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Thread: Crank Tech
09-13-2011, 11:32 AM #1
For those readers just coming to this thread, I have moved it from Allen's (Broke7) thread about broken timing chains because it was hijacking Allen's thread and I was the hijacker!
Keith, Scott and other interested folks,
I have attached several pictures of a crank modification I have recently done to correct some of the problem. The challenge we face with a larger bolt is the tug it places on the snout. When we tighten a bolt it clamps the damper and internals (gear, & timing wheel) to the front of the crank by exerting pressure on the components and squeezing them against a crank snout shoulder just in front of the oil pump drive journal. Essentially what the big bolt solution I used to use does is try to pull the snout straight forward off the crank.
I have come up with an interim solution that uses a stud that anchors in #1 main journal and sandwiches all the pieces without puling on the crank snout. I will do another post just on this technique in a week or so and include a second article on building your own dual disk clutch. For now here are some pictures of the parts and the process.
Here are the ARP parts needed:
This is the crank set up in a lathe ready to drill the stud hole:
This is the tapping step while the crank is still in the lathe:
And this is the finished crank:
When you have completed the modification the entire package of goodies on the front of the crank will get sandwiched between the nut and washer on the front of the crank and #1 main journal, making for a very tight and durable assembly. I've got to go back to my notes and check but if my memory is correct the torque on the 9/16 stud/nut package is about 165 or 170 ft/lbs and the clamp load on the package is more than double the stock clamp load - without any attempt to pull the crank snout off the crank.
I've got to see a vampire about a blood test right now so I'll get back with the other details when I return later today.
Last edited by eschaider; 02-16-2018 at 11:18 AM. Reason: repair pic broken links
09-13-2011, 01:05 PM #2
I'm trying to get my head around the clamping physics in this.
The stud, being installed in the new threads inside the crank snout with zero torque during the installation, deform the snout
in a different way then having the threads screwing in under load?
So the stud/snout thread interface has more metal to metal contact to spread the load during the clamping phase, causing less distortion to the crank/snout interface. And the actual snout/thread interface is taking place at a different depth in the snout.
09-13-2011, 02:07 PM #3
The magic (if there is any) is in where the stud or bolt anchors to produce the clamping force. The bolts all anchor in the crank snout before the oil pump journal. The stud goes all the way through the snout to the #1 main journal and sandwiches everything between the journal and the nut.
When you use a bolt that terminates in the snout it can produce similar levels of clamping force but it is doing so by placing the anchor point in the snout where it effectively tries to pull the snout off the front of the crank (at the juncture with the oil pump drive journal) as you tighten the snout bolt and raise the clamp load. The problem is the bolt anchoring point - it is forward of the register the timing gear(s) are being clamped against. By increasing the load applied by the snout bolt you are effectively trying to pull the snout off the front of the oil pump drive journal because the snout bolt anchors in the crank snout forward of and not behind that journal.
The stud approach goes even deeper into the crank past the oil pump drive journal and anchors in the #1 main journal producing a nice sandwiching effect with out placing any tensile load on the snout. If you have an aftermarket Kellog crank you can actually go back to and anchor in the front most counterweight of the crank. OEM Ford cranks use a cross drilled main style of oiling instead of the Kellog straight shot method. With a cross drilled main you can not get all the way back to the counterweight without obstructing part of the oil passage for #1 main.
It's easier to see than explain. I've got some drawings somewhere (if I can find them) from when I began the project. They illustrate the front of the crank. When you see the drawings, the lights go on quickly. I'll do a search and see if I can find an uncluttered one.
FWIW a grade 10.9 12mm bolt at 75 ft/lbs TQ produces less than 14,000 lbs of clamp force. An ARP 2000 9/16x18 stud at 170 ft/lbs TQ with a hardened steel washer, produces a little over 30,000 lbs of clamp force. This is a nice step up in the clamping department.
Found the drawing! Like they say a picture is worth a thousand words.
When you use a bolt instead of a stud the crank wants to break at the under cut fillet between the snout and the oil pump drive journal just behind the timing gears where it can do the greatest harm. This type of failure is very expensive because of all the damage it does to the heads, valves, guides and potentially cylinders especially if a valve breaks off and falls into the cylinder.
Last edited by eschaider; 06-12-2016 at 05:19 PM. Reason: found the pic
09-13-2011, 02:18 PM #4
09-14-2011, 02:40 AM #5
Ed, your right, the pic explains it quite well.
But does anyone know why Ford undercut the fillet at the snout/crank interface.
That seems to be the last place you would want a decrease in cross section.
I remember looking at a Kellogg aftermarket crank and was impressed at how nicely they rounded all the stress risers
and polished and shot peened their cranks. I think that alone would make it worth stepping up to Kellogg or Scat crank.
Even though you have to make sure that you get the correct rod and main bearing shells to accommodate the fully rounded fillet.
And have you seen any of the billet cranks. Real works of art but a ton of money.
09-15-2011, 02:32 AM #6
The answer is actually a two part answer Drew. The first part is that is the way (with a few exceptions) Detroit OEMs have always built their cranks. The second part is why. The answer to the why question goes something like this, when you finish a crank with full radius fillets the cost is deceptively high. To maintain the radius within the design specs the crank grinding wheel requires more frequent dressing - much more frequent. The cost associated with this is viewed by the bean counters as being unacceptably high.
Remember in Coletti's book he commented that the Manley rods were several times more expensive than a block. According to Coletti, Ford paid $56 for a single gold plated (Manley) rod and in the book he says, "...two of these rods cost more than a whole [Terminator] block." The incessant drive to control cost in Detroit's manufacturing phase frequently demands compromises. The Manleys got through because of approaching deadlines and a lack of credible alternatives. The cranks were lucky but not as lucky.
Kellogg who has been the source of untold high end, high performance cranks in Detroit OEM circles for decades, was an easy fix for crank durability that more or less still fit inside the bean counter's envelope of acceptable pricing. Part of the ability to meet target costing was the use of undercut instead of full radius fillets. The cranks still had the center counterweights which many production cranks lack - again for costing considerations. The crank material was the now famous micro alloy that mimics a lot of 4340 characteristics. The finished product however still had the undercut fillets.
When you buy an aftermarket Kellogg you still get undercut fillets but you do get a better oiling system than the OEM Ford specified cross drilled mains. The undercut fillets on either crank are a real problem for Terminator engines and the undercut between the snout and the oil pump drive journal is Public Enemy #1. That is not only where we break the cranks it is the worst possible place to fail because it separates the timing chains from the crank killing the valvetrain.
The crank snout stud is an interim fix for the snout problem and, while it really isn't as much of a fix as it is an improvement (because I think it will potentially still fail) I don't have the time yet to produce the real fix. The "real" fix involves increasing the snout diameter, modifying the snout journal lengths to work with a new splined lower gear, trigger wheel and damper. The finished crank will have a snout that resembles the drive snout (albeit different dimensions) on the P-51 engine in the pic below.
This new crank will obviously be a billet effort. It will use a new dual lower timing gear that is independently indexable for each cylinder bank within less than 1˚(crankshaft). There will be full radius fillets used everywhere on the crank and the material will be one of the super alloys for ultimate strength. I have a an initial proof damper design completed and manufactured along with a lower blower pulley drive that is secured by both fit and six 10MM fasteners. The lower blower pulley drive has been designed with provisions for an external wet sump (or dry sump if you prefer) auxiliary drive.
At first a billet seemed like overkill but the further I got into my project the more it screamed for a custom crank. I will probably get to the crank in the early spring. As I do I will post up pics so you can see more of it. In a few weeks (more or less) I'll do a write up on the studded OEM temporary fix.
p.s. The billet is currently planned to use a 5/8" ARP 2000 stud in the snout, that will anchor in the #1 counterweight and provide right at ~40,000 lbs of preload when you torque it down to 235/240 ft/lbs. Rememebr the stock 12MM grade 10.9 bolt is 14,000 lbs (or less) preload at its torque spec.
Last edited by eschaider; 06-12-2016 at 05:29 PM.
09-15-2011, 02:59 AM #7
I really enjoy getting the inside history on why and how cost cutting at the development level has turned out a good, but could have been great, product.
Are any of the forged aftermarket cranks fully rounded, or is the only way to get rid of that stress point at the snout to have a custom billet crank made.
And that RR aircraft engine is absolutely fantastic. I always love looking at them. Too bad they would create too much nose weight in our cars. Too bad they would carve a groove in the ground and stick up out of the hood three feet at the same time.
09-15-2011, 03:25 AM #8
Sadly the forged aftermarket crank manufacturers are currently sourcing their inventory from Kellogg. As a result they all have the undercut fillets which (along with some other considerations) is what drove me to the billet alternative. The easiest test to see if they are really manufacturing the crank themselves is to ask for it to be finished with full radius fillets everywhere. The moment you ask for that the manufacturer starts to back down. The most amazing part of this is that all of their other forged racing cranks (which they actually make) will come with full radius fillets as a standard feature. Only their least expensive entry level products may not have full radius fillets - which begs the question why would you use undercut fillets on a high end crank like a Cobra?
Today no one makes a forging for our engines that has full radius fillets. The undercut fillets are a significant part of the problem but snout diameter is also a bad boy. A 1.25" diameter snout with undercut fillets on a engine with a crank driven blower, like ours, is an accident looking for a place to happen. The reason Ford went to the front support for the crank on the Terminators, I suspect, was that it was cheaper (cost again) than a new crank design for a limited production vehicle with an engine approaching its end of life. Our crank snouts were designed for normally aspirated engines at the very beginning of the ModMotor's life cycle more than a decade before Terminators saw the light of day. The engine is (especially with big blowers and high boost) asking for a more robust drive system than you need for the n/a version of the engine.
I understand the thrill from the Merlin, they really wind my spring up also. Whoever thought of the Merlin name for them hit the nail right on the head, because they certainly are magic! As old as they are, they employ a lot of design decisions that are amazingly applicable today.
p.s. Now with a little more tire pressure and some magnesium components ... maybe?!!!
09-15-2011, 06:04 AM #9
- Join Date
- Dec 2004
- Hildebran, North Carolina, United States
Good stuff Ed... those pics look familiar for some reason. What would be nice would be to see some pics of the "old skewl toy" that lies in pieces in your garage
09-15-2011, 10:04 AM #10
09-15-2011, 12:15 PM #11
Rd, I am going to bring a spare crank over to a friend's dad. He is a metallurgical engineer at McDonnell Douglas.
I want to get his opinion on Tig welding in the undercut, machining and polishing it down, and re heat treating the crank.
Then aerospace shot peening it. Should be doable unless the heat of welding compromises the snout interface strength.
But then when you ad up all the costs involved how close does it get to your billet crank?
Last edited by oldmodman; 09-15-2011 at 12:15 PM. Reason: spelling
09-15-2011, 12:18 PM #12
09-15-2011, 12:49 PM #13
09-15-2011, 12:58 PM #14
You're right about Gilroy and Garlic Drew, at least in years gone by. Since the Chinese began selling garlic at retail cheaper than these guys could grow it the industry has effectively closed down over here. When they were at their prime you could put a steak out on the close line in the morning and have it marinated by noontime for dinner. Essentially the only thing left today is the Annual Garlic Festival and you have to be careful over there because they have Garlic in everything from toothpaste to soda. Some of it is really tasty, some of it will set you back on your heels!
Not too sure about the brain cells part but the aroma back in the heyday would leave you in a continual state of hunger - all day.
09-15-2011, 02:21 PM #15
I believe it is from the Mustang. The valve covers say Rolls Royce although I don't think I see the two stage supercharger on this one. The supercharger is hard to see because they mounted it on the back of the engine. Here is a picture of the Packhard manufactured version of the engine taken from behind so the two speed two stage centri is visible.
I believe the large round casing behind the propeller shaft is the gear reduction unit used to maintain constant propeller speed.
The centri supercharger was capable of producing upwards of 39.5 psi if my information is correct and used (on the Griffin version of the engine) a water methanol injection system at takeoff. Pretty impressive technology for WWII era engines.
Last edited by eschaider; 06-12-2016 at 05:34 PM.
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