Cloud9...68

I need to replace the bolt that attaches the trailing arm's inner bushing to the torsion bar tube. The stock bolt is a grade 8.8, but all I can find at the local specialty bolt store in the correct size is a grade 10.9, which of course is stronger than the 8.8 grade, but also tends to be more brittle. The car is a track car, driven on a track that includes a sharp downhill right-hander, where the car can get airborne, so it takes a real beating. It has a full race suspension, including Racers Edge spherical bushings for the connection of the trailing arm to the torsion bar tube, and 800 lb/in coilovers in back (torsion bars deleted). It's a 968 with pretty much a stock engine. I assume Porshce used a "lower grade" bolt for this application for a reason, but for the metallurgy experts out there, do you think I'm OK using a grade 10.9 bolt, or should I order an 8.8? If so, is it urgent, or is the 10.9 OK to use for awhile? Thanks.

V2Rocket

http://rtstools.com/are-grade-8-bolt...s-10-9-metric/

Cloud9...68

Thanks - a great, concise explanation. Looks like I'm fine (actually better off) with the 10.9 grade bolt.

Droops83

Thanks for the link, that site has some cool tips and tricks.

Yep, just make sure you increase the torque value commensurately.

Cloud9...68

Yep, I did. And yes, that is a very cool site - I think I'm going to ask for the head lamp they recommend for my birthday!

V2Rocket

Curious about this - is it a standard practice to use higher torque on higher grade bolts for a given application?

Most steel alloys have similar tensile properties but the specific makeup and manufacture can give higher elastic/ plastic deformation points, hence the higher grade.

But I would think a 10.9 bolt would stretch the same (same clamping force) as an 8.8 at the same torque... just that a 10.9 could TOLERATE a much higher torque value before falling.

Would love to be corrected if my thinking is wrong here.

Droops83

I think you are correct in your thinking. The answer is "it depends." For the most part, an automaker will only use a higher-grade bolt if they really need to (CV joint, diff to ring gear, etc), and in that case they will be utilizing the extra clamping force and will publish a torque spec that is consistent with this. It also depends on the materials----with the exception of cylinder head bolts threaded into an aluminum block, you won't see a high-grade bolt threaded into aluminum or magnesium. No point in that.

As a 944 example, for years I had issues with the bolts for rear caster blocks of the front suspension coming loose after heavy track use, and it got worse when I went to monoball bearings instead of bushings. I was tempted to try higher-grade bolts with more clamping force, but I was reminded that these bolts thread into what is essentially a nut welded to the body, which is extremely difficult to repair if it breaks out, and probably would if repeatedly torqued to a higher value. The ultimate solution in this case ended up being the Brey-Krause lightweight brace between the two mounts, which spreads the load and keeps the bolts from loosening.

The bottom line is, it depends on the intended use and the materials in question, and what sort of structure you are threading into.

Humboldtgrin

Thread locker is always an option. There are many types for many applications.

Cloud9...68

Yes, depending on the application. It might be a good option for the caster block bolts, but the trailing arm bushing through-bolt uses a lock nut, so it ouldn't make sense there.

Droops83

Yes, blue Loctite is good extra insurance for suspension bolts, but it won't prevent the problem of a very heavily loaded bolt coming slightly loose---red Loctite might be better, but of course you'd never use that on a suspension bolt because you want to be able to easily disassemble the parts for repair/maintenance.

The issue with caster block bolts is they were loosening just enough to cause a popping sound during initial hard application of the brake pedal, which was the caster blocks moving. The brace is a great solution, though it does reduce the ground clearance of my already very low 951 even more!

Another area where this is an issue on the 951 is CV bolts----the factory 8MM bolts simply don't provide the clamping force necessary for a higher-hp 951 during track use, and they come loose (even though they are grade 12.9). The Porsche factory installed 10MM CV bolts on 911s/930s of the same era and it's never an issue. That is an expensive solution, though, so we have to band-aid the issue with safety wire or Stage 8 locking CV bolts. Neither of these maintain clamping force, but they do keep the bolts from backing out if they do loosen.

Regular users will probably roll their eyes as I post this for the 10th time, but Carrol Smith's book about this topic is a must-read for anyone who touches a wrench, and has much more insight on this subject:

V2Rocket

Droops - I wonder if drilling the CV/stub axle (if there's a suitable place) for a pin or two (like on the flywheel) would take some load off those bolts?
CV bolts have to hold up in tension (torque) and shear (rotation of the axle).

Droops83

The Porsche factory actually did this for the early 911/912 CV axles: there six holes as in a normal CV joint, with two 10MM lugs 180 degrees apart, plus four bolts at each flange. This went away when the 915 transaxle was introduced.

Six 10MM bolts at each flange is the ultimate solution---even the latest 991 GT2RS does quite well with this configuration .