ARP Bolts
Ok, so I have read all the posts since notra4re first posted his intention on getting ARP to make us owners some special bolts for our transaxles…but are the bolts available, soon to be available, or put on hold pending some other “research”??? The people at ARP know bolts, how to process them and the correct material selections. Note, I have no financial interest (or otherwise in ARP). I have just used their products in the past and studied their technical catalogue. They are a premier bolt producer and well recognized. Nota4re, what is the story as to availability? Incidentally thanks for the great pictures in posting #387 showing the various combinations. Very helpful.
Some of the other contributors have tried to help but some of you miss the mark in understanding fasteners.
Shadowman (“The harder the bolt the less it will bend..... in fact at the extreme end of being hardened they will not bend at all but rather snap like a dried twig.”)…well maybe,… it depends on the loading. True, higher strength bolts tend to have LOWER elongation (not bending) than lower strength bolts, however any bolt will “snap like a dried twig” at a high enough load. Tensile load that is, not fatigue loading. Typical engineering practice is to torque a fastener to a high enough preload such that the bolt never experiences any fluctuating load at all. In a fluctuating load bolted environment you DO NOT WANT separation of the two bolted surfaces and thus transfer of all of the fluctuating load into the bolt. That is highly undesirable. Thus you preload the bolt to the maximum load just short of yield. Typically 90% of bolt material yield strength (ref Shigley, Mechanical Engineering Design). Certainly do NOT back off of recommended torques as some have posted (post #386).
Gimbal (“only thing I observed was that the 2 bolts seemed over torqued”). What does this mean? Was the break-away torque measured? Break-away torques will always be higher than torque up due to: Static friction under the bolt head is higher than “rolling friction” during torque up. It takes more torque to get the head turning when static than keep it turning while being turned during initial torque. These bolts may have locktite on the threads from the factory which skews the break-away torque necessary or feel.
(“The fully threaded bolt is also a no good thing and I replaced with bolts with a filleted radius at the bolt head.”) The fully threaded bolt DOES have a transition fillet from the thread to the integral washer surface. See picture #5 (posting #387) from nota4re showing the transition fillet on the M8 G12.9 factory Allen head bolt. This fillet is a very important aspect of a bolt design. Typically bolt manufacturers put as large or generous a fillet in this transition region as possible to increase fatigue resistance of the bolt in addition to “rolling” this fillet to impart compressive residual stresses in the fillet which again benefits fatigue resistance. ARP does this (as do others) as well as rolling threads which again benefits fatigue resistance. From a tensile overload perspective the bolt is limited in load capability to the “tensile area” of the thread. This area is well known and tabulate for each thread and is slightly larger than the thread area using the root diameter and less than using the mean thread diameter.
(“would assume both axle and hub flange expand at about the same rate as temp increases. In fact it might not be a good idea for one to expand more than the other, this itself could cause high stresses, etc. I am sure Ford knows or could easily figure out if the splines and slop is excessive per the design.”) I am certain Ford as well as Riccardo engineers have accounted for the thermal expansion of their constituent components. Incidentally that is why we have difficulty initially shifting from first to second gear with the transaxle below operating temperatures. The designers know about this and for the transaxle accounted for the thermal growths in the gear meshing to be precise AT temperature. I do not think the transaxle bolt problem is temperature related.
Analogdesigner (“Fortunately, the ARP 300 alloy is very similar to 17-4 ph stainless. It's acceptable to machine any amount from the outside of the integral head/washer, since the material properties will not be affected. This is not a heat treatable alloy, so this is why we can get away with doing this.”) The ARP catalog says “Stainless Steel: Ideally suited for many automotive and marine applications because stainless is tolerant of heat and virtually impervious to rust and corrosion. ARP ‘Stainless 300’ is specially alloyed for extra durability. It’s polished using a proprietary process to produce a beautiful finish. Tensile strength is typically rated at 170,000 psi”. Although the catalogue does not comment on the material heat treat (undoubtedly proprietary) the typical solution heat treat for 300 series stainless is 1850 – 2050F / cool rapidly in air or water followed by 1550 – 1650F / air cool. 17-4PH is a “Precipitation Hardening Stainless Steel” with a typical heat treatment (for AMS 5643 bars and forgings) of 1900F 30 minutes air cool or oil quench to below 90F, followed by a precipitation hardening temperature of 900F – 1200F for 4 hours. Not clear as to your statement that these metals are “not a heat treatable alloy”. Clearly they are.
(“The integral washer in the 12 pt. fastener will be far more vibration resistant and fatigue resistant than a socket head fastener. This is due to the increased surface area compared to the socket head fastener. This is probably good news for all of us!”) The fatigue resistance of the bolt comes from its preload, material selection, thread properties and shank fillet properties, not the type of torquing head on the bolt. It is the tensile load carried in the bolt itself which resists component separation. I have not see in literature any perceived fatigue benefit of a threaded fastener prefaced on whether the head was a 6-point, 12-point or internal hex head. Could you cite the basis?
(I will carefully calculate the proper torque value, because on a car with a "good" spline fit, the fastener should be torqued to >75% of it's yield strength. Note: this is much higher than Ford recommends...”). You are CORRECT!! Now get the load up to +90% of YTS and we are cooking. Be careful though of what thread lube is used as the lubricant can significantly alter thread friction and thus load into the bolt such that you can unintentionally exceed the 90% target!
I do not mean to pick on any contributors, just trying to clarify as to certain technical issues. As one contributor pointed out, the more thoughts brought forward for thought and discussion, the better we are in finally determining the “root cause” of OUR problem.
