I recently had six bolts shear off my rear CV/hub assembly, and I want to replace the bolts with the best ones possible. I have some questions about materials.
I need to use allen screws, and these are the materials available through McMaster Carr in standard screws:
Black-Oxide Finish Heat- Treated Alloy Steel These screws are heat treated for strength and have a Black-Oxide finish. This is our most common socket cap screw material finish.
Ultra-Coated Alloy Steel Ultra Coating provides nearly twice the corrosion resistance of hot dipped galvanized screws. The clear, silver coating also known as Armor Coat, is applied to Cadmium-Plated Steel and does not affect the thread. Screws with this coating act as effective replacements for Stainless Steel screws.
Zinc-Plated Alloy Steel Alloy Steel with a bright Zinc-plated finish that resists corrosion.
18-8 Stainless Steel Offers excellent corrosion resistance. May be mildly magnetic.
316 Stainless Steel More corrosion resistant than 18-8 Stainless Steel and just as strong. It's nonmagnetic, and contains Molybdenum that helps it withstand many industrial chemicals and solvents, as well as pitting cause by Chlorides.
300 Series Stainless Steel Screws are corrosion resistant and meet military specifications that are more stringent than standard screws.
Nickel-Chrome Plated Stainless Steel 18-8 Stainless Steel polished to a mirror-like finish. Finish resists corrosion and wear.
I used the 18-8 stainless bolts, and they failed. Any idea if there's a better choice here, or a better choice that might be available elsewhere? I need to stick with Allen screws since there's not clearance to get a socket, etc., around the head of a normal bolt.
Thanks in advance for any advice.
Rick is correct, stainless is not a super strong material. Your best bet is to find a grade 8 material. I just did a quick search for "grade 8 socket cap screw" on the internet and they are available. Don't know if McMaster carries them though?
Sand Diego Duner wrote:You might want to check out aircraft bolts. How are the bolts breaking anyway?
It only happened once, and they just sheared off. It could be that the CV seized up, but I don't think so. When I got it all apart and cleaned the CV up, it seemed undamaged.
I changed the original bolts from black oxide to 18-8 stainless when the black ones started to rust up on the heads.
quad1100, thanks for the tip. I'll try to track down some grade 8's and swap 'em out.
Got the grade 8's at Brown Nut & Bolt in Paramount. Swapped 'em out and I'm feeling better about the chances of that right wheel staying put! Steve, where are those bolts available?
I understand that Stainless is softer too. Softer than a grade 5 I think. Stainless is only good for low stress applications to prevent rust especially on high temp components like exhaust. That's for sure why they broke.
The strongest type I know of is Grade-9 (maybe that's F-911) but they are harder to find than Grade-8. I know a place in Garden Grove (HillcoFasteners.com) that does nothing but fasteners and carries Grade-9 hex head bolts that I bought from but I don't know if they have allen head Grade-9. They seem to cater a lot to offroaders even sponser some SCORE trucks and all their bolt needs. They know everything about bolts and if you told them you need them for a 930 CV, they would know exactly what you are talking about.
BTW - Using Allen Head bolts is the other reason why they broke. Allen heads are not as strong as regular bolts because the hole for the allen wrench doesn't leave much material to attach the head to the shank. Are you sure you can't use 12-point heads. That is what I got on my 930 CVs with boxed arms. They are only 10mm across the head. I think that's what most people use on their CVs since normal bolts wont clear.
It's amazing how much I have learned about bolts from building my own buggy. A couple of months ago, I couldn't even tell you what a Grade-8 bolt was exactly
Not sure about 12-points bolts. The ones that let go broke on the other end, basically where the bolt went into the hub, not at the head end. Even with the allen-head bolts, it's tight. The zip-tie that holds the boot on gets pinched by the bolt heads.
A few years back when I was a Porsche/Audi tech. new CV's came with new bolts and you didn't re-use the old ones. At the time it seemed like a waste but since then I have learned that the manufacturer is not in the habit of giving away anything more than necessary. From the description of where they failed, it sounds like they were over torqued and stretched.
Not only is the material important, but the bolt body tolerance to the CV body is critical. I have gone to two local race shops and they have recommended bolts that were INCORRECT!! The factory bolts are the ONLY ones that should be used. The U.S. size bolts that are close are not close enough. There is no shoulder and the body of the bolt is substantially smaller than the factory bolt. This not only makes for a weeker bolt, but allows the CV to rotate ever so slightly if the bolts are even a little loose. This allows the axle CV assembly to get some momentum before it hits the bolts and enhances the probability of shearing them off.
The best bolts I have found are from Coast Fabrication in Huntington Beach. They can get ANYTHING you need if they don't have it.
For the record, NAS bolts are the highest quality that I have found. These bolts are rigorously tested and inspected, and are of the highest quality materials.
Not only is the material important, but the bolt body tolerance to the CV body is critical. I have gone to two local race shops and they have recommended bolts that were INCORRECT!! The factory bolts are the ONLY ones that should be used. The U.S. size bolts that are close are not close enough. There is no shoulder and the body of the bolt is substantially smaller than the factory bolt. This not only makes for a weeker bolt, but allows the CV to rotate ever so slightly if the bolts are even a little loose. This allows the axle CV assembly to get some momentum before it hits the bolts and enhances the probability of shearing them off.
For the record, NAS bolts are the highest quality that I have found. These bolts are rigorously tested and inspected, and are of the highest quality materials.
Thanks for the bringing this one back up! Of course you are absolutely correct as we have seen many different failures brought on by not using the proper fastener for the application.
Hmm.. That could apply to many things, couldn't it?