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Old 11-01-2016, 01:12 PM   #1
sorka
 
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Re-use flywheel torque to yield "stretch bolts"

I'm in the middle of a clutch job. Was about to re-install the flywheel when I ran across a post by someone saying that the bolts had to be replaced because they are stretch bolts.

However, TIS says nothing about replacing the bolts anywhere. It just says to torque to 38 ft lbs, wait an hour, and then turn 90 degrees tighter.

I called a local Toyota dealership here in the Bay Area and they had none in stock. They checked the stock of all the other stores in the south bay and none of them have any in stock. They checked the dealerships all along my way home and none of them had any in stock.

Given that I'm inclined to believe dealerships just re-use the bolts. Any thoughts?
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Old 11-01-2016, 02:30 PM   #2
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I was going to say not to reuse them, but the manual doesn't specify them as non-reusable. Very strange if they are in fact stretct/torque-to-yeild but can be reused. I guess for the small cost of a new set, it's worth my peace of mind to not have to worry about one shearing off and destroying the clutch.

*EDIT*
I was looking at the xD repair manual, which doesn't specify the flywheel bolts as non-reusable, only pre-coated. See my post below for details from the 2007 Yaris factory manual.

Last edited by CrankyOldMan; 11-02-2016 at 01:16 PM.
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Old 11-01-2016, 02:49 PM   #3
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Originally Posted by CrankyOldMan View Post
I was going to say not to reuse them, but the manual doesn't specify them as non-reusable. Very strange if they are in fact stretct/torque-to-yeild but can be reused. I guess for the small cost of a new set, it's worth my peace of mind to not have to worry about one shearing off and destroying the clutch.
My local Toyota dealer says they can get them by tomorrow morning at 8:30 AM. Was going to start putting it all back together tonight but will hold out if they actually get the bolts. Cost $2.71 each which actually isn't a total ripoff as that was more than a $1 cheaper than the dealer near my work. Not that I wouldn't have paid the $4 but it's nice to see a local dealer not trying to ream me. The online price range from $1.97 on up.
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Old 11-01-2016, 03:27 PM   #4
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I have about a dozen clutch jobs on various cars and always reused the bolts. I'm also a GE trained machinist and I've never heard of "stretch bolts".

As far as I'm concerned the service manual takes precedence. Whatever it says I would do that.

But just for fum...

Bring a dial caliper with you and measure the new and old bolt lengths. Measure all the used one and compare them.

Tell us what happens!
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Old 11-01-2016, 04:07 PM   #5
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Originally Posted by bronsin View Post
I have about a dozen clutch jobs on various cars and always reused the bolts. I'm also a GE trained machinist and I've never heard of "stretch bolts".

As far as I'm concerned the service manual takes precedence. Whatever it says I would do that.

But just for fum...

Bring a dial caliper with you and measure the new and old bolt lengths. Measure all the used one and compare them.

Tell us what happens!
I'll compare them with my Mitutoyo digital caliper. I'm sure the old ones will be a little longer but it will be interesting to see by how much.
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Old 11-01-2016, 05:14 PM   #6
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Torque angle stretch clutch flywheel bolts yield

They probably don't start to yield/stretch(plastically) at 38 ft lbs. Or 39 ft lbs.
The factory didn't wait an hour.
I'd look for a Toyota tightening torque for similar diameter bolts in a similar application...then probably reuse if not corroded.
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Old 11-01-2016, 06:04 PM   #7
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They probably don't start to yield/stretch(plastically) at 38 ft lbs. Or 39 ft lbs.
The factory didn't wait an hour.
I'd look for a Toyota tightening torque for similar diameter bolts in a similar application...then probably reuse if not corroded.
They probably have a different method for achieving the same results in mass production. For instance, they could store flywheels at a very cold temperature and then rely on the expansion to finish clamping force as it expands rather than rotating 90 degrees after an hour.
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Old 11-02-2016, 12:50 AM   #8
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I think you should re-read the manual. The hour wait is probably after doing the 90 turn (before starting the car). The bolts get warm when stretching and they probably want them to cool down before getting shear loads? Not too sure about that. There is no reason to wait after torquing to 38ft/lb though.

Stretch bolts are almost always considered non-reusable, and again I'd suggest to read the manual. The thing with stretch-bolts is that it's sometimes fine to re-use them (even if you're not supposed to) and sometimes they will fail. I often re-use stretch bolts, but I have enough experience to know when they can handle it or not. Flywheel bolts are not something to mess around with: If you are unsure just replace them.

Cheers.
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Old 11-02-2016, 12:52 AM   #9
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Just to clarify. In the automotive world stretch bolts are almost always non-reusable. In other fields (larger bolts) they often measure them and reuse.
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Old 11-02-2016, 07:27 AM   #10
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Put them back in & use thread locker.
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Old 11-02-2016, 08:57 AM   #11
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Has anyone heard of "stretch bolts" in other assemblies besides the clutch?

I havnt.
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Old 11-02-2016, 09:11 AM   #12
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Quote:
Originally Posted by sorka View Post
Cost $2.71 each...
Just a quick cost vs risk assessment.

New bolts:
$16.26 + tax
Waiting one day to get them.

Grenaded clutch/flywheel repair (cheapest aftermarket I found in 10 second search):
$63 Replacement brand clutch kit
$50 Luk brand flywheel/flex plate
$16.26 for a new set of bolts (old ones will probably not be salvageable)
Hours/days downtime tearing out engine/trans to replace

Even if you skip the OEM clutch and flywheel and go super low quality, you're out at least a weekend to pull everything apart. Investing the $20 while you've already got it apart seems to be a no-brainer to me.

That said, I agree that elastic deformation bolts can be re-used, plastic deformation cannot. But without a length spec in the manual to verify against, or a good understanding of what a plastic deformed bolt looks like, it's a crap shoot.
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Old 11-02-2016, 10:02 AM   #13
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Yes yes....but what's the truth?

If it's true they can't be reused then there must be a spec.

No spec...leads me to believe it's all a myth.
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Old 11-02-2016, 11:32 AM   #14
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Quote:
Originally Posted by CrankyOldMan View Post
Just a quick cost vs risk assessment.

New bolts:
$16.26 + tax
Waiting one day to get them.

Grenaded clutch/flywheel repair (cheapest aftermarket I found in 10 second search):
$63 Replacement brand clutch kit
$50 Luk brand flywheel/flex plate
$16.26 for a new set of bolts (old ones will probably not be salvageable)
Hours/days downtime tearing out engine/trans to replace
A flywheel has a huge amount of inertia. If the bolts were to suddenly shear at 6500rpm you would need a new car.

https://www.youtube.com/watch?v=n0TxZFDLa3w

This is why it is not something to play around with.

Cheers.
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Old 11-02-2016, 12:00 PM   #15
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Well my Yaris has only 106 hp and I don't drag race (or exceed 45 mph most days) so we can rule out the engine blowing thru the hood.

If you re drag racing you ve got a lot more than clutch bolts to worry about.
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Old 11-02-2016, 01:12 PM   #16
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I took a look at the OEM Yaris repair manual and it seems that truth is stranger than fiction! The A/T flex plate CAN reuse the bolts, but the M/T flywheel CAN'T. Even stranger than that, is that the xD repair manual (not pictured) only shows the M/T bolts as "precoated", and not "non-reusable", but also has a post-torque turn requirement.

It seems that the difference, then, is in how the joint is engineered. The flywheel experiences significantly higher shear loads and is safety critical, so it needs a tighter joint, and it was engineered with Torque Turn to Tighten (TTT) joints. The flexplate has a lot of "slush" factor from the torque converter, so it can rely on standard torque to friction joints. (I read an article on it, for reference: http://www.enginebuildermag.com/2001...rque-to-angle/)

In this case, I would specifically follow the OEM procedure and replace them for a M/T vehicle, instead of following "shop lore" about if you can or can't, based on other vehicles' requirements.
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Old 11-02-2016, 01:18 PM   #17
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Quote:
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A flywheel has a huge amount of inertia. If the bolts were to suddenly shear at 6500rpm you would need a new car.
I was thinking more of a single head shearing off and tumbling around inside, but yes, a total failure at redline would be catastrophic for everything around it. That's why real racecars are required to have a shroud around the flywheel that protects the driver and bystanders.
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Old 11-02-2016, 01:31 PM   #18
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Originally Posted by bronsin View Post
Has anyone heard of "stretch bolts" in other assemblies besides the clutch?

I havnt.
The Yaris head bolts get replaced after use.
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