Another good source is an agricultural equipment parts counter, providing you don't get an idiot who is the owners son. We run shear bolts in our old square baler where a small 3/8" bolt will handle the entire torque load of the tractor and baler's flywheel and rotating assembly. Believe me when I say a LOT of torque is handles and it takes a TON to shear the bolts, though not impossible.
Lucky Mod
- Thread starter dsltech83
- Start date
Anytime you drill a hole and are depending on shear force, it is always better to ream that hole so that bolt is a tight fit. A couple of thousands loose fit will eventually fail. I would suggest that you increase the bolt size to 7/16" by useing a reamer to enlarge the 3/8" hole. Use only a six (6) fluted reamer. Take a carbide tool and scrape each flute cutting edge just once will result in an undersize hole of about ~ .001.
Sorry to hear that this has happened to you. I've just completed and installed my third lucky conversion, this time on the Moosestang. So the Moosestang, Night Moose, and Lady Moose are all in service with lucky modded flywheels, using nothing more than the standard directions, and bolts sourced from the same hardware store. The bolts I get are kinda gold in color if that means anything, I don't know. Another variable is your torque wrench. Unless you've had it calibrated in the last year, all your doing is getting a rough guess of what the torque really is. These things fail when the bolts get loose, and the two flywheel halves begin to move independent of one another. This starts to egg out the holes, and then the impact shear becomes greater on acceleration. Failed lucky mod conversions are, in my opionion due to the any of the following mistakes.
1) insufficient bolt torque
2) wrong or bad bolts
3) holes drilled oversize
4) DMF guts not fully removed.
5) Insufficient locknuts and/or locktite.
I'm curious also as to how you put the bolts in. Is the nut facing the engine or the trans? It should always face the engine.
1) insufficient bolt torque
2) wrong or bad bolts
3) holes drilled oversize
4) DMF guts not fully removed.
5) Insufficient locknuts and/or locktite.
I'm curious also as to how you put the bolts in. Is the nut facing the engine or the trans? It should always face the engine.
Mel as a stickler for details.....YOU are saying "it" as the operative word when you mean the NUT SHOULD always face the engine? Please confirm and leave no room for error...
antiqueford
Full Access Member
Excellent explanation of bolt load modes and moments by Old Blue, I could never explain it so simply! Also, another good tip from Oldbull. Keep in mind, whenever you lift your foot off the throttle, the shear force on the bolts changes direction, and that fatigue stress comes into play! The holes should be as snug as possible, and at best a drill bit will cut oversize by at least .002"! Reaming the holes is an excellent option, even if you can't "adjust" the reamer the way Oldbull suggested 
If you're unclear about anything or something doesn't seem right when you put it all together, take it to a machinist. We're pretty good at making holes
If you're unclear about anything or something doesn't seem right when you put it all together, take it to a machinist. We're pretty good at making holes
Nut is facing engine. I will check when I disassemble to see if I can tell if the bolts came loose before they sheared, but by the looks of the nuts that came out of the inspection cover the bolts did not come loose. The ends of the bolts were still in the nuts. Holes were drilled with a 3/8" bit on a drill press. The bolts didn't just fall into place so the hole isn't oversized I think..... Loctite use was wurth red high strength nuts torqued to 40 ft-lbs, 3/8-16 UNC grade 8 bolts.
Mel as a stickler for details.....YOU are saying "it" as the operative word when you mean the NUT SHOULD always face the engine? Please confirm and leave no room for error...
There is always a chance of a loose nut behind the flywheel, huh. And to be honest, I don't think your wording changes anything. 
turbodiesel94
Full Access Member
McmasterCarr has quality bolts.
and THAT!! is a SIGNIFICANT FACT!!!!! theres a whole LOT less strength at the treaded portion,,than at the solid shank....and once the back and forth torque of the engine,,starts to rock the bolt,,bye bye...
hairyboxnoogle
Full Access Member
Something im not sure on, but was told once. Grade 5's supposedly have a higher shear point than grade 8s. Apparently the 8s have more tensile strength but are more brittle thus the lessened shear capacity. Came from the guy at Tacoma screw company, he suggested 5s over 8s when i fishplated my frame.
how about some shoulder bolts to avoid the whole thing Al was talking about with his awesome pic
http://www.mcmaster.com/#shoulder-screws/=hrdhtn
these things have a machined shank to fit in the hole much more precisely then you run of the mill bolt.
http://www.mcmaster.com/#shoulder-screws/=hrdhtn
these things have a machined shank to fit in the hole much more precisely then you run of the mill bolt.
Grade 5 and Grade 8 bolts are both medium carbon steels that have been hardened, and have the same practical ductile (non-brittle) range. The Grade 8 bolt is alloyed with other materials to provide more strength in proof, shear and tensile ratings while not needing to be excessively hardened or "carboned up" to a state where steel would become brittle. Rockcrawler.com wrote a short article that covers this misconception in the offroad crowd too. This generality has even been published in automotive magazines, and generally results in a weaker bolt being used without sound reasoning by hobbyists.
how about some shoulder bolts to avoid the whole thing Al was talking about with his awesome pic
http://www.mcmaster.com/#shoulder-screws/=hrdhtn
these things have a machined shank to fit in the hole much more precisely then you run of the mill bolt.
I saw this and thought heck yeah, even the chart I put in showed that the shoulder has much higher ratings in shear that the threaded portion of a bolt, even though either can be used in shear applications as long as the ratings are respected. I clicked on that link though, and immediately saw that the "shoulder screws" linked to are NOT rated for shear loads, at all (it states in the text)!
I went digging and on McMaster and found that the bolt you need if you want to use a 'shouldered' area for higher shear loads are called "High Strength Steel Grade 8 Cap Screws, partially threaded" - these bolts (cap screws) are rated for shear and tensile loading. The way all this jargon is worded with bolts and such, you have to be really, really careful about what you chose for your applications.I was re-reading this thread after the last couple posts, and I noticed the part I highlighted the second read through. The -16 thread per inch bolts will shear sooner than the -24 fine threads. The thread root is cut deeper, and your cross section is smaller on the coarse thread bolt, try to spec the fine threads for shear applications if you can. The best bet as stated above, if you can find the partially threaded bolts to use and put the non-threaded smooth part in the shear interface where the parts would slide against each other. It's a case of good (coarse), better (fine), best (smooth shoulder). Also note that torque specs for proper clamping will generally be lower for the coarse threads, although 40 ft lbs. falls within the safe range for both coarse and fine grade 8 3/8 bolts when assembled dry.
Liquid loctite does also act as a thread lubricant during torquing as well, although not to the same degree as moly or assembly lube. You might consider a value between the dry and lubricated specs for fasteners while using loctite. Just one more thing to add to the never ending compilation of Lucky Mod & Fasteners 101, lol.
+1 - I use McMaster for all sorts of stuff I can't find local, the fact that an industry quality supply house will even deal with individuals and no minimum orders is a HUGE advantage for us all! Their shipping is usually very good as well.
9 times out of 10 the bolts on your truck probably don't need anywhere near this level of **** retentive attention, but since you've had a failure once, might as well cover as many bases as we can think of. Flywheels, tow hooks, winches - these areas usually deserve a little extra legwork I guess. I have a PDF of torques that I want to archive here for future reference too.
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This site is the bomb for all things related to steel fastener stamps and strength ratings, for bolts AND nuts!
Ataylor
Full Access Member
Old Blue shoud change his handle to "BoltMan" Very informative, I had to read it twice.
Took the flywheel back out of the truck last night, dad took it by a local retired machinist who doed small jobs for farmers at home, he's gonna mill 6 more holes in the flywheel halves for a total of 12- 3/8" bolts holding the halves together. Got some bolts from fastenal, Dad came home with 12 Grade 9, 3/8-16x2(no 24 tpi in stock) bolts and metal locking nuts. If it shears those off he will just have to buy a single mass kit.
I would never use anything other than a fine thread bolt in this application! No lie! Doubling up on the number of bolts may not help if the thread pitch is such that they are difficult to maintain in tension.
