Need opinion on thread engagement, head marking

[MrJTP2001]

In preparation for installing new shocks on the front of my 68 911 I removed the strut and was giving it a good cleaning. Then I noticed something I wasn't comfortable with. In the industry I retired from, a threaded fastener must expose 1.5 threads beyond the threaded part it is being attached to, be it a nut or, in this case, the caliper carrier plate. All four bolts holding the caliper carrier plate to the strut are called out as M10x25 with no washers being called for. The two bolts at the front of the carrier didn't quite come flush with the threaded portion of the strut. The two rear bolts, having to pass through the splash guard mount as well as the lock plate, failed to extend at all through the mount on the strut. In fact the bolt was 3 to 4 threads short of even coming flush with threaded hole on the on the strut mount. I measured all four bolts and they were all within 0.5mm of being the specified length.

I was tempted to just use longer bolts and forget it since there would not be an interference problem if the bolts were 5mm longer. But these bolts (4 on each front strut) have head markings unlike any others I've seen on this car. Besides the standard "Kamax 8.G" markings there is also a circle deeply pressed into the center of the head.

So, I'm hoping a thread engagement/head marking guru will explain the situation and discuss a possible solution if one is needed.

TIA

[gled49]

That hardware is of the old style and markings. I call them 356 hardware to distinguish them from newer hardware. the markings: 5g, 8g, 10k, and 12k were the earlier markings and were replaced with the newer: 8.8, 10.9 and 12.9. If you can't find hardware of the proper length you want, with the proper marking, then you need newer hardware. 68 was right at the time they were changing over to the new markings so you will find both. Gordon

[MrJTP2001]

I realize the bolt markings are not current. I am more interested in any significance to the circle in the head marking and whether, due to any significance of the circle, there might be a problem if I simple used 5mm longer normal 8.8 bolt. And, I am quite uneasy about the lack of full thread engagement on something like the mounting of the caliper carrier plate. The picture shows the worst of the 4 bolts as far as thread engagement but none of the bolts even came flush with the back of the mounting plate. All parts in the image are original and match the PET.

[304065]

Tony,

If it makes you feel better, go ahead and use a longer, modern screw. You are correct that those are M10x25, no washer is used. However, just double check that you actually have 25mm from under the head to the tip of the screw-- an inch should be able to capture all of those thicknesses.

You are correct about the old headmark, this is before DIN/ISO changed the nomenclature for property-class. Which means you have a genuine, 40+ year old German screw right there. And the trouble with old fasteners in critical applications is that you're trading off 40+ years of corrosion and unknown stress/strain in the fastener for the sometimes shaky QC and Taiwan/China provenance of a modern fastener.

No judge will EVER see that location. And yet every time you hit the brakes, those fasteners come into play. Why not replace with a modern fastener that provides you thread engagement you are comfortable with? And save the Kamax headmark for where it will matter.

[chris_seven]

Tony,

There are many opinions on this subject and I think that some common ground is needed as I have heard in some cases that 1 thread is a minimum and others that say 3 threads. I think that these suggestions are simple methods to try to ensure that some arbitary assembly standards are being met but they have little in the way of foundation.

In the case of a nut ASME PCC-1 (Guidlines for Bolted Joints used in Pressure Vessels and Pipeline Assembly) recommends that the end of the bolt should be flush with the nut. It is permitted to have the bolt longer by about 3 threads but this is not advised as corrosion of the exposed threads can cause damage to the nut if a bolt is removed and the nut is not replaced.

ANSI Standards permit that 1 thread of the nut can be exposed but I am not sure that I agree with approach as this may reduce load carrying capability.

ASME PCC-1 also suggests that any free threads should be at the opposite end of the joint.

When you consider threaded holes in forgings I am not sure that the same rules apply. If the length of thread in the forging is significantly greater than the length of an equivalent nut then there is no need to have full thread engagement as there will be adequate strength.

I wouldn't worry about exposed threads providing you have sufficient engagement of the bolt. An M10 hexagon nut would typically be 8mm long.

It would clealry do no harm to fully engage the thread but it won't improve the performance of the joint and I wouldn't use a fastener that protruded any further than being flush.

If you think about a stud being fitted into a threaded blind hole the stud can never protrude so again also long as there is sufficient thread engagement to support the required load all is well.

If you use the stock length fastener I would apply a little paint into the exposed thread once it was tightened as this will hold back corrosion in this area and avoid it looking scruffy over time and won't really do any harm.

[MrJTP2001]

Thanks John. I was wondering if the circle in the headmark might mean something like special heat treatment, or .....? During full dismantling of the car there were only 8 bolts with that headmark and they were all on the caliper carrier plates.
I realize the bolt will be in shear so full engagement isn't as critical but I haven't checked to see how many threads are engaged.
I did pull the bolts and they are all 25mm +/- 0.25. I could next go measure each component in the stack but instead I think I'll use 30mm bolts and get on with mounting the new shocks. I'm trying to get the car into the paint shop this month, rebuilt engine due back soon; I gotta keep moving.

Tony

[MrJTP2001]

Chris, thanks for your reply, we must have been typing at the same time. Your reply was very informative and gave me a couple of things to measure before pushing on.

Tony

edit:
I took a couple of measurements and the bolt does engage 8.7mm of the threaded portion of the forging. It may not look good but it apparently does the job.

Thanks again to both.