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Discussion Starter #1
We've always accepted that head bolts need to be replaced anytime they are disturbed but I don't see that requirement in the build manuals. I might be missing it...

Are there any other bolts that are mandatory replacement?
Con-Rod bolts seem logical...other possibilities off the top of my head are flywheel and maybe the crank pulley. Am I missing anything or is there some black and white guidance somewhere for me to read?
 

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I’ve reused flywheel bolts numerous times. But always new head bolts. Basically anything whose torque specifications call for a number plus another x degrees are not to be reused.
 

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So, the way metal yields is defined by a graph for each metal. When you see "torque to X and then Y degrees", it's saying torque to the high end of the elastic region, then turn till you're in the plastic region.

Anything in the blue region is "elastic" and causes no permanent deformation. Anything in the green region results in permanent deformation of the fastener. Interestingly enough after being plasticaly deformed steel will follow it's original stress curve back down parallel to the original curve from the point on the plastic curve.

In theory if you can get a torqued TTY fastener exactly to where it was when you removed it on a stress curve you can reuse them almost indefinitely. The problem is actually doing that. Torque is only an analog of stress, and further, because you've elongated a portion of the fastener from how it was when you initially torqued it, the same torque applied to a used fastener creates a different stress level from a new fastener, which is greater. And no, there is no conversion factor to use here, if such a thing did exist it would be highly dependent on the exact material used in the fastener.

There is also some science behind deformation being an important part of not backing out, but stress/strain is what governs clamping load.

If you do reuse a TTY fastener, you're probably best off measuring the breakaway torque when removing it (with a digital torque wrench this is pretty easy and not too time consuming) and retorquing it to the same number. If you don't want to bother to do that, you can probably multiply the torque value specified in the manual by 1.15, for an additional 15% of torque to reach the same strain as originally applied by the "torque to X and then Y degrees" pattern.

For the most part it's really a question of "Is it worth the cost of replacing the fasteners to not have to do the job again." On my VW when I did the camshaft the bolts for the camshaft caps came out to $200 through the dealership after my aftermarket bolts got overtorqued by accident and I decided that yes, I probably should replace and not reuse the fuel injector rocker bolts as the aftermarket kit assumed you should. That was the borderline point for me, where I was strongly considering just reusing the over torqued bolts and hoping for the best. (Torque spec was 7ft-lbs and then turn 90 degrees 3 times in a pattern, the 7 ft-lbs was exceeded by a screw driver.)

Head bolts are what, $70? Totally worth it. Now, bottom end bolts OTOH are very hard, if not impossible to find, resulting in more of a case for reusing them. When I make a choice to reuse a supposedly TTY fastener I look at a few things. How critical is the part? Will I die if this fails? Is this fastener ON a rotating assembly where it's more likely to back out? Is there obvious evidence of elongation on any of the fasteners already? Are replacement fasteners easy enough to source? Could I just replace this with a non-tty fastener and still meet my general safety requirements.
 
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Discussion Starter #5
...For the most part it's really a question of "Is it worth the cost of replacing the fasteners to not have to do the job again." On my VW when I did the camshaft the bolts for the camshaft caps came out to $200 through the dealership after my aftermarket bolts got overtorqued by accident and I decided that yes, I probably should replace and not reuse the fuel injector rocker bolts as the aftermarket kit assumed you should. That was the borderline point for me, where I was strongly considering just reusing the over torqued bolts and hoping for the best. (Torque spec was 7ft-lbs and then turn 90 degrees 3 times in a pattern, the 7 ft-lbs was exceeded by a screw driver.)

Head bolts are what, $70? Totally worth it. Now, bottom end bolts OTOH are very hard, if not impossible to find, resulting in more of a case for reusing them. When I make a choice to reuse a supposedly TTY fastener I look at a few things. How critical is the part? Will I die if this fails? Is this fastener ON a rotating assembly where it's more likely to back out? Is there obvious evidence of elongation on any of the fasteners already? Are replacement fasteners easy enough to source? Could I just replace this with a non-tty fastener and still meet my general safety requirements.
Glad you popped in Stuart, thanks for the perspective.
The bolts that triggered the question are indeed the lower end bolts that fasten the bulkhead section to the block, and your right, they aren't necessarily easy to find, especially the double ended ones. Other bolts on the engine like rod bolts are an easier choice.
81419
 

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so 4 ugga duggas is the correct spec for reusing a TTY bolt then?
Eh, this isn't a drain plug, give 'er at least 7 ugga duggas.
 

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Discussion Starter #7
It looks like the Factory torques are around 10%ish below the max service torque so when in doubt I'll just go to max torque. Its not quite Stuarts +15%, but it should be +a little bit so I'll call that good. A little loc-tite should keep them tight.
 

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Discussion Starter #8
I took a quick scroll through the build manual and here's what I found:

-The only fasteners specifically described as TTY in the whole book are the con-rod bolts.

-List of fasteners with a "torque then turn some more" spec:
-Pipe plugs in the block
-Bulkhead to block bolts (inside the engine)
-The nuts on the oil baffle plate
-The nut on the oil pickup tube
-Head bolts
-Crank pulley bolt
-Water pump mounting bolts

Since some of those are steel bolts in aluminum threaded holes, I'm going to venture a guess that they are not actually TTY, just particularly important or prone to vibration.
I'll replace the head bolts because every engine builder since the age of dinosaurs says you have to, and I don't want to be the asshat who doesn't do it and gets a head gasket leak, and I'll replace the con-rod bolts but the rest are going to be re-used.
 
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