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Thread: Manifold bolt/stud dilemma
01-02-2020, 12:08 PM #1
I touched a bit of my concern with available exhaust bolt/stud length, thread engagement area and material for sealing turbo manifolds.
I was currently running a 3/8” (9.5mm) flanged header manifold from hp performance. The tfs heads allow approximately 25mm thread depth/engagement. I also used a copper gasket from SCE that measures at 1.1mm. The Percy speed lock bolts length are 25mm, therefore I’m only getting 14.4mm thread engagement in the heads.
The overall problem I saw was exhaust leaking fairly badly using this combination. I was wondering if there was a longer bolt applicable in the m8 range, so I called ARP and started talking to them. They don’t have any direct applications, and the greatest tensile strength in that size was the 8740. An application they suggested was an LS intake manifold bolt that was longer. The problem they stated was that you are only able to torque these bolts ~25ftlbs.
Not knowing for sure any proven combinations or any issues anyone sees with that setup. Perhaps ditching the gasket and going with a straight copper rtv, or buying the longer studs to ensure more thread engagement? Or like ARP stated, the clamping force can only be so much, therefore I’ll have to look elsewhere for a solution.
01-02-2020, 12:19 PM #2
ARP recommended their 771-1003 (30mm) or 771-1004 (35mm) 12pt stainless series bolt for the added engagement in the head, but beings that they were still only rated for 180,000psi respective and a torque rating of 24ftlbs, not sure if that’s going to provide enough clamping force.
Any help would be greatly appreciated!
01-02-2020, 04:29 PM #3
You don't need 180Kpsi test hardware, Matt. That said the easiest way to get long studs is go to McMaster Carr and search on set screws. They are usually grade 12.9 or 14.2. This is way over the top for the clamp loading you need. If you want corrosion resistance go to stainless.
My heads have exhaust stud holes that are 30mm deep. Add 9.5 mm for a ⅜" flange, 8 mm for the nut, 1.5mm for the gasket and give yourself 2 mm more to protrude out the top of the nut and you will want basically a 50mm long fastener. If you want to use a washer add that in also.
Check your dimensions to validate the length you need. 50 mm long set screws usually go for about $1 per item at McMaster.
01-02-2020, 05:10 PM #4
Thanks for the response Ed, the bolts/studs maybe only part of the issue, maybe not the root cause of the leaking. Surely the 8740 ARP hardware is overkill for the application, however the call was to merely understand clamping force, tensile strength, available torque spec rating, and how deep thread engagement is involved with the equation. I am now a little more versed on ruling out the actual bolts, and you are correct in using McMaster to order them, I don’t think that is the underlying issue.
I have verified to the best of my abilities with a straight edge and feeler gauge....the collector flange is straight and true, and the heads are as well. Is the copper gasket soft enough to conform at the 25ftlbs spec, will the additional thread engagement of the longer stud allow for more torquing and clamping of the stud/bolt, or would it be better ditching the gasket for another material such as the oem mls stuff, or not run a gasket at all and rely on an rtv copper gasket. Everyone seems to have an opinion and I’m trying to absorb all aspects of this issue.
01-02-2020, 06:37 PM #5
IMO, if it doesn't leak then it's the correct process for solving an issue: I use a steel gasket with RTV on both sides of the gasket on all my gaskets and nothing leaks. Nobody will convince me I did it wrong.
01-03-2020, 12:29 PM #6
I tried to reach out to SCE again to ensure the assembly was installed as per they’d like to see...
From what I read on the installation instructions, and from laying the gasket on the head and on the manifold flange, it seems like their embossed lip that should compress, does not do so as the lip should be compressed between two flat surfaces. May not be noticeable on the picture, but it seems like the exhaust flange is larger than the embossment lip of the gasket. Am I reading the tea leaves correctly on this one?
As far as I can see, the general consensus is to run the longer stud/bolt to engage maximum threads in the head for insurance purposes, and run the stock mls stuff, and if totally necessary, a thin coat of the copper gasket rtv on both sides, and torque to 25ftlbs.
01-03-2020, 05:13 PM #7
I'm in the same space you currently are on this problem, Matt.
A thread engagement rule of thumb for aluminum is usually 2.5 to 3.0 diameters of thread engagement. If you look at ARP head studs the are slightly over 4 diameters of thread engagement in the block as are the OEM TTY bolts. Considering the elevated temperatures in and around the exhaust port I would look for at least 3 diameters of thread engagement. On my own heads the use of a bottoming tap gave took me from about 3.5 to a little over 4 diameters of thread engagement — very similar to the head stud / bolt thread engagement.
As you correctly observed the additional thread engagement is essentially insurance but, well spent insurance monies. An grade 12.9 or 14.2 8mm fastener with 24 ft/lbs of torque will produce about 6600 pounds of tensile loading at the fastener. When you spread it out across the area of the exhaust flange it drops significantly. If you have a gasket that is at the edge of or slightly intrudes into an exhaust port, this point will be the primary point of failure. The OEM gaskets have a slightly larger margin around the ports and could end up providing a better seal.
The increased thread engagement up to or above 4 diameters is good insurance. The aluminum casting around the exhaust ports gets hotter than anywhere else on the block with the exception of the actual exhaust port. The additional diameters of thread engagement provide welcome insurance against thread pull out or other heat and stress induced failures. Couple of turns on a bottoming tap is all it takes.
Wen you select your fastener hardware I would suggest a corrosion resistant stainless for the set screw, the nut and the washer. It will make any servicing you need to do in the future much easier to accomplish w/o the usual rust build up these items seem to collect.
01-03-2020, 06:32 PM #8
01-13-2020, 06:15 PM #9
Ed, places my order for the 40mm stainless medium strength (70,000) studs. Ideally I’d go with 45mm, as the nut would be almost flush with the stud bottomed out, however, until you jump up to 50mm (2”) the option are very limited. I believe the 30mm engagement in the head should be sufficient without excessive stud showing.
01-14-2020, 01:47 PM #10
You can use stainless set screws you don't need to use actual studs. The set screws are also available in more length choices than the studs are.
01-15-2020, 04:18 PM #11
Ed, I’ll get some of both, I know there are a couple cylinders that require use of a narrow head bolt for clearance.
The 40mm studs I have are in the head 21.5mm with enough clearance (18.5mm) to mount the exhaust flange and gasket with enough thread to engage all threads on the corresponding nuts.
To put it in perspective, the under head thread length of the stock bolts is 24mm versus my 32mm, if all else stays the same, I’m gaining about 8mm head depth. Using the math of the flange and the gasket of 10.6mm, stock bolts only thread in 13.6mm into the head to my 21.5mm.
This should be sufficient enough I would hope!
01-16-2020, 12:35 PM #12
Whenever you are anchoring in a soft metal like the aluminums you want to shoot for 3 diameters of thread engagement minimum. For an 8 mm fastener that would look like 24 mm of thread engagement. By way of comparison Ford and ARP use a whisker over 4 diameters of thread engagement in the block for the head bolts or head studs if you use ARP H/W.
In the vicinity of the exhaust port I would try to obtain the maximum thread engagement physically possible. Each time the engine runs the highest temperature of any location in the head or block casting (other than the combustion chamber surface) is obviously the exhaust port. The greater the anchoring depth of the exhaust header fastener the more to your advantage it is.
Most aluminums begins to soften around 400˚F. By 600˚F aluminum will have lost 50%, possibly more, of its room temperature strength. With proper cooling a head casting should never see this temperature. If the coolant is low and the exhaust porting in the casting is not submerged in coolant you can approach or exceed this temperature threshold easier than you might suspect. I would allow discretion to be the better part of valor here and use every thread available on the exhaust flange face of the head casting and always keep the cooling system adequately filled.
Ford typically leaves a small, perhaps two possibly three threads, of unused thread space at the bottom of each of their thread wells. Although it is untapped the use of a bottoming tap will make this additional threading available for fastener anchoring purposes. If you use this approach you will typically be able to achieve 3 to 3.5 diameters of thread engagement — which will serve you well. Remember that CNC machines, like Ford uses to machine these heads, use thread forming taps not thread cutting taps. If you run a thread cutting tap into a hole tapped with a thread forming tap you will reduce the percentage thread engagement and also the strength and life of the threaded hole.