Use of high tensile bolts. ( RCR40 )

[Neil]

When I answered OP's question, I had not intended to write a book about fasteners- Carroll Smith has already done that. The general idea is use the best quality fasteners that you can, especially in critical areas. Stainless (303/304) bolts might be entirely acceptable if they are big enough but that isn't generally the case so alloy steel bolts are generally used due to cost & availability-- and they are just fine. I don't encourage anyone to rush out and copy what I've done- your applications are probably different than mine.

In his book Carroll Smith also recommends against using commercial bolts in his cars, I noted my experience with ARP only as an example of how even a respected company can get things wrong now and then. My problem with commercial bolts is that you really don't know what you're getting. Other than size and ultimate tensile strength rating ("Grade 8", etc), head style (hex head, etc), and plating, everything else is unspecified.

An aircraft, military, or aerospace bolt is covered by a VERY comprehensive set of specifications- all of the previous specs plus the material (alloy steel, CRES, etc), shear strength & UTS over temperature, class of threads (3A, etc), notch sensitivity, fatigue specs, and on & on. Uncle Sam pays a lot for those bolts but they are the best you can get. AN/MS/NAS bolts are inspected over & over again. You're right, surplus bolts do not have certification paperwork but I don't care. Meeting their AN/MS/NAS specs is good enough for me.

You are right about a failure at 130 mph being certainly more catastrophic than one at 50 mph, but I'm looking at not having that failure at 200 mph.

About my rivets- the layout patterns are per the FAA handbook. 75ksi A286 or Monel rivets are needed to achieve the full strength of my 7075-T6 stressed panels (yes, they are stressed, forming part of the chassis structure itself). Pop rivets are fine for fastening non-stressed panels and they cost a heck of a lot less.

Your point about leading a novice down the garden part is well taken. Building a car is a learning experience and learning as much as you can about the "how & why" a car is designed & built the way it is is the key to gaining vital knowledge & experience. Sharing that knowledge & experience on forums like this is a good thing. Since there are so may forum members with so different levels of expertise it is difficult to answer a question posed by someone you don't know.

 

[Ed McClements]

Hi Edmund

Fortunately all of the GT40 replica manufacturers have used relatively large diameter fasteners (eg. 1/2") in most suspension areas...not the piddly little 3/8" fasteners which Colin Chapman used to specify on his race cars!

This pretty much means that providing you utilise a known-quality steel fastener of a suitable grade you will be OK. I'll qualify that statement further:-

You don't need an aircraft-spec steel fastener for a 1/2" wishbone bolt
You shouldn't use stainless fasteners in these areas, though they're fine for non-structural uses, especially in areas like bodywork retainers where they'll be visible
You don't need titanium fasteners, as their use really requires special attention with regard to thread lubrication, installation and subsequent maintenance
You shouldn't use eBay fasteners on your suspension or brakes, as there are so many cheap counterfeits on the mass-market
Find (and form a relationship with) a supplier of branded fasteners who knows their products and can advise accordingly

With a fastener of these diameters, and if used in double-shear (such as the inboard end of the wishbone mounts) I would personally suggest a grade 8 bolt will be sufficient, and they're widely available in a BZP finish with or without a trivalent chromate passivation for corrosion resistance. WRT the single-shear bolt between your steering rod end and upright - I'd be tempted to modify this arrangement to double-shear, but the current grade 10.9 bolt (sorry - this is the metric grade...you'd need to look up the equivalent...maybe A490?) is a good choice for that location.

So yes - change all those stainless fasteners for grade 8, bought from a known source.

Cheers,

Eddy

Disclaimer:- I'm just a hobbyist, like you. I have read the Carroll Smith book (and a Forbes Aird title, along much the same lines). I built a car and raced it a number of years ago, and it didn't fall apart! Carroll Smith was writing about high-end race cars, so some (but not all) of the considerations apply to a GT40 replica. I don't know if you have 350 or 600hp in the tail, or if you want to bang over the rumble strips without thinking about the consequences...like all the hotshoe drivers who don't have to maintain the cars. Other internet opinions are available!

Edited to add: I should have read the thread fully before replying. There are already some great responses, and mine adds nothing new. I'll leave it here for completeness, though.

 

[Edmund Lawson]

Thanks for the advice everyone .
Looks like I need to do some shopping then . I would think they were used because 8s are near impossible to find over here.
Most of the fasteners look like 304 apart from the two on the lower rear wishbones , brake discs and caliper mountings .
Engine mounts too.
On the same subject of fasteners , I noticed the front hubs and steering arm holes are larger than the holes in the part they fasten to ( using 3/8" bolts ) . Should I leave this as is or enlarge the holes and use 7/16" bolts ? Any thoughts ?
In the second pic, is this an acceptable way of doing this ? Usually bolt goes right through and uses a locknut , not threaded into the hole.
Other four pics are just examples of fasteners in various places

Ed.

 

[Howard Jones]

I use Fastenal. They will sell small quantities, seam to offer good quality (both control and material), will ship your online order and are local. I always buy a few extra (if I need 6 I buy 10) and leave them in their original packaging so I know where they came from.

I have been using their ecoguard option when available:

https://www.fastenal.com/content/documents/2012/09/EcoGuard_Snapshot_Apr12.pdf

Last thing, sort of. These are race cars. Rule number one of racecars is everything falls off if you don't keep it tight. That means inspections. Over and over until you find all the things that will fall off because the particular frequency of vibrations on your car loosen them. Most of the time lock nuts are the solution. If the piece gets hot then jet nuts will work, and sometimes the only thing that works is safety wire. I use low strength lock tight almost on everything as a matter of course.

There is no reason to use anything except grade 8 on any part that will kill you. I call them itwillkillyou parts. But they are more or less all the suspension pieces that keep the wheels on, steer, brake, power transfer, and safety equipment.

On pic two: I would use a longer bolt and a 1/2 height lock tight nut on the outboard side with a washer under it. Pic 3 is close to too much threads showing on that rod end. pic 3 and 4 you need these: Porsche part

 

[Neil]

Ed;

Here is a very useful article about bolts: http://tinelok.com/grade-5-vs-grade-8-fasteners/

For a street driven car I wouldn't rush out and replace all the 1/2"-20 stainless fasteners since the sheer size of those bolts makes the joint reasonably strong but I'd replace them when it is possible to do so with something better. I wouldn't risk running on a track with SS bolts, though. Notice that the chart in that website shows that even a grade 5, 1/2"-20 bolt has an ultimate tensile strength of over 18,000 lbs and over 14,000 lbs shear strength so no need to panic- just replace them and hope the previous owner used Never-Sieze on those threads.

Regarding your question about mismatched bolt hole sizes: Never fasten two parts together with a bolt that is undersized in one hole! You have two options, first, as you suggested, machine the 3/8" hole to 7/16" or sleeve the 7/16" holes down to 3/8". This is not a good idea. Having the larger bolt will give you a stronger joint, plus the sleeve would need to be an interference fit (be pressed into the hole).

As long as I'm writing this, let me warn everyone about using aluminum bolts and rod end bearings. They are available, strong, lightweight, and are anodized in pretty colors. The problem is with the fatigue properties of aluminum. In cyclic stressed applications such as suspension or steering, they are good for only a limited number of vibration cycles before they crack and eventually break. For the same reason don't use aluminum connection rods in your engine. Don't do it!

 

[Howard Jones]

here:

https://www.ecstuning.com/b-genuine-porsche-parts/spacer-plate-priced-each/9p1501375/

 

[Cam]

Mmmm ... I promise I’m not picking on you Neil but I’m feeling compelled to post a counter to what you’ve advised.

If I were reviewing Ed’s car I would be very concerned. It seems to me the car was built with sub-standard fasteners in critical areas. I would personally go through every single bolt and verify each one makes me comfortable - and that would be alloy steel in all structurally stressed locations. You’re (Neil) taking on some personal responsibility by recommending that Ed keep his stainless fasteners. I don’t know where you get the data to support this recommendation. It’s completely unknown what loads any of these locations were designed for and saying “it should be ok” just isn’t a good enough reason (to me) to recommend that someone take the more dangerous path - I think we can all agree that failure of a structural joint is dangerous. If the original spec was for a 1/2” bolt then the designer obviously thought this joint was going to be highly loaded and therefore went with a larger, more expensive bolt diameter. The larger diameter should make you think stresses here are going to be particularly high, and therefore your strongest available bolt material should be used, not the lesser.

No one here knows how Ed plans to use his car or what his driving cycle is but without knowing this or the design factors of safety, the conservative and responsible advice to offer would be to go with the alloy steel bolts. Maybe his daily commute includes 170mph blasts on the autobahn.

And while I agree that track duty is generally more stressful on a car, peak loads may actually be higher for a street car. Tracks are typically smoother and pothole free whereas the street is filled with a ton of things that could create enormous momentary spikes in load - pot holes, curbs, fender benders, road debris, etc. I would disagree this should be used as the criteria for standard vs sub-standard hardware.

Ed - before going out and machining your upright or making bushings, take a close look at how those parts are to be bolted together. The bolts there may take zero shear load and therefore don’t need to be sized the same - I can’t say for sure since I can’t tell how they all go together. I’m guessing the hub is piloted into the upright and that all shear loading is taken up by the bearing and pilot, not by those 4 bolts. There’s also an aluminum upright being mated to a steel hub; different materials, different CTEs. Ed - If you don’t understand what I mean by this paragraph then find someone who does and have them review your car. Either way, I’d check with RCR if you don’t understand what’s going on here.

It’s exceptionally difficult to manufacture an assembly using multiple sources and assume that every piece made by everyone will be exactly the same each time - every component will have some type of manufacturing tolerance. So RCR likely designed the upright to use a 3/8” bolt knowing that the hub would be machined with 7/16” holes of unknown positional accuracy - the smaller bolt ensures that any hub you purchase will fit, now and forever. If I were designing this assembly I would make the hub fit almost exactly and allow the bolts to vary - shear loading taken by the pilot, axial loading taken by the bolts. Making all 5 exact would be an exercise in futility and extremely expensive.

I can think of a thousand places on my car where my holes are purposely mismatched (every single one). Every commercial and aerospace product I’ve ever designed also employed non-identical hole sizes. I’m sure my manufacturing/production teams would have screamed for my resignation had I made any bolted joint a zero tolerance match.

Neil - I can’t tell if you’re just trolling us but the lack of consistency in your advice is a bit troubling. On the one hand you’re stressing the importance of aerospace spec/certified fasteners and on the other you’re advocating the use of stainless bolts on structurally critical joints. Without sounding too harsh or rude, your statements on hole sizing tell me you don’t understand design for manufacturability or best practices for bolted joints. Your recommendation to machine the upright without first understanding the assembly is just ... bad advice.

 

[Neil]

Cam;

Re- read my post. I said that for street use I would not run out and replace all the 1/2" SS bolts immediately but I'd recommend replacing them when he could. There is no point in going crazy about SS bolts. In one respect, they have a characteristic that is a saving grace- they have a long yield curve. That is, they will bend a lot before breaking. Bending absorbs energy. In any case, SS bolts should be replaced with steel alloy or better.

Whatever the considerations others may put forth, I would not put a 3/8" bolt into a 7/16" hole. If precision is required, drill & install dowel pins & match-drill the bolt holes.

I have not said that aerospace bolts are required. They are nice to have because they have great advantages but they are hard to find and are usually $$.

One point that has not been addressed here is the issue of grip length. This is the length of a bolt from under the head to the start of the threads. In a typical double shear suspension application where a rod end bearing is bolted between two "ears" on the chassis, you need a bolt that has a grip length of the thickness of the width of the bearing ball plus the thickness of the "ears" and perhaps one or two flat washers. An AN/MS/NAS bolt is available in grip lengths that are specified by a suffix denoting the grip length in 1/16". For example, an AN8-24 bolt would be a 1/2"-20 with a grip length of 24/16" or 1 1/2". This allows the bolt shank (body) to bear on the "ears" rather than the threads. You never want the threads to be extending into the hole in the bracket. A commercial grade 8 bolt has a 1 1/4" thread length specified (US standard) so its grip length is whatever the bolt length is minus the thread length. The 1/2" grade 8 bolts are generally available in length steps of 1/4" so, unless this length is what is needed, a user may faced with two choices: allow the threads to contact the bracket holes or stack up washers to take up the extra length. Not optimum in either case but people do it anyway.

Let's remember that we are not designing anything for manufacturing, we are just building a one-off street/race car. I'm not advocating anything, I'm just pointing out that other choices are available to the builder.

 

[PhillipM]

Lots of fancy talk about exotic materials and engineering. Who cares. Tell me what race teams use and be done with it.

Id be surprised if above grade 8 or 10.9 from reputable sources

10.9 for anywhere corrosion/crack sensitive, 12.9's everywhere else. Aluminium rivets since the majority of rivet work is just aluminium panelwork anyway.
Clearance holes for all bolted assemblies as otherwise you're cutting things out with an angle grinder after any on track/stage bumps and rubbing, split dowels if they need perfect alignment (they'll flatten in a bump instead of damaging the expensive machined bits) - and if you're relying on shear strength for your bolted assemblies you're doing it wrong.

 

[Cam]

Sorry Neil - but you’re advocating the use of stainless fasteners in structurally critical joints, that’s just not responsible (IMHO). I read your post very carefully and you say nothing about duty cycle or preventive maintenance. You don’t know that Ed wouldn’t just leave those fasteners in place till they failed, and why is it acceptable to have a graceful failure at all? Better to hedge bets and never have a failure, period.

Street vs track, doesn’t matter - I don’t want to have a failure in either use case. Why is it ok in one but not the other? Track conditions are a lot more controlled versus on the street. To me, a street failure with all its unknowns is much more dangerous. Upright failure on the freeway? What about that semi you just passed, probably not a stretch to think it possible those 18 wheels could do a lot of damage to a car and it’s occupants.

Re: grip length - thanks for the education but that’s not a topic on discussion here, no one’s asked about grip length so I don’t know why you’ve brought this up twice now. What’s with the education on double shear suspension ears? No one asked about that ... or for a recipe on how to read bolt designations.

Going to match drilled or pinned holes without understanding what’s happening is also another terrible idea. Let’s say Ed match drills his uprights as you suggest - you did recommend that he modify his hardware to match hole sizes. 2 years from now it seems his hub has excessive play and he needs a replacement. Wouldn’t you know, the new hub no longer fits because his previously match drilled holes don’t line up. Should he sleeve his upright to match the new hub, or should he go to en even bigger bolt. 4 years from now, what should he do?

Different materials expand at different amounts based on temperature. Do you know how much shear stress you’re putting into a pinned or match drilled joint? I doubt it.

Yeah, these are one-off cars, but serviceability is of supreme importance. I’m certainly not building a car that only has a usable life of 2 years before I have to tear everything down and buy all new uprights so I can match drill with a new set of hubs.

So you say you’re not advocating anything but you’re just letting us know there are other choices available ... from what I can gather your choices are ... sub-par, for a lack of more elegant wording, and would lead those less conversant on the topic down potentially dangerous paths. What’s the point of letting us know we can do something ill-advised just for the sake of you letting us know you know a bunch of stuff but you don’t necessarily recommend we do what you’re saying?

Aluminum bolts? No one asked about aluminum bolts or aluminum connecting rods. Wut?! Why was this even mentioned?

I think the usefulness of this thread is done. My take - you have interesting anecdotes but I want to be very far away from whatever machine you’ve worked on. I would caution any reader that takes you up on whatever choices you’re throwing out there for the sake of letting us know you know. It’s probably the path not taken by most for a reason. Best of luck to you and your build!

Edit: sorry Ed - I’m not picking you out for a particular reason, I think you represent a good percentage of the builders out there who aren’t well versed in fastener theory. You don’t need to have a degree in engineering, just a passion for cars to build one of these! The factory and builder experience base does a good job of helping you sort the detailed stuff out so long as you go with the “tried and true” solutions.

 

[Neil]

True, no one asked ... but forums like this are useful vehicles (no pun intended) for exchanging information that may be something new to someone.
Frankly, there is no need to argue- you build your car the way you want and I'll do likewise. If you are happy with your result, fine. No criticism from me.
I found it educational to not only research the published literature on the subject but also to look closely at other cars to see what they have used successfully. I'll leave it at that.

 

[Neil]

Phillip;

Did you mean to say this? "...if you're relying on shear strength for your bolted assemblies you're doing it wrong." How else can you fasten your suspension arms to your chassis?

 

[Howard Jones]

HoAaaaa my friends. Everybody's got to learn this stuff somewhere. We are lucky, VERY LUCKY, to have people who have built and used their cars enough to have learned a few things and they are trying to help a guy who is just getting going on a unfamiliar build. Ed was asking a pretty broad based question about his understanding of what he has in hand, what he maybe should have as a alternative, and generally what do to do now about it.

I bet he is better informed now than when he started typing. Lots of stuff to confirm for his own comfort, professional sources such as Carrol Smiths books (I have them all) and generally a better breath of the question he asked. Just what a Forum like this is for.

Ed, ask anything and you will get a number of answers, Many saying the same thing, some so far out there that even knowing you don't know the original answer you know THAT can't be right, and unrelated information that will add to your knowledge base.

I have posted pictures of my ideas on this forum and got PMs from members explaining why it was dangerously engineered along with simple improvements to make it correct...……………..more that once. If I hadn't asked...………………… Posting pictures of what you are doing is the best thing you can use this forum for and that is why I do it all the time.

Both Neil and Cam are trying to help an have offered some good information, BUT it is your responsibility to filter and confirm everything you read here. I am more or less in line with Eddy. That is really good hotrodder/armature racer advice and it won't steer you wrong. The good news is that this whole forum process is where you will learn some really cool stuff and make some new friends. You are welcome to PM me so we can exchange Ph info and we can talk anytime.

PS, the hub bearing outer body sits into the upright and that is what locates it in position and takes all of the lateral loads fore, aft, up and down. The four bolts clamp it in place (in and out) and prevent the bearing from loosing it referenced positioning in the upright. Use grade 8 American made bolts, locknuts, and washers for the four bolts and you will be fine. HOWEVER!!!! they must be kept tight! Check them a lot at first until you known they remain that way.

PS2. I don't really see anything WRONG with anything in the above posts justs a bit of different takes on your question.

PS3. your information does not denote your location but I bet you can get grade 8 bolts online. Where are you?

 

[PhillipM]

Phillip;

Did you mean to say this? "...if you're relying on shear strength for your bolted assemblies you're doing it wrong." How else can you fasten your suspension arms to your chassis?

Yes. The friction from the clamp load should be taking all the design loads. In an impact the assembly can move with the hole tolerance without immediate failure, and the bolt can bend and distort in heavier impacts to take energy out without snapping and causing a bigger problem. And you don't get fretting failures and bolts coming loose as you do if you try to use just the bolt itself in shear.

 

[Neil]

Phillip;

Now I see what you mean. Yes, torquing a bolt will clamp two or three (or more) surfaces together tight enough to generate friction forces that take the load but technically it is a shear application. A loose (untightened) bolt, 100% shear, is a seldom seen thing.

 

[PhillipM]

Perhaps, but the bolt should be loaded in pure tension, the inserts in your suspension joints/bushes should be taking any and all shear loads.

I also saw someone mention spring washers but I can't see it now - split spring washers on structural bolts are a complete waste of time - they won't stop a bolt coming loose (in fact they can encourage it as they make the assembly slightly less stiff) - they will only stop an already loose bolt from falling out as fast. But a loose structural bolt is generally already a problem.
They're great for panel bolts, etc, where you can't use the full bolt preload (but you really should just use more, smaller bolts, ideally) - but for anything structural if you're wanting bolt retention/locking then stick to threadlockers and positive lock washers like Nord-Locs

 

[Neil]

I agree, Phillip. Some time ago, "Machine Design" magazine published an article on fastener locking devices. There is also a video somewhere that I saw showing a bolted joint with various locking devices being vibration tested on a shake table. All of the usually-encountered mechanical locking devices allowed the nut to turn and eventually spin off the threads. A fully-torqued bolt & nut was better that a split or star lock washer but it too eventually failed. The best were prevailing-torque locknuts and Loctite. Castle nuts & cotter pins are OK but fully torquing a castle nut usually results in the hole not lining up with the slot in the nut so the nut needs to be loosened just a little. Not bad, though.

 

[PhillipM]

I swear by Nord-Locs, they've solved a lot of issues for me with CV bolts, skidpan bolts, etc, coming loose over the years with the offroad impacts on them.

 

[Neil]

I have not tried Nord-Locks but they look good, Phillip.

This may be helpful reading, including the comments & videos:

https://engineerdog.com/2015/01/11/10-tricks-engineers-need-to-know-about-fasteners/