So, did the lucky mod to my dad's 93 this morning all was well for a couple of hours, he backed up into the yard at my brothers house to turn around, went to take off sheared all 6 bolts that held the 2 pieces of flywheel together. I used grade 8 bolts and nuts, torqued them to 40 ft-lbs like the article said and used red loctite. Anyone else have problems with this conversion. Used a Sachs rep set for the DMF with a sprung hub, worked for 50 miles then sheared them all off. Any ideas or sugestions?? This is a single cab 93 7.3 na zf 4x4 so it's not like it's a 8,000lb truck or something crazy like that. Do I need more bolts?
Lucky Mod
- Thread starter dsltech83
- Start date
flareside_thun
Reviving The General Tao
The only time I've read on here of the bolts shearing was an instance of "grade 8" bolts there, in fact, not. I read that one guy was going to attempt welding his DMF in addition to the bolts. Whether he did or not, I have no idea as I don't recall any followup about it.
I recall hearing of this happening to a few people, and this is the main reason why I don't advocate the Lucky Mod...
I should ask, though, where did you source the bolts? I've heard that garden-variety hardware store Grade 8 bolts are of varying quality and might not be enough to handle this sort of force. I recall people having sources of higher-quality bolts, but I don't remember what those sources are...hopefully one of them will chime in soon...
I should ask, though, where did you source the bolts? I've heard that garden-variety hardware store Grade 8 bolts are of varying quality and might not be enough to handle this sort of force. I recall people having sources of higher-quality bolts, but I don't remember what those sources are...hopefully one of them will chime in soon...
flareside_thun
Reviving The General Tao
Fastenal is one manufacturer/supplier that comes to mind.
justinray
Contributor
I always beat up the guys in the machine shop at work for various high grade bolts.
TWeatherford
Full Access Member
I assume you drilled out the holes to take a 3/8" bolt? I've got about 20k on my lucky mod, no problems yet. Yours failing so soon suggests something was very wrong.
Out of curiosity, making lots of conservative assumptions for unknowns, I calculated the actual strength needed to transmit the torque our engines can produce, and also what the six grade 8 bolts are capable of withstanding. The result was that the bolts are 9x as strong as they need to be.
Out of curiosity, making lots of conservative assumptions for unknowns, I calculated the actual strength needed to transmit the torque our engines can produce, and also what the six grade 8 bolts are capable of withstanding. The result was that the bolts are 9x as strong as they need to be.
Devon Harley
Full Access Member
If someone has an idea an then you don't here from them it's probley because they did it and it didn't turn out so good just thought I would add that
f-two-fiddy
Registered User
Devon, not to be a spelling ****, but that last post was almost impossible to comprehend.
Punctuation is your friend!
Punctuation is your friend!
Matrix37495
The Amish Wolfman
If someone has an idea, and then you don't hear from them, it's probably because they did it, and it didn't turn out so good. Just thought I would add that.
Better Rob?
the replys from people,,that did this,,and using garden variety bolts,,wasnt good. like he said,,GOOD grade 8 bolts,,have wayyyy more than enough strength for the torque the engine produces. get them at any QUALITY supply place.."lucky" did his,,and never had a problem...
Yes, I drilled out the bolt holes on a drill press, bolts were grade 8's, albeit from the local hardware store, maybe will try fastenal, I think dad can get to one not too far from work. Been thinkin about it though, the flywheel is held to the crank with 8 or 9 3/8" bolts, what does everyone think about drilling a couple more holes in the whole mess, or even one between everyone making a total of 12??
Bolts are funny things, and more complicated than you might imagine. Over design is always a good thing in amateur applications! Take, for example, the following chart:
That chart is for grade 2 NF fine thread bolts. You can see the safe working limits are much lower than the absolute shear strength when calculated as 60% of tensile strength for a given bolt area. For example, that 3/8 bolt in grade 2 the calculated shear strength would be 2897.4 lbs actually is at its safe limit at 510 lbs. Bolts that are subjected to shear loads that are variable, intermittent (cyclic) also have another failure mode called fatigue failure which lowers the bolt capacity even further - you will often see this discussed on airplane propeller bolts, since it is important that the propeller does not fall off while in the air! The design of the threads actually has an large impact on fatigue strength of a bolt - a round, smooth thread root is much stronger than a thread that is cut, or has a sharp thread "root." Unfortunately I could not find you a simple calculation for fatigue strength, probably because the forces are dynamic, and the calculations more complex and rely on other factors beyond simple strength calculations.
Grade 8 bolts have approximately 2x the absolute strength of a grade 2 bolt (55,000 psi vs 120,000 psi). If you design within grade 2 parameters, you will always have an extra margin of safety if you use a grade 5 or 8 fastener.
So, we know you use 6 bolts in a "lucky mod" flywheel. I also went out and measured my old DMF and found the bolt circle radius to be about 2.5" for those 6 bolts. We commonly measure engine torque in ft-lbs or spoken "pound-feet". Simply, this is how much force the engine can apply 12" (one foot) from the axis of rotation (center of the crank). This is important because when move a bolt closer to the axis of rotation (crank) the more force it must be able to withstand, think about how much torque you can apply with a 3 ft cheater bar vs a stubby wrench - you need a lot more oomph to move the same bolt with the shorter wrench for the same bolt.
Static torque of a 600 ft lb engine (safe overestimate) on those bolts would be 12" / 2.5" = 4.8 times the shear force on those bolts, or 600 x 4.8 = 2880 lbs. If you divide that by 6, you see that each bolt needs to take 480 lbs of torque each in this appication for a 600 ft. lb. engine. Pretty close to that 510 lb static limit for a grade 2, eh? Now put those bolts in a dynamic, cyclic environment like a flywheel and you can imagine you are operating pretty close to or even exceeding their fatigue strength specs. Now, of course we drilled our holes so accurately that each bolt is sharing the load equally and one is not subject to more force than the others, what if one bolt hits the "wall" of the drilled hole before the others? It has to take all the force until it deforms enough that the other bolt(s) can then share the load, assuming it doesn't break first.
So, ensure you have very highly rated, traceable strength bolts in there to give yourself some more margin, or like you said, add more bolts! Also make sure you put the proper torque on each fastener which would be 22 ft lbs. for a dry grade 2 3/8 bolt, or 50 ft lbs. for a dry grade 8 bolt. This ensures that the clamping force exceeds the shear force on the bolt, in which case you can avoid bolt fatigue all together (the bolt creates friction between the two surfaces that is greater than the shear torque load on the joint) A grade 8 3/8 NF bolt torqued to 50 ft lbs can create 7900 lbs of clamping force, well in excess of the calculated safe shear loads for the bolt.
Pretty cool, if not extremely boring! I always found this kind of stuff fascinating, hope it helps!
You must be registered for see images attach
That chart is for grade 2 NF fine thread bolts. You can see the safe working limits are much lower than the absolute shear strength when calculated as 60% of tensile strength for a given bolt area. For example, that 3/8 bolt in grade 2 the calculated shear strength would be 2897.4 lbs actually is at its safe limit at 510 lbs. Bolts that are subjected to shear loads that are variable, intermittent (cyclic) also have another failure mode called fatigue failure which lowers the bolt capacity even further - you will often see this discussed on airplane propeller bolts, since it is important that the propeller does not fall off while in the air! The design of the threads actually has an large impact on fatigue strength of a bolt - a round, smooth thread root is much stronger than a thread that is cut, or has a sharp thread "root." Unfortunately I could not find you a simple calculation for fatigue strength, probably because the forces are dynamic, and the calculations more complex and rely on other factors beyond simple strength calculations.
Grade 8 bolts have approximately 2x the absolute strength of a grade 2 bolt (55,000 psi vs 120,000 psi). If you design within grade 2 parameters, you will always have an extra margin of safety if you use a grade 5 or 8 fastener.
So, we know you use 6 bolts in a "lucky mod" flywheel. I also went out and measured my old DMF and found the bolt circle radius to be about 2.5" for those 6 bolts. We commonly measure engine torque in ft-lbs or spoken "pound-feet". Simply, this is how much force the engine can apply 12" (one foot) from the axis of rotation (center of the crank). This is important because when move a bolt closer to the axis of rotation (crank) the more force it must be able to withstand, think about how much torque you can apply with a 3 ft cheater bar vs a stubby wrench - you need a lot more oomph to move the same bolt with the shorter wrench for the same bolt.
Static torque of a 600 ft lb engine (safe overestimate) on those bolts would be 12" / 2.5" = 4.8 times the shear force on those bolts, or 600 x 4.8 = 2880 lbs. If you divide that by 6, you see that each bolt needs to take 480 lbs of torque each in this appication for a 600 ft. lb. engine. Pretty close to that 510 lb static limit for a grade 2, eh? Now put those bolts in a dynamic, cyclic environment like a flywheel and you can imagine you are operating pretty close to or even exceeding their fatigue strength specs. Now, of course we drilled our holes so accurately that each bolt is sharing the load equally and one is not subject to more force than the others, what if one bolt hits the "wall" of the drilled hole before the others? It has to take all the force until it deforms enough that the other bolt(s) can then share the load, assuming it doesn't break first.
So, ensure you have very highly rated, traceable strength bolts in there to give yourself some more margin, or like you said, add more bolts! Also make sure you put the proper torque on each fastener which would be 22 ft lbs. for a dry grade 2 3/8 bolt, or 50 ft lbs. for a dry grade 8 bolt. This ensures that the clamping force exceeds the shear force on the bolt, in which case you can avoid bolt fatigue all together (the bolt creates friction between the two surfaces that is greater than the shear torque load on the joint) A grade 8 3/8 NF bolt torqued to 50 ft lbs can create 7900 lbs of clamping force, well in excess of the calculated safe shear loads for the bolt.
Pretty cool, if not extremely boring! I always found this kind of stuff fascinating, hope it helps!
Last edited:
ONE Critical thing that's perhaps been missed, the load on the bolts should be only on the shoulder portion not the threaded portion in a shear situation.
as soooo poorly depicted in my crude drawing....
The one on the Left has threads in the shear area of the bolt, thus a point of failure as the the threaded portion is not designed for shear......
The one on the Right has a proper design where the shoulder is the only portion in shear thus significantly more resistant to shear.
I hope this is clear...
as soooo poorly depicted in my crude drawing....
The one on the Left has threads in the shear area of the bolt, thus a point of failure as the the threaded portion is not designed for shear......
The one on the Right has a proper design where the shoulder is the only portion in shear thus significantly more resistant to shear.
You must be registered for see images attach
I hope this is clear...
