A question for the Mechanical Engineers:

[MrJTP2001]

As I start putting my project car back together I am concerned about bolt torque.

There are several conditions that exist when tightening a bolt:

1) New bolt going into a newly tapped hole

2) A new bolt going into an old hole with minor effects of age

3) An old bolt going into an old hole, both with the effects of age (in my case the car is 41years old).

Take for instance the M12x1.5x30 bolt that attaches the caliper to the carrier plate. Lots of rust. The hole has been cleaned by running a tap through it, then flushing the hole. The bolt (for the most part NLA) is in fairly good shape. A new bolt would be used if found, but the PET and the shop manual don't give a clue as to the specs for the bolt. I could assume its a 10.9 fine thread but is that all I need to know? So take this an example case.

I'm pretty certain the the specs in the factory manual are for condition 1 above. When putting these back together we're at condition 3 above. There is significant resistance when threading the bolt into the hole. This will give a false torque reading and possibly result in under tightening the assembly.

One could lube the threads with oil to reduce this friction. On a new assembly, oiling the threads reduces the required torque by about 20%. Ignoring this can result in over tightening the bolt resulting in possible damage to the assembly.

So, what does one do when faced with conditions 2 or 3 above? In this case I cannot get a new carrier plate so my best case is condition 2.

Any ideas?

[davep]

I can check to see what size the spare caliper bolts I have are if you want some new ones.
Tapping old threads can cause them to be slightly undersize I have found. Bolts tend to have the threads rolled onto them when made, but I'm not certain of threaded holes. The different manufacturing methods tend to create slightly different threads.

Lubricating threads will reduce the torque required for intalling. However, getting things apart after often requires some anti-seize lubricant when installing. A catch-22. I many cases I will use some anti-seize or some Loctite. Some caliper bolts use a lock plate to keep the bolt from rotating after installation. Racers use safety wire to do much the same. You do have several options there.

[MrJTP2001]

Thanks for the answer Dave. The specific bolt in my example does have a lock plate so there is no problem with it staying in place. I just used that particular example as an illustration. Indeed, running a tap through a threaded hole does reduce the amount of engagement for the bolt. The tap leaves shiny metal behind so some metal is lost in the process. I have used anti-sieze lubricant on several of the threaded fasteners so I won't (hopefully) need to use my air tools to get them off next time.

I guess it comes down to using an intelligent 'feel' when coming close to the specified torque for each location. Obviously the metal lost to the tap or to rust will reduce the maximum axial load on the assembly but lets assume Porsche's engineers took that into account. I'm not too concerned about my project car since it (and I) have accumulated all the track time we're going to see.

Being an aero/electrical engineer I could see a wide range of possibile outcomes for each of the various fasteners on a 40 year old car and just wondered if anyone with knowlege in that area had given it enough thought to hand out some guidelines.

I do have bolts for use on my calipers. Mine cleaned up pretty well and, after a lot of googling, I found some leads on DIN10.9 fine thread bolts.

Thanks.

[dvkk]

..., I found some leads on DIN10.9 fine thread bolts.

Don't get hung up looking for "fine thread" bolts. Such a designation does not exist in the DIN standard. Your M12x1.5 thread is just the common M12 thread.

[MrJTP2001]

Hi DVKK,

Yes, that's true for an M12 DIN Class 8 bolt but I was contemplating replacing the bolt with an M12 class 10, wherein the 1.5 pitch is a fine thread per DIN 934.

Oops. Should be DIN 960

[304065]

First a question: What is your use for the car? Concours at one end of the spectrum, racing at the other. Where are you in the middle?

For example, most of the bolts in an SWB 911 have a headmark of 8G or 10K. This predates the "property-class" spec promulgated by DIN and later ISO-- it didn't come out until the late 1960s. So to be original, you have to have the "old" headmark which precludes you from using modern hardware.

Second: the factory specified a higher property class when they wanted to, and it's in the parts book, you have to know how to find it. I think the electronic version of PET has deleted all that information, but the paper books still have the annotations such as "phr" for phosphate coated, "verz" for "verzinkt" or zinc-plated, etc. On my SWB resto about the only time they deviate from an 8G (modern 8.8) property class is for critical bolts like the uppermost pinch bolt in the steering u-joints and the bolts that hold the banana arms on. There are a few others.

You can't just switch from an 8.8 (I'll use the modern ISO lingo from now on) to a 10.9 and call it cool-- the bolts have different phyiscal properties. Certainly you would never go from 10.9 to 8.8 but there are examples where the brittleness of the higher tensile strength bolt may be unsuitable.

So back to your issue-- first, ask yourself these questions:

1) Is anybody every going to SEE this bolt in normal installation? If not, jump to the end.

2) How critical is this bolt to the operation of the car? If the bolt failed would death, serious bodily injury or property damage result? If it's a bolt holding the caliper carrier to the strut, I think the answer is yes. So jump to the end.

3) What is the physical condition of the "old" bolt? Is it reusable-- has the hex been buggered, has the bolt necked down (measure with calipers) or is there obvious pitting from corrosion? How are the threads? If any of these is present, skip to the end.

4) When bolts are tightened beyond the yield point there's usually a necking down, or a reduction in diameter. Since you don't have the original KAMAX or VERBUS specs (or whatever headmark is in the car) it's impossible to test, but you should be able to do some basic measurements and figure that most fasteners get narrower toward the tip, and don't have an hourglass shape. If you have made it this far, good, but if the dimensions don't seem right, skip to the end.

5) The thread fit is a concern, particularly for bolts that have suffered corrosion and been processed (see my thread on headmarks for a picture of bolts that have been tumbled)-- plating will put a tiny bit back, but I wouldn't use a tap to clean out threaded holes, you are probably removing metal. This increases the tolerance which complicates things.

6) The torque spec is an approximation of the force needed to overcome friction and stretch the bolt to the ideal preload. The ideal preload is a function of the force trying to pull the fastened parts apart. You are correct that rust will drive the required torque up resulting in insufficient preload. So, you want to have the conditions as close to "new" as you can. This means removing the rust by blasting, tumbling or wire brushing and assembling without grease or oil.

7) If you lubricate the friction goes down, but now you have another problem, which is that you may be stretching the bolt beyond the yield point. Again, you can get a feel for this if there's a sickening breakaway during the torque application (often accompanied by a "popping" sound as the fastener breaks . If everything is smooth you're probably good to go.

Here's the end part: if you're in doubt and the fastener is critical, get a new one from PORSCHE. They still stock everything with good headmarks, probably not original, but it's their chain of custody from the manufacturer, which means you won't get a counterfeit copy (more relevant in your discipline of Aerospace, but still can happen in the stream of commerce).

EVERYTHING has a DIN Spec. Hell, the Germans even have a DIN spec for the typeface they use. The whole fine thread/coarse thing is an Imperial convention, the DIN spec sets forth the appropriate thread pitch. You should use the exact same pitch and property class as specified in the parts book. Let me know if you can't find something.

Hope that helps. . . I'm not an engineer (I'm a lawyer by training who reads a lot) but I sure do like bolts.

I apologize to Jim Sims for saying "bolt" instead of "screw."

[MrJTP2001]

[QUOTE=304065;204804]First a question: What is your use for the car? Concours at one end of the spectrum, racing at the other. Where are you in the middle?

No concourse, no racing. Aim is to repair 40 years of damage and return it to a well running, nice looking 911 for casual driving.

...You can't just switch from an 8.8 (I'll use the modern ISO lingo from now on) to a 10.9 and call it cool-- the bolts have different phyiscal properties. Certainly you would never go from 10.9 to 8.8 but there are examples where the brittleness of the higher tensile strength bolt may be unsuitable.

That's the heart of my questions. If one upgrades to a higher class will you be gaining (for example) higher axial loads will sacrificing susceptibility to shear damage. That's where I was looking for some guidance.

So back to your issue-- first, ask yourself these questions:

Jump to End

Here's the end part: if you're in doubt and the fastener is critical, get a new one from PORSCHE. They still stock everything with good headmarks, probably not original, but it's their chain of custody from the manufacturer, which means you won't get a counterfeit copy...).

...You should use the exact same pitch and property class as specified in the parts book. Let me know if you can't find something.

There are always procurement problems with these SWBs so I might be in touch from time to time. I read your other post about HeadMarks. You mention that you 'chase' the threads. To me that means using a tap or a die. How do you do that without losing metal? After the walnut shells I use a very light wire wheel to clean out the threads. Is that the purpose of your 'chasing'?

Thanks John, I'll re-read your post and make sure I heard everything you said.