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Torque To Yield

General engine tech -- Drag Racing to Circle Track

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lewy-d
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Torque To Yield

Post by lewy-d » Wed Sep 11, 2019 8:54 am

I am rebuilding a Toyota 1ZZ engine. It has Torque to Yield main bolts. What do I do about checking main clearances? Will they close up with new bolts? Can NEW TTY bolts be reused? Buy 2 sets of TTY bolts ($37 per set)? Spring for ARP bolts ($115)? I am not opposed to the ARP but this issue will surface again, and some folks aren't gonna want to spring for the ARP stuff.
thanks,
Lewy


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Re: Torque To Yield

Post by strokersix » Wed Sep 11, 2019 10:14 am

How it works:

In a typical bolt the underhead radius sees a significant stress concentration. TTY actually causes the material in that radius to yield. If you then removed the bolt and could look inside you will find residual compressive stress in that radius, not unlike rolled fillets in a crankshaft.

TTY takes advantage of this. After assembly and in service parts settle and the bolt tension drops a bit. What this does is keep cyclic stresses in the underhead radius below the fatigue limit while maximizing the overall fastener tension. I expect the OEMs have done a lot of testing to refine the design.

Pressure vessel manufacture uses a similar idea. Fill the vessel with water and pump it up until it yields just a bit. This washes out left over stresses from welding and fabrication with result of a more fatigue resistant vessel in service.

That said, maybe it's fine but I personally would be hesitant to reuse TTY after in service. I like the suggestion to use old bolts for mockup then new ones for final assembly.

LoganD
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Re: Torque To Yield

Post by LoganD » Wed Sep 11, 2019 10:25 am

Yes, just use the old TTY bolts for clearance checking, then use the new TTY bolts for final assembly.

David Redszus
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Re: Torque To Yield

Post by David Redszus » Wed Sep 11, 2019 10:52 am

By definition, a bolt should be torqued into its elastic range but not into its plastic range.
If within its elastic range,the bolt should return to its original length. If it does not, it's use is
problematic.

Some OEMs will specify a used bolt critical dimension; either length or diameter, within which the bolt
may be reused. Lacking specifics, I find it useful to carefully measure and compare a new bolt to a used bolt.

Sometimes, the bolt stretch occurs in the threaded area which is more difficult to measure. Running a new nut down the bolt to test for thread binding can often indicate thread stretch.

Beware of using bolts that are too high in grade for the application. If they cannot be properly stretched into their elastic range, they will not hold, no matter how strong they are advertised.

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woody b
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Re: Torque To Yield

Post by woody b » Wed Sep 11, 2019 11:39 am

Some bolts are "torque to yield", and some are "torque plus angle". They're both usually called torque to yield. The 2nd article Kevin linked mentions LS chevy engines. It states the head bolts can't be reused, but rod and main bolts can. The head bolts are true torque to yield, but the main and rods are torque plus angle. There is a GM document about this, but I can't find it. I don't know which the Toyota bolts are. Like others said, I'd use old bolts for all checks until final assembly.
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Re: Torque To Yield

Post by strokersix » Wed Sep 11, 2019 12:50 pm

As I expect most here know, using torque to achieve fastener stretch is notoriously variable due to inconsistent friction effects underhead and threads.

Yield point of a fastener is likely less variable. OEMs use this to optimize the bolted joint.

Local yield underhead is a guarantee in my opinion. Stress concentration in the radius is probably 3:1. Some stress concentration in the threads too.

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Re: Torque To Yield

Post by BillK » Wed Sep 11, 2019 12:53 pm

Lewy,
There is an AERA bulletin that basically says you can reuse the bolts as long as they are not stretched and there is no thread damage. If you PM me your e-mail address I will send it to you or you can have your machine shop print it out for you. It is AERA TB 2141
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BigBlockMopar
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Re: Torque To Yield

Post by BigBlockMopar » Wed Sep 11, 2019 1:23 pm

Here is a PDF link to that bulletin.
http://enginetechcatalog.com/images/Bul ... 202141.pdf

lewy-d
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Re: Torque To Yield

Post by lewy-d » Wed Sep 11, 2019 1:47 pm

Thank you everyone. BigBlockMopar thanks for the bulletin! I'm a bit less worried now. New factory bolts, measure them, check clearances, measure them again. I think that will tell me a little more.
Lewy

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Re: Torque To Yield

Post by allencr267 » Wed Sep 11, 2019 3:32 pm

They're fasteners that use the Torque-Angle method of tightening.
If they were TTY's, they'd look very different, with at least one if not more necked down narrow areas along the bolts length that do the Yield part of its name, stretching to near breaking and must not be re-used.

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Re: Torque To Yield

Post by midnightbluS10 » Wed Sep 11, 2019 4:36 pm

David Redszus wrote:
Wed Sep 11, 2019 10:52 am
By definition, a bolt should be torqued into its elastic range but not into its plastic range.
If within its elastic range,the bolt should return to its original length. If it does not, it's use is
problematic.

Some OEMs will specify a used bolt critical dimension; either length or diameter, within which the bolt
may be reused. Lacking specifics, I find it useful to carefully measure and compare a new bolt to a used bolt.

Sometimes, the bolt stretch occurs in the threaded area which is more difficult to measure. Running a new nut down the bolt to test for thread binding can often indicate thread stretch.

Beware of using bolts that are too high in grade for the application. If they cannot be properly stretched into their elastic range, they will not hold, no matter how strong they are advertised.

I asked about measuring a different, new bolt to compare to a used bolt length. Useless unless you originally measured the length of the original bolt because not every bolt is the exact same length. I specifically asked Felpro about their TTY LS headbolts and that was their response.

In the end, I bought new bolts. It wasn't worth the risk. Bolts are cheap compared to a new engine. $60 ain't worth the hassle. Even on the stock rebuild of the I6 in my trailblazer, I used new bolts and those were like $90/set. It's not worth the risk to reuse $50 worth of bolts on a $10k engine, nor a $1000 engine, either.
JC -

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Re: Torque To Yield

Post by modok » Wed Sep 11, 2019 9:03 pm

Used bolts for checking, new bolts for assembly.
You may be able to figure out a torque value that you can for checking, if you do a lot of them.

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Re: Torque To Yield

Post by MadBill » Wed Sep 11, 2019 10:06 pm

I can't imagine anyone being able to measure the clearance difference between a main assembly torqued to a few lb-ft. less than yield vs. yield... :-k
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Re: Torque To Yield

Post by modok » Thu Sep 12, 2019 12:44 am

And even if you could, the difference it would make is predictable.

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