I am using a Spec aluminum flywheel on a 3800 Series 3 SC motor and would like some input on the best way to bolt it up.

I got some Grade 9 1" bolts and washers from McMaster Carr. I think using the 11 ft lbs + 50 degrees method is a good way to go. GM uses this spec to get the proper clamp load on the flywheel to crank joint. At 11 ft lbs there isn't much effect from friction and using the 50 degree additional rotation all the bolts should be very close in tensile load making the flywheel to crank joint pretty close to what GM intended it to be.


I found the manufacturer specs on the bolts and they give a torque spec on their Grade 9 5/16-18 bolts of 36 ft lbs with a thick nut and 14 ft lbs with a waxed locknut. I used a spacer the same thickness as the Spec flywheel with a new bolt, washer and standard Grade 8 nut and tightened it with a bending beam torque wrench. The torque increased to about 44 ft lbs and remained at that point as I kept turning until the bolt broke. I repeated this four times with new bolts and nuts with the same result. Based on a 75% load factor that gives a torque of 33 ft lbs for these bolts with the Grade 8 nuts at the same thickness as the Spec flywheel.

Next I used a new bolt, washer and nut with the flywheel spacer and tightened it to 130 in lbs. I put a torque angle gauge between the torque wrench and the bolt, with the wrench set to 32 ft lbs, and tightened it. The torque wrench clicked over close to 50 degrees every time, sometimes a few degrees short of 50 and sometimes a little over 50. Based on this experiment I think the GM specs with some Loctite would be the best way to install the Spec flywheel with these Grade 9 bolts and washers.

Any thoughts would be appreciated, I only want to do this once.

If you do a search you will find that aluminum flywheels are real bad actors for sure. Many many of them seem to come loose causing lots of grief. Seems the aluminum sort of cold flows from under the bolts and they then shake loose. Good luck, I am not sure anyone has found a fool proof solution. Larry

The engineering term is "creep". The constant load from the bolts tends to make the aluminum flow out from under the bolt heads, relaxing the clamp load. The high temperatures to which a clutch is subjected accelerate this process.

Use the largest diameter washers you can in order to minimize this effect. There's also a ring shim with the bolt holes in it which does an excellent job of spreading the load from the bolts out. If this fits under your clutch disk, use it.

Thanks for the input.

I have spent a lot of time reading about aluminum flywheels that came loose and some that had no problems. There is a lot of confusion about what bolts to use, washers or no washers and how to torque the bolts. There seemed to be more problems with Fidanza flywheels than Spec.

It makes me think that using washers and locktite and the proper torque make it more likely I won't have any problems, he said with his fingers crossed. I plan on using the torque angle method in order to get as close to the GM clamp force as possible.

I think the torque angle is really used because it is easier to do with robots. I have done both and you will see that you will end up the the same torque doing it either way. If you use a torque wrench to pull in the additional angle you can get a reality check on that method when you see the ending torque of the fastener. Larry

I am going with torque angle. I think it eliminates any variation in the actual clamping force caused by friction affecting torque wrench readings.

25ft lbs with ample loctite has been good luck for my group.

quote Originally posted by trotterlg: I think the torque angle is really used because it is easier to do with robots. I have done both and you will see that you will end up the the same torque doing it either way. If you use a torque wrench to pull in the additional angle you can get a reality check on that method when you see the ending torque of the fastener. Larry

The angle process allows bolt stretch to be gauged more precisely. This allows using smaller bolts due to lower margin for error. Robots can measure torque just as easily as they measure angle. The use of angle takes uncertainties relating to friction out of the equation.

I use tight fitting dowel pins on my crank/ flywheel interface to help prevent bolt breakage. Its best done by a machine shop to get a very tight fit. I've never had a problem since using them.

"I use tight fitting dowel pins on my crank/ flywheel interface to help prevent bolt breakage. Its best done by a machine shop to get a very tight fit. I've never had a problem since"
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this makes a lot of sense.