Need length of hex bolts to replace mounting studs

pedal pusher · Aug 30, 2009

I have the motor mount from Sick Bike Parts to try again to mount my GasBikt 66cc engine on the oversize downtube of my Huffy Santa Fe. I have removed the studs from the engine and want to use hex head bolts to mount the engine. I guess most things on this are 6mm x 1.0 pitch. The overall length of the removed front studs is approximately 24mm, and the oal of the rear is approximately 75mm. I don't think I will need that much length expecially in the rear mount. The bike is a Huffy Sante Fe which appears identical to the Cranbrook, but an older model. Has anyone installed engine and bolts like this and can furnish or recommend the correct bolt length and the gradeto make for a nice installation?
All information and suggestions greatly appreciated.
Thanks

2door · Aug 30, 2009

First of all I moved your post to the mounting techniques section where it will get more attention.
I've no experience with the Sick Bike Parts front mount but you should have received the right fasteners with it. If you want to switch from studs to bolts that's a good idea in my book but as far as length for the front, I can't help you. The stock mount when used on a standard 'V' frame bike will require 6mm X 20mm. You'll fine it easier to use Allen head capscrews instead of hex head bolts due to the limited space available. As for the rear mount, if installed on a standard diameter seat tube, will be a 6mm X 50 to 55mm. Again, use Allen head capscrews. You'll notice the kit supplied nuts for the engine studs are smaller in outside diameter than a standard metric nut. That is because of the limited space around the mounting areas of the motor mounts, intake and exhaust manifolds. Ace hardware carries all the necessary sizes you'll need and in a grade 8 or the metric equivalent of a grade 8. Some will tell you that a grade 8 fastener is too hard and will break. That just is not true. You can run a grade 5 fastener and be okay but the extra assurnace is worth the few pennies difference in price. Grade 8 fasteners are usually black. Do not use stainless steel fasteners for anything other than decorative work.
Tom

pedal pusher · Aug 31, 2009

2Door,
Thanks very much. I meant Allen head instead of hex head and even have a set of Allen wrenches where I can find them. It was a senior moment. Again, thanks very much. This has been a challenge thus far as nothing seems to fit.

Scotchmo · Aug 31, 2009

2door said:
First of all I moved your post to the mounting techniques section where it will get more attention.
I've no experience with the Sick Bike Parts front mount but you should have received the right fasteners with it. If you want to switch from studs to bolts that's a good idea in my book but as far as length for the front, I can't help you. The stock mount when used on a standard 'V' frame bike will require 6mm X 20mm. You'll fine it easier to use Allen head capscrews instead of hex head bolts due to the limited space available. As for the rear mount, if installed on a standard diameter seat tube, will be a 6mm X 50 to 55mm. Again, use Allen head capscrews. You'll notice the kit supplied nuts for the engine studs are smaller in outside diameter than a standard metric nut. That is because of the limited space around the mounting areas of the motor mounts, intake and exhaust manifolds. Ace hardware carries all the necessary sizes you'll need and in a grade 8 or the metric equivalent of a grade 8. Some will tell you that a grade 8 fastener is too hard and will break. That just is not true. You can run a grade 5 fastener and be okay but the extra assurnace is worth the few pennies difference in price. Grade 8 fasteners are usually black. Do not use stainless steel fasteners for anything other than decorative work.
Tom

I would recommend against replacing the mounting studs with bolts. The studs are typically threaded securely into the cast aluminum motor casing. When tightening the mounts, the nut is rotating on the steel stud. If you use bolts, the bolt threads are rotating in the relatively soft aluminum case at fairly high preloads. Doing this repeatedly will cause galling of the aluminum and will eventually destroy the threads. The bolts can work but be aware of the potential problems.

2door · Aug 31, 2009

Scotchmo said:
I would recommend against replacing the mounting studs with bolts. The studs are typically threaded securely into the cast aluminum motor casing. When tightening the mounts, the nut is rotating on the steel stud. If you use bolts, the bolt threads are rotating in the relatively soft aluminum case at fairly high preloads. Doing this repeatedly will cause galling of the aluminum and will eventually destroy the threads. The bolts can work but be aware of the potential problems.

Not true. Sorry. In the first place the studs are not typically "threaded securely" into the aluminum castings. They are often only finger tight from the factory. As for your theory that the bolts are being threaded in at high preloads, if you torque them correctly and don't over do it as is too often the case, you are in no danger of damaging the aluminum threads in the casting. Where people get into trouble here is from tightening the fasteners too much and too often. As for "repeatedly" tightening, why is there a need for it? Unless you are constantly tightening, which is a mistake, you're not adding any wear to the aluminum threads. The factory studs and even the nut, which some people say are junk, will suffice if installed correctly. The weak area I've seen are the kit supplied lock washers which flatten too easily and do not function as locking devices after the initial tightening. If you're going to replace the fasteners always replace the lockwashers with good quality items. One of the problems I see on every engine I've delt with is a uneven mating surface between the head of the bolt or nut where it tightens against the intake and exhaust manifolds. The weld bead often extends into this area which prevents the undersurface of the bolt head (nut/washer) from having a full contact surface. This will cause the bolt or nut to be cocked when tightened against the flange which can weaken and cause failure of the stud or bolt under stress (vibration) Don't believe me? Check your exhaust and intake manifolds where the bolt head or nut tightens against them and look for a flush fit. If there is any interference between the head of the bolt or nut with the weld bead you have the potential for eventual fastener failure no matter what you use, grade 5, grade 8 studs and nuts or Allen head bolts. All of this is also true of the engine mounts. There is absolutely no need to tighten and retighten these fasteners over and over again. Use good lockwashers, tighten once and then again after the engine has reached operating temperature the first time then visually check them for flattened lockwashers. If the offset ends of the lockwashers are not aligned then the fastener has loosened if they are flush, nothing has come loose so leave them alone. Pepeated tightening is the biggest cause of fastener/thread failure with these engines. Not poor quality fasteners.
Tom

Scotchmo · Sep 1, 2009

2door said:
Not true. Sorry. In the first place the studs are not typically "threaded securely" into the aluminum castings. They are often only finger tight from the factory. As for your theory that the bolts are being threaded in at high preloads, if you torque them correctly and don't over do it as is too often the case, you are in no danger of damaging the aluminum threads in the casting. Where people get into trouble here is from tightening the fasteners too much and too often. As for "repeatedly" tightening, why is there a need for it? Unless you are constantly tightening, which is a mistake, you're not adding any wear to the aluminum threads. The factory studs and even the nut, which some people say are junk, will suffice if installed correctly. The weak area I've seen are the kit supplied lock washers which flatten too easily and do not function as locking devices after the initial tightening. If you're going to replace the fasteners always replace the lockwashers with good quality items. One of the problems I see on every engine I've delt with is a uneven mating surface between the head of the bolt or nut where it tightens against the intake and exhaust manifolds. The weld bead often extends into this area which prevents the undersurface of the bolt head (nut/washer) from having a full contact surface. This will cause the bolt or nut to be cocked when tightened against the flange which can weaken and cause failure of the stud or bolt under stress (vibration) Don't believe me? Check your exhaust and intake manifolds where the bolt head or nut tightens against them and look for a flush fit. If there is any interference between the head of the bolt or nut with the weld bead you have the potential for eventual fastener failure no matter what you use, grade 5, grade 8 studs and nuts or Allen head bolts. All of this is also true of the engine mounts. There is absolutely no need to tighten and retighten these fasteners over and over again. Use good lockwashers, tighten once and then again after the engine has reached operating temperature the first time then visually check them for flattened lockwashers. If the offset ends of the lockwashers are not aligned then the fastener has loosened if they are flush, nothing has come loose so leave them alone. Pepeated tightening is the biggest cause of fastener/thread failure with these engines. Not poor quality fasteners.
Tom

A screw fitted directly in the aluminum thread in lieu of using a stud can work well enough but the galling caused by torquing screws in aluminum is just one of the problems. You can replace the poor quality stud with a good quality screw and it could be an improvement. But a good quality stud used to clamp to an aluminum casting will outperform a good quality screw when each are installed correctly. And the length tolerance of the stud is less critical. The depth of the threaded hole in the aluminum casting is designed to allow for the full tensile strength of the fastener. If the length of the screw is a little too short, you do not get the designed engagement and therefore cannot utilize its full strength. If the length of the screw is a little too long, you will not get any clamping force since it will bottom out in the hole. The length of the stud can vary a lot with no affect on the integrity of the connection. Since the quality control on these motors is lacking, it is up to you to insure that the stud is of good quality and installed correctly. You could use a bolt instead, but if you’re going to change it anyway, you are better off using a good quality stud. Applying torque to a nut on a stud also prevents the torsional wind-up that is present when you torque a screw. The longer the screw, the more of a problem this becomes. In general, the nut on a stud in aluminum can be safely torqued to a higher level than a screw directly in aluminum. And the residual torque will more closely match the installed torque.

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Need length of hex bolts to replace mounting studs

pedal pusher

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2door

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