S-Prihadi

Gents,

I actually asked this question like 3 years ago, but at that time the reply was not from someone who actually understood in great detail such process.
I am hoping more machinist or anyone in the specific industry who can explain better.

I will remove my rear subframe for bushings replacement next year.
I been reading the WIS again and hence the question I had back then, have resurfaced again.





The bolts with number 1a and 1s are the bolts in question. The bolts 2a is not too critical.

Here is what WIS of 2017 and 2020 explained on the process.










This is how the bolts looks like for 1a ( item #130) and 1s (item #140 ), same exact bolts.





The shorter 2a bolts for stop plate










ASK
If a thread forming screws they are :
So, .... when I remove the bolts, do I use new bolts or I re-use old bolts with some thread sealant ?
MB WIS expects that I cut away current female thread/bore on car body, weld a new one with still a virgin bore and do thread forming again..... that is what I understood from the WIS yes, no ??


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I then look at the front subframe, to see what kind of bolts are used.
Here MB called it THREAD FORMING BOLTS.



Bolts/Screws item 340 and 350




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The WIS only mentioned replace ( one time use ) for item 350 the long bolts.
No 17 in wis is bolt #340 and no 18 in WIS is bolts #350.



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Thanks so much guys..............

Jeedie

Use new bolts from MB, re-tap the existing thread in the floor to clean out any remaining thread lock/rust/whatever, blow clean to remove any metal debris and install the new bolt.

Key here is that the bolt has a preload and then an angle-based tighting, indicating these are stretch bolts. As MB does not specify tolerances for used bolts, you cannot reuse the old ones. Further, if there's any debris on the thread, this could lead to wrongly stretching the new bolts, hence the need to have fully clean threads.

Don't cut out / weld anything, just make sure the thread is fully clean before tightening the new bolts.

S-Prihadi

Yes, that is what my Indie advised me to do too. I will use loctite though.

I will still buy new bolts, for just in case.

bbirdwell

If the new one -time use bolts already have thread lock on them; do not add liquid thread lock as it can act as a lubricant and reduce friction between the threads. To reach recommended torque the bolt might be over-stretched.
I concur with above advice on cleaning threads with a tap.

S-Prihadi

Yes, new bolts come with its own "dry" thread sealant...the one MB uses. Micro capsules or something they call it.
If I do re-use old bolts for fear the new one being a thread forming bolt and somehow minor damage the existing female thread, I will use loctite.
When the time comes, I will inspect the female thread and will inform u guys yah.
Its like 6 months more approx .....

Jeedie

Micro encapsulated bolts. Below some specific info, including the need to re-cut:

S-Prihadi

Aha........I realized it is language barrier on my part.
I define a CUT as a total removal ..DUGHH !!!!!!
So I thought the female thread must be gone completely and install a new blank bore for re-forming the thread by new thread forming bolt...LOL. what a dork I am.


I found a more suitable word. It is called thread restoring or thread chasing some guys call it.

kajtek1

I learned mechanics long years ago when such technology did not exist, but from my experience:
-this is not a stretch bolt
-you are not recutting threads in the metal, but cleaning old glue from previous bolt.

IMHO the technology has a assemble speed in mind and not really strength.

S-Prihadi

Yes those are not a stretch bolts. Only 80 NM and then 90 degrees value on them, is not even as tough as the front Camber & Caster Arm at higher values..which they are also not using stretch bolts.

chassis

Use new screws.

Do not apply additional thread locking compound.

Do not drill out the old thread.

"Chase" the existing threads using a blind tap.

Rickman30

Like Jeedie said. It is not reforming any threads, it is cleaning any and all debris from existing threads for proper torque application of the bolts in question.

I have reused old bolts before, as required by a cheap shop owner.

S-Prihadi

Thank you all..... wheew...I feel much better now , that I get the "CUT" interpretation correct.

Quint22

Any bolt that has a torque then a predetermined turn radius (say 20ftlbs then 90degrees) is a stretch or torque to yeild bolt and needs a new one technically. It sucks that MB uses these everywhere. Another money saving area that screws the end user for longevity.

Long story short buy new bolts/screws, clean out the old threads and reapply new hardware. Same scenerio on my cummins head bolts. When replacing them you buy new, ya chase out the threads in the block to clean them out, install new bolts with a torque then 90 degree rotation I believe. You can replace them with good bolts that are just torqued in stages but much more expensive.

ygmn

Just because it gives a torq to some value and then (Turn of Nut) an angular rotation does not mean stretch to yield bolt that should be replaced every time.

Normally in bolt world any USA Grade 8 or above and DIN 10 or above should be replace after use but depends on how bolt is installed, static loaded and dynamic loaded (Fatigue) and also sometimes no redundancy to save Life or Limb.
All this depends on engineer and company engineering specs on how they tell assembly line and then service techs how to assy bolt.

This new world of torq to value and then turn certain angular amount came about because the engineer wants a certain PRELOAD that comes from stretching bolt normally up to 80-90% of yield strength and then the design loads never exceed 70% and he can be sure joint does not separate and can handle fatigue issues.

Torq alone is a crappy way to tell tech what we want for preload so the bolt world decide they needed better method that was simple.
TURN of the NUT has been around for decades in AISC steel building codes
BUT issue is when do you start measuring angle - used to be run nut up until SNUG then rotate 30deg etc.

Some smart guy came up with idea of low pre-torq using wrench to ensure snug joint and is not too much to have a large stretch error if friction is off (not enough or too slippery of torq friction) then they can TURN of NUT.

I can say for super critical bolts we would require bolt tensioners - but these would be large 1 1/2" OD bolts.
Another method for accuracy is measure length after torqing in stages (Think Piston Rod bolts)
Yet a company called SUPERBOLT came up with a dial on the bolt head which you spin while tightening and once it stops spinning the bolt is proper.

kajtek1

Not.
When I was replacing head gasket on OM603 engine, the WIS gave me tolerances for reusing old bolts.
Bolts are relatively cheap, but the engine had almost 50 of them, what would add the cost.
I set my caliper at 50% of the given tolerances and all bolts passed the requirement, so I reused them all.
It is last years, when manufacturers push for new bolts all the time. In my younger days we have been reusing bolts dozens of times just fine.

Quint22

ygmn the torque to yield is that unless your talking about some random torque value that is generated by the snug then turn amount but I cant recall that being used since before I learned what a torque value was. Grade 8 is getting down there near chinesium bolts so not saying much. Every bolt that I have ever torqued if it was a torque then rotate it was a stretch bolt because they couldnt give a torque value and they were using smaller lower rated bolts to come in under cost for the situation but still meet the requirements.

Good ol high standard American bolts use a torque value and can be reused although techincally like kajtek1 said they should be measured. No one really does that much anymore that I have experienced.

Anyhow in the real world high torque applications and precise equipment its a torque value or replace upon removal torque to yield type. Not sure about the construction world, imagine thats alot more forgiving.

ygmn

Kajtek:
I said normally they should be replaced which does not mean every time even though I would replace head bolts - as cheap insurance and who knows if one or more of the old bolts has permanent stretch as I would not have measured them beforehand and/or bad threads.


Quint22
Turn off nut is when they say torque to 45 ft-lbs then turn another 90degrees.
Torque to yield is tighten bolt until it just passes the yield point and has permanently stretched a bit - this gives a consistent clamp load required for todays specialized and different materials used in engine assembly.
Those thermal properties (expansion rates) can be a real PIA when designing bolt joints that have huge temp swings - Think pipelines in a refinery - Below freezing to hottest summer, high pressure and a nasty chemical inside pipes connected by bolted ANSI flange joints.

Gr 8 bolts is the highest USA SAE spec bolt- made from high strength hard material and can be loaded more then a gr 5 of same diameter. these are not junk and have been and will around for as long as we use USA specs.
10.9 is DIN (German) bolt spec similar to gr8 but Slightly higher strength.

Many ways to tighten bolts and torque is main one as cheap and good enough for most part on bolts 1-1/4" and smaller.
For bolts 1-1/4" (1-1/4" can go either way) and above; bolt tensioning is preferred method as torque values are crazy high and not reliable due to massive friction in joint.

Not all bolts should be re-used. And if you do not know for sure and they are tension bolts then replace - cheap insurance versus andy life or limb.
Bolts in shear can be re-used as long as no visible deformation on shank or threads.
torq to yield bolts should be replaced as they have been "stretched" past their yield point and if used again the force to "stretch to yield" is less due to mechanics of the material.

Normal Bolt torq values tend to be 70% of Yield strength of male thread material - this does not mean all - Up to engineer

chassis

Where is this coming from? The global auto industry uses 8.8, 10.9 and 12.9 designations. SAE has no relevance for the past 30 years in the auto industry.

Suspension, steering and brake system fasteners are the most safety-critical fasteners on a car. Subframe-to-body bolts, the subject of this thread, are suspension fasteners and are therefore safety critical. Replace them.

Always replace safety-critical fasteners. It is false economy to do otherwise.

S-Prihadi

Awesome guys....... awesome. Thank you all

ygmn

Other industries still use gr 8, sorry for confusion.
I was discussing bolts in general not just on cars.

Quint22

Yeah different areas different specs and techniques. Have torqued many a large fastener with hundreds or thousands of ft/lbs torque. Obviously using a hydraulic torque magnifier and what not. These are usually torqued in phases and many times (usually) required special lubricants to ensure proper/even torque application. Oh and yes many of these were greater than 1.5" fasteners.

Grade 8 is good for many things but most of the applications I experienced grade 8 was basically the bottom of the barrel fastener and if found being used resulted in a serious poop show.

That being said working on subs and power plants is a different beast. Heck even the helicoptor arresting gear we had a contractor use grade 8 bolts because they didnt have the proper ones. Had to go back and replace and retorque all 1500 of them......