Re torqueing head studs/bolts

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justinp61

justinp61

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I posted this on Moparts too.

I've been SLOOOOWLY assembling my new small block and last night got the heads on it. Last summer I read an article in CC or HR where they fixed some problems with a guys (I think) Olds. Whatever it was it only had ten head bolts like a SBM. In the article after installing the heads they let it sit over night, the next day one by one they broke the nuts loose and re torqued them. The nuts were marked after the original torque so they could see any movement on the re torque and there was.

Before I left the shop I marked some of the nuts and both bolts on both sides. This AM I broke them loose and re torqued them one by one. and they all moved. The nuts on the studs moved more than the two bolts. The bolts are in the holes that oil the rockers and were torqued to 100 ft lbs and the nuts at 110 ft lbs. All the marks on the nuts were at the 6 o'clock position and the bolts were at 12 o'clock.

BTW the head gaskets are Fel Pro 1008s.

Studs.


Bolt.
 
I always retorque the head fasteners with aluminum heads after they're heat cycled. Usually after the cam break in, when I'm adding the inner springs. Things always move a little.
 
Always a good idea to retorque after a running the engine a bit. Maybe do it again when you do your first oil change 500 miles etc... Not a break them loose and retorque just put the torque wrench on the and hit the number
 
I would not loosen them before retorquing them.
 
The reason to break them and then re-torque is that an accurate torque reading can only be made while the bolt/nut is moving. You can't get an accurate reading without it moving.

I have pretty much re-checked head bolt torque on every engine after a few heat cycles for 4 decades. But I don't loosen and re-torque anymore; I just take the torque a bit beyond the spec to see if the bolt/nut moves and it not, it's good. Just looking for loosened bolts/nuts or gaskets that have flowed or settled, and they seem to show up that way.
 
tnts72 said:
Who comes up with this break it loose and retorque thing...? Good way to start a headgasket leak if ya ask me :wack:
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How's that?

Why break them loose? It's called break away torque. In my case the nuts are torqued at 110 ft lbs, it will take more than 110 to move them.

As far as the re torque goes there may be some head gaskets still made that require it, Fel Pros don't, PermaTorque. I've been using them for 40 years and have never re torqued one, even in lean times when I reused them and never had a leak. This may be the first, who knows. Bottom line, keep doing what works for you.

I posted this because I found it interesting, specifically the fact that the bolt with it's smaller shaft diameter moved less than the nuts on the studs did. It may be the fact that the nut is fine thread and has to turn more than to move the same distance as the coarse thread, plus the little extra radial tension at the washer. I'm some sharp character out there can explain it. LOL
 
Every time that you tighten a nut/bolt, it makes the bolt/stud stretch a little. Some of the stretch is permanent.

If you loosen the bolt/nut then retorque, then you now restretch the bolt again.

If you just torque it to the torque again and not loosen it, you don't introduce another clamp cycle of stretch into it...

After 5 clamp cycles, the bolts should be replaces as they may now be more prone to break...
 
nm9stheham said:
The reason to break them and then re-torque is that an accurate torque reading can only be made while the bolt/nut is moving. You can't get an accurate reading without it moving.
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No, there are two kinds of torque, static and dynamic.

Static is higher and what you call "breakaway" which is how much torque it takes to start the bolt turning.

Once it turns, then you are measuring dynamic torque, the amount needed to keep it turning, which is slightly lower.

But if you verify the torque by just tightening only, you won't stretch the bolts again and the verified torque should be close enough if it was static vs dynamic... It's better than stretching the bole more if you don't have to...
 
I imagine anyone who turns a wrench will find this interesting. I'm interested in hear what AndyF and other knowledgeable professional builders or aircraft mechanics/machinists think.

I understand the concern about loosening the bolt or nut too much. I've often backed off a nut or bolt a small yet effective enough amount like a 1/8 turn or less to maintain some high percentage of the installed torque that still allows for a smooth re-torqueing of the item back up to spec without having to overcome the break-away torque.

I think the key is not to over loosen the fastener. Perhaps, a simple test using a rod bolt, a torque wrench and a micrometer to measure the change in length during the process would be enlightening.

Now, the real question. Did you follow the same procedure on the head AND main bolts before you bored and finish honed your bores.

I just read Krazycuda's response as I was typing this. He's on track. Temperature also plays a role to some degree. Dough!
 
It would be interesting to hear ARP's take on this.

The article had some interesting stuff, this afternoon I'll se if I can find it.


The mains don't have a gasket under them and I suspect the movement has more to do with the gasket than bolts or studs stretching.
 
justinp61 said:
It would be interesting to hear ARP's take on this.

The article had some interesting stuff, this afternoon I'll se if I can find it.


The mains don't have a gasket under them and I suspect the movement has more to do with the gasket than bolts or studs stretching.
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80-90 % of the torque is to overcome friction on the threads and under the head surface.
 
krazykuda said:
No, there are two kinds of torque, static and dynamic.

Static is higher and what you call "breakaway" which is how much torque it takes to start the bolt turning.

Once it turns, then you are measuring dynamic torque, the amount needed to keep it turning, which is slightly lower.
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We're saying the same thing.....BTW the extra torque needed for 'breakaway' is due to stiction which is always higher than the friction of moving. It is like sliding a heavy box along the floor: you need to put more force on it to get it moving and it takes less force to slide once it is moving.

As far as the bolts stretching, the old original types in these engines can be re-used because they are not designed to stretch beyond their elastic limit and become permanently stretched when torqued to spec. (Although some may indeed permanently stretch at times.) A lot of modern head bolts (maybe all?) are torque-to-yield bolts that do indeed permanently stretch, and are not to be re-used. Those are the ones to not loosen andre-torque...IMO.

JustinP, I'll guess that the gaskets 'settled' or flowed a bit on you. It is an interesting observation and thanks for posting it. And I suspect you are right on the thread pitch.
 
nm9stheham said:
We're saying the same thing.....BTW the extra torque needed for 'breakaway' is due to stiction which is always higher than the friction of moving. It is like sliding a heavy box along the floor: you need to put more force on it to get it moving and it takes less force to slide once it is moving.

That's the classic definition of static vs dynamic torque. ^^^

As far as the bolts stretching, the old original types in these engines can be re-used because they are not designed to stretch beyond their elastic limit and become permanently stretched when torqued to spec. (Although some may indeed permanently stretch at times.)
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Even the old ones stretch a little at a time when torqued. I still would not use them over 5 times. I have a rocker arm bolt that is stretched to where you can see it necking down in the threads. If it is used and torqued, it's sure to break.

I would not take chances with critical bolts like rods, mains, rockers, and heads. 5 clamp cycles and throw them away or use them for mock up only and replace with new when done.
 
So kk, you're saying you would throw away a set of ARP head bolts after 5 clamp cycles?
 
Ironmike said:
So kk, you're saying you would throw away a set of ARP head bolts after 5 clamp cycles?
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Yes, after 5 clamp cycles you are taking a risk...

What are you going to loose if they fail???
 
My old Mazda says retorque after every valve adjustment (12/12,000 miles) and the procedure is to break them and then retorque them one by one. Your mileage may vary.....Modern head gaskets dont need it though, refer to your gasket instructions.
 
Ironmike said:
So kk, you're saying you would throw away a set of ARP head bolts after 5 clamp cycles?
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Or you could measure them for stretch and keep using them as long as they are not stretcehd. ARP's are still steel and will stretch like any other bolt if tensioned beyond their elastic limit. Lacking that, some limit on the number of uses makes good sense for any critical use fastener. I have reused some stock head bolts 6-8 times.... not knowing any better LOL!
 
To do a proper bolt stretch, you have to have both ends of the bolts ground flat and parallel to each other with centers drilled.

Then use a ball micrometer to measure the bolts before the first stretch cycle, and then every one thereafter for accurate results...

I do not know of any way to pick a used bolt and measure how much it has stretched.

I've set up the multiple spindle machines on assembly lines to tighten them.
 
Yes, it takes some prep and record keeping. I would think any consistent surfaces can be used if you are doing your own.

The only way I can imagine for used bolts is to do a comparison amongst all of them, and just toss the ones significantly out from the others. True, not all that accurate.....kind of a 'better than nothing' check...
 
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