling arriving at 157.31!!

 

[stephen9666]

Originally Posted By: artbuc
Really now, does Toyota expect their techs to have ultra precision torque wrenches?


Yes. I have 5 torque wrenches of varying sizes and I'm just a DIY'er

Originally Posted By: artbuc
If I had this job done by a first rate skilled mechanic, would there actually be a 1 ft-lb difference between intake manifold and camshaft bearing cap fasteners?


I would expect my mechanic to properly torque them to spec, or as close to spec as a quality torque wrench can get them.

 

[Merkava_4]

Not to highjack this thread, but would any of you guys buy a torque wrench with laser etched numbers?

 

[Scott_Tucker]

The truth of the matter is, the fasteners are not likely to fail or fall off if an experienced tech installed them by feel. Torque does is not the final word in whether a fastener is tightened properly, there are a whole bunch of other factors.

However, on something like that cam cap, if you over-tighten it the bearing bore may end up being out of round. On the intake manifold, there is usually a beefy enough flange that you will not warp it if you don't use a torque wrench. It totally depends on what the fasteners are being used for.

I always use a torque wrench on intakes and cam caps. But I get as much time as I want to do any job. A tech working flat-rate may see it as only important to torque the cam caps. This is a good argument against flat-rate as a lot of techs might torque neither - just to make enough money to stay alive.

 

[JHZR2]

Ill bet that at the factory, there are a ton of torque instruments that are set for the right number, and are routinely calibrated.

Id expect the stuff to get put back together with the right values too. It takes no time at all to change a torque wrench from 11 to 12 ft lb.

Where the calibration lies is another thing, and Id bet that being 4-8% off would be considered acceptable. But Id guess that having that error be consistent is also key.

 

[fsskier]

Originally Posted By: Scott_Tucker
The truth of the matter is, the fasteners are not likely to fail or fall off if an experienced tech installed them by feel. Torque does is not the final word in whether a fastener is tightened properly, there are a whole bunch of other factors.


Hmmm... I lack your confidence in this. Our SHO Taurus returned with a split mag, a sheared off security lug bolt, and everything else torqued well past 200 ft lbs. I have seen a semi full of warped valve covers, caused by overtorqueing, often by paid mechanics. The aforementioned BMW called for regular detailed service, the exhaust manifold bolts survived the orginal factory torquing, my 12,000 mile and 24,000 mile service performed at home, then were sheared off by the "factory trained, highly experienced) dealer mechanic who was tightening them by feel for the 36,000 mile service.

In all cases above, the customer paid. So, maybe you are right, trained paid mechanics should go by feel.

Also it increases income from roadside service, lug bolts torqued past 200 ft lbs cannot usually be removed by the owners spare tire tools.

 

[fsskier]

This is an anecdotal report, but I just wanted to include it...
A local ASE mechanic graduated from our local community college with top honors, and related the following story. Some of the students considered themselves very experienced. The instructor had set up a test set of bolts to be torqued, some with 1/4 inch drive some 3/8 and some with 1/2, some with torque wrenches (reading covered), all different lengths of course. Do one upside down, one around the corner, one nearly impossible to reach, a couple right in front of you.

The best students had about a 2/1 error sometimes i.e. 15 could be 7.5 or 30 ft lbs. The worst were way worse. Needless to say, an impression was made.

 

[Scott_Tucker]

Originally Posted By: fsskier
Originally Posted By: Scott_Tucker
The truth of the matter is, the fasteners are not likely to fail or fall off if an experienced tech installed them by feel. Torque does is not the final word in whether a fastener is tightened properly, there are a whole bunch of other factors.


Hmmm... I lack your confidence in this. Our SHO Taurus returned with a split mag, a sheared off security lug bolt, and everything else torqued well past 200 ft lbs. I have seen a semi full of warped valve covers, caused by overtorqueing, often by paid mechanics. The aforementioned BMW called for regular detailed service, the exhaust manifold bolts survived the orginal factory torquing, my 12,000 mile and 24,000 mile service performed at home, then were sheared off by the "factory trained, highly experienced) dealer mechanic who was tightening them by feel for the 36,000 mile service.

In all cases above, the customer paid. So, maybe you are right, trained paid mechanics should go by feel.

Also it increases income from roadside service, lug bolts torqued past 200 ft lbs cannot usually be removed by the owners spare tire tools.





I am not saying they are doing it right, I'm saying that is what a flat rate tech must do to survive and that is why flat rate is terrible.

I should say a *good* experienced tech. There are very few good techs out there. I hear you about overtightened lugs, broken off or cross-threaded bolts, etc. I see a lot of shoddy work. Some people should have never gotten in this business. Most mechanics know zero about how fasteners work. This is where theory really comes in handy because it helps you visualize how a fastener should be tightened. The thing I have noticed about lug nuts is that it's not necessarily that they have been overtightened, but that the mechanic did not lubricate the threads, so they gall. When you try to take them off they feel like they have been torqued to 200 lb/ft.

 

[Scott_Tucker]

Originally Posted By: fsskier
This is an anecdotal report, but I just wanted to include it...
A local ASE mechanic graduated from our local community college with top honors, and related the following story. Some of the students considered themselves very experienced. The instructor had set up a test set of bolts to be torqued, some with 1/4 inch drive some 3/8 and some with 1/2, some with torque wrenches (reading covered), all different lengths of course. Do one upside down, one around the corner, one nearly impossible to reach, a couple right in front of you.

The best students had about a 2/1 error sometimes i.e. 15 could be 7.5 or 30 ft lbs. The worst were way worse. Needless to say, an impression was made.


A student is not qualified to say whether he is 'experienced' unless he's been working on the job for years and is just in night classes. However, I do not doubt the scenario you described. Most good techs I know would love to torque every single nut and bolt but the business model does not allow this. Techs are constantly hammered on about how long it takes them to do the job, no matter where they work. That is, except me. I restore cars for rich people so I get to take as long as I want and the customer will always pay his bill.

 

[artbuc]

Originally Posted By: Scott_Tucker
The thing I have noticed about lug nuts is that it's not necessarily that they have been overtightened, but that the mechanic did not lubricate the threads, so they gall.


I thought lug nut torque specs were for dry bolts only. Lubing leads to 20-25% excess torque. There is alot of info on the web about this abd it is fun to read the debate among seemingly very competent mechanical engineers. Seems like there is some agreement that using anti-seize on the threads only will not cause excessive bolt tension but do not use it on the lug nut head.

 

[fsskier]

thanks for your response.... I was getting ready to avoid service in California even if I had to ship my car home!!

Further notes: We have 3 cars that get switched annually from summer tires to ice tires. We are a family of commuters and snow skiers. Just being curious, I often not only torque on my winter and summer wheels, but I also "untorque" them as I remove them 6 months later. If installed at 85 to 100 ft lbs (recommended) they come off with very similar torque readings.

If a tire shop does them, the numbers may double overnight. The shops with the most dummies (Walmart??) require their techs to hand torque them, and they come off at the proper setting.

If it is at an upscale tire store, with top mechanics.... ok, those are the ones that ruined the SHO Taurus ... they were torqued past 200. The ones done at Firestone exceeded any torque wrench I could find, and took two guerillas to remove a few days later.

I spent almost 50 years in the FAA regulated airline industry, it is remarkable how "over regulation" and "too many rules" have vastly improved our durability, reliability, longevity and safety, and great driven DOWN costs.
Yes, you absolutely fly way safer and cheaper (adjusted for inflation) now that you did ever.

If you crack all the bolts and fittings while bolting a JT9D on to a jet passenger plane it will cost way more than a mag wheel, and actually our customers do not really appreciate falling 35,000 feet to their deaths ever!!

I bet you are a great mechanic, Scott..... keep that torque wrench handy though.

 

[mechtech2]

Normally, bolt holes strip rather than the bolts fail. At least in aluminum.

You do not want to go over the tq specs.
Yes, there is some rooms to play with, but tq specs are usually a range - avoid helping out with more twisting force!-