I have always used new fasteners on all internal parts every time I build something.I recently got some new ring gear bolts at Summit and was thinking I just changed them 5 years ago when I put the 3.54's in.Do you guys change ring gear bolts,flywheel bolts,pressure plate bolts etc. when you have them apart? I just think it's just cheap insurance to use new top quality fasteners when you have it apart
Now I know an aircraft tech and he says sometimes it's better to know what you have instead of using new.
Do you guys use new bolts every time you change out rotating high stress parts like flywheels and ring gears?

Gus

No.
Old parts have proven themselves to be good.
New parts haven't yet...

No, I don't trust fasteners anymore, how do you know for sure where they are made? The original fasteners worked just fine, if they still look good I use them.

I toss any critical fastener previously used, be it engine, trans, rear end, suspension, braking system, a fastener can only survive just so many stretch, stress, along with heat cycles before failure...I generally prefer to increase the grade/tensile of the fastener if I can when replacing it

On this topic, I would say one of the most re torqued, without at least a torque wrench, possibly over tightened with an impact, cyclically stressed, non redundant, critical fasteners, is also one of the least replaced items on a high performance car, the wheel studs.

ARP

Gus, if it ain't broken, don't fix it
if marginal or suspect replace it with better parts

New ARP bolts inside the engine. Other less important stuff is replaced if it shows wear and tear and looks suspect.

My last engine rebuild was based from an engine that came from a van that burned to the ground so I bought all new fasteners throughout.

I don't replace bolts unless I see a problem with them. I throw bolts and nuts away if they have any damage but otherwise just clean them and put them back into service. Bolts are basically springs and we reuse springs over and over until they get damaged or lose tension.

Gus, I have changed gears in my Dana at least 5 times if not more. I am going to change them again and this time I will change out all of the bolts. May not need to but after this many times why take the chance -

Never

Absolutely spot-on...

That's what most people believe. Without any metallurgical background, of course...

Actually I have 35+ years experience in a metallurgical/engineering field

Send me all your wore out bolts, I'll get rid of them for you.

Which is the exact reason why I agreed...

Ok, so explain better your statement. By the way I was the senior metallurgist in one of the major independent test labs in the south for 34 years, so we speak the same language.

So in your expert opinion is a visual inspection all that is needed to reject a fastener? I have been a technician for over 30 years and have rebuilt probably 100 engines of all types. I always replace head bolts and main cap bolts on an engine that has been in service for 50 years and is going to be whooped on repeatedly. On regular passenger car engines that have come in to the dealership that need a rebuild due to head gasket failure I always replace the head bolts. What is your professional opinion on reusing plastic region stretch bolts.
I've been doing a lot of rebuilds lately and sometimes the head bolts don't seem to pull that 90 degree turn all the same, some feel like they pull down easier than the others. The bolts are too long to fit my calipers so I have never tried to measure them for comparison.
Thanks
Gus

Obviously if a bolt has stretched or is corroded, bent, or visually damaged, by all means throw it away. If you are not sure, you can measure the hardness, check its structure and grade (destructive tests, in general) and chemistry to see where you are. And as far as fatigue one must know if the stress on it is in the fatigue range, and so is temperature, that has to be high enough to affect the metal. All of these tests make sense in a lab, but practically it's much cheaper and easier to use new fasteners, so I don't have a problem with that.

I look at it this way, there are certain fasteners in an engine that, if they fail, the cost to repair far outweighs the cost to initially replace. Rod and main bolts come to mind. Head bolts don't quite fall into that category for me but the thought of hanging over a fender wrasslin' a head off because the headgasket failed due to a soft headbolt puts those into the replace category for me.

The rest, not so much.