AN/MIL vs. SAE Gd. 8 fasteners

[John Powell]

In his “... to Win” series of books, Carroll Smith seems not to be enamoured of SAE Gd. 8 fasteners in racing, one of his comments is that they are brittle, and he recommends that those meeting AN/MIL specifications be used instead. On the other hand, many still use Gd. 8, and it must also be recognised that his books, while very informative, are mostly written with purpose-built open-wheel and sports-racing cars in mind, such as F. Atlantic and Can-Am, and not production-based cars modified for racing.

His recommendation may be OK for those who live in or near large metropolitan areas where surplus aerospace parts and supplies are readily available. For example, when I first raced out of Toronto in the 1960s, we had (if I remember correctly) at least three to chose from out near the airport who stocked all sorts of surplus aircraft hardware, including Aeroquip hoses and fittings. For others, however, supplies are difficult to obtain. Again as an example, since I returned to racing last year and now live on the rural fringes of Ottawa, Ontario, which is basically a government and computer town, I find that AN/MIL fasteners seem to be only available new through special order, and they’re expensive.

Anyway, I thought that this might be a good topic for discussion (and to bring me up to speed a bit after what was a thirty-three year absence). So, as they say here in the Ottawa Valley “pitter-patter, lets get at ‘er.” :D

[John Powell]

OOPS! I just realised that maybe this should be on the Race Tuning and Information forum (well, I am an Old Fart after all ;) ), maybe a Moderator can move it?

[John Powell]

Hmmm. I though that this would have started a discussion by now - I've already had seven responses on the Mazspeed forum. :confused:

[rmicroys]

Whatcha mean John? You mean bubble gum and duct tape aren't good enough for ya ;)

[John Powell]

Hey! I love Red Green as entertainment (and a grain of truth in humour) but when my life could be on the line .........:rolleyes: I may be an old fart, but I'm nowhere near ready to go yet. :D

[slucas]

Smiths books do specialize in the open wheel/sports racer world but the info applies to all race cars in general.
I would say the SAE vs. AN/NAS quandary comes down to weight.
A formula car has to be light and that ain't easy, whereas a sedan usually has to carry alot of weight. Your nuts & bolts can be quite a bit smaller if you go AN. So I think SAE is OK but you must use the right grade , and the hardware will be bigger & heavier. Ti bolts are commen now in some racing but ...

By the way John , check out the the local farm equipment dealer, they sell real good hardware, but you'll need to go up to 1" drive rachets :D

[John Powell]

Wow, we now have a discussion! :) Steve, (it is Steve, isn't it?) I farmed part time for about 18 years, and you're right about machinery dealers. The one in Sarsfield, about a mile and a half up the road, has a fair supply of Gd. 8 stuff, mostly coarse thread, as well as some metric 8.8. He can also get higher grade metric through special order. What I'm trying to get at is what people think are the pros and cons, such as in your reply. One that Smith mentions is that Gd. 8 hardware is more brittle than aerospace stuff, and breaks under excess load instead of yielding first. The trade-off here may be availability (and cost) vs. desirability for an intended use. 'Tis a quandary. :confused:

[wpfri]

John your dream come true.I found a guy who is a small plane hardware supplier who is going out of business.e-mail Ken Koven sales@wpfri.com and he will give you the numbers.

William

[John Powell]

William,
Thanks for the info, but I don't really need any right now - I was just trying to get a discussion started for future reference and for the information of others who had a similar question. The place where I would probably consider AN/MIL fasteners is for the suspension, and I haven't made a list of sizes, etc. yet. There's another issue here, and that is that they're all metric, and I know nothing about the availability of metric AN/MIL equivalents. Maybe we should add this to the discussion?

[slucas]

What we need in this thread is an engineer that knows how to screw things up good .
Part of the equation with suspension bolts is knowing that point where they brake when you need them to,such as when you bang a wall.Better that the bolt break off than the frame get torn up because the bolt was too good. That being said ,it is best not to guess and to find out from somebody that knows.

[malcolm]

hey, I am just an engineering student... so I'll need some time to learn how to screw things up good, as you say. ;)

(actually, I am taking a course this semester that deals with fractures, I believe. should be interesting) :D

[James Mewett]

A grade 8 bolt has an ultimate tensile strength of at least 150,000 psi (150 ksi). An AN bolt (which is a general purpose aircraft bolt) has an ultimate tensile strength of 125 ksi. The next higher strength aircraft bolts (many MS or NAS series bolts) have an ultimate tensile strength of 160 ksi. Higher strengths than this are available but they are only used where they are specifically required. It would appear that from a strength point of view grade 8 bolts should suffice as general purpose race car bolts.

General purpose structural aircraft machine screws are also 125 ksi and are identified by an "x" on the head and have a non-threaded shank. Non-structural machine screws are about 55 ksi and have no "x" and are generally fully threaded.

Aircraft bolts have short threaded ends because any shear planes must bear on an unthreaded shank. If you use Grade 8 bolts you should chose them by the unthreaded shank length not the overall length such that the threaded section is not loaded in shear. You will need to cut off the excess threads if you want to save weight and make them look decent.

All hardware should meet a specification appropriate to its intended function. Similarly, bolts subject to rotation in service should use castellated nuts and cotter pins not self-locking nuts. On the subject of specifications, many factors aside from ultimate tensile strength are relevant to a fastener's performance. Relevant criteria include dimensional tolerances, surface finish, plating spec (and embrittlement relief if applicable), alloy and material quality spec and fatigue strength. Aircraft hardware will be "better" than commercial spec hardware because it is more tightly controlled (eg consistent) and it is available with traceability to the manufacturer.

The next point is torque. Every critical fastener should be torqued. Aircraft references indicate the standard torques which are a function of the bolt strength, diameter, thread pitch and whether the fastener is loaded in shear or tension. If you don't torque your particular application, there is not much point in using an aircraft fastener there.

My two cents - on highly loaded critical fasteners (which is to say if they break, something bad happens) use aircraft quality hardware. Everywhere else grade 8 is good enough provided shear is taken on the unthreaded shank and the bolt is properly torqued. For whatever its worth I am an aeronautical engineer.

[DareBee]

As far as things go a standard grade 5 is min 120,000 tensile strength, which for ultimate shear and general fastening it doesnt make sense to use aircraft spec.
A properly designed part that uses fasteners should be designed so the fastener does no more than hold the part together ie should not have side load whatsoever therefore ultimate tensile strength is usually the important factor.

John for all sorts of goodies including specialty fasteners any DIY guy needs to check out McMaster Carr

http://www.mcmaster.com/ (http://)

Yes its US, ships from Cleveland.
I find that I can order at 4 pm and I have my stuff noon next day. They are not my cheapest supplier but the are my MOST reliable. The specialty and hard to find stuff the have is unparalleled.
There is no duties or brokerage (they look after it) and shipping of 10 x 10 box usually about $15. They take credit card.

[John Powell]

DareBee. The link didn't work for me.

Anybody got any comments on metric fasteners equivalent to AN/MIL?

[wpfri]

Originally posted by James Mewett
A grade 8 bolt has an ultimate tensile strength of at least 150,000 psi (150 ksi). An AN bolt (which is a general purpose aircraft bolt) has an ultimate tensile strength of 125 ksi. The next higher strength aircraft bolts (many MS or NAS series bolts) have an ultimate tensile strength of 160 ksi. Higher strengths than this are available but they are only used where they are specifically required. It would appear that from a strength point of view grade 8 bolts should suffice as general purpose race car bolts.

General purpose structural aircraft machine screws are also 125 ksi and are identified by an "x" on the head and have a non-threaded shank. Non-structural machine screws are about 55 ksi and have no "x" and are generally fully threaded.

Aircraft bolts have short threaded ends because any shear planes must bear on an unthreaded shank. If you use Grade 8 bolts you should chose them by the unthreaded shank length not the overall length such that the threaded section is not loaded in shear. You will need to cut off the excess threads if you want to save weight and make them look decent.

All hardware should meet a specification appropriate to its intended function. Similarly, bolts subject to rotation in service should use castellated nuts and cotter pins not self-locking nuts. On the subject of specifications, many factors aside from ultimate tensile strength are relevant to a fastener's performance. Relevant criteria include dimensional tolerances, surface finish, plating spec (and embrittlement relief if applicable), alloy and material quality spec and fatigue strength. Aircraft hardware will be "better" than commercial spec hardware because it is more tightly controlled (eg consistent) and it is available with traceability to the manufacturer.

The next point is torque. Every critical fastener should be torqued. Aircraft references indicate the standard torques which are a function of the bolt strength, diameter, thread pitch and whether the fastener is loaded in shear or tension. If you don't torque your particular application, there is not much point in using an aircraft fastener there.

My two cents - on highly loaded critical fasteners (which is to say if they break, something bad happens) use aircraft quality hardware. Everywhere else grade 8 is good enough provided shear is taken on the unthreaded shank and the bolt is properly torqued. For whatever its worth I am an aeronautical engineer.

Thanks for the Info!What university did you grad from.

William

[James Mewett]

U of T

[wpfri]

Thank you.The reason I asked.My father used to teach and I asked him If he knew you.

William