V-8 Pressure plate for Model A

fordgarage said:
BTW, the bolts I bought 25 years ago also require that the flywheel holes be counterbored slightly for the bolt shoulder. This allows all the load path in the driveline to transfer shear forces through the shoulder, not through the bolt threads.
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Vince is correct and I did c'bore each of the holes 0.3125" dia. x 0.20" deep. I did this in the same set up when I drilled and tapped the mounting holes to maintain the accuracy of the holes. Don/WI
 
It seems that this is the season to pull engines and replace clutches. I am doing the same. I finally found the Mr. Gasket blister pack at a speed shop, $10. I noted that the bolts had significant shoulders. I also found 6 5/16-18 x 1 1/4 Grade 8 at a hardware store that had shoulders. I cut off about 1/2 in to get the correct length.

I have some questions.
Why not use Gr 8 instead of the Gr 5 that is supplied by Bratton? I noted also that their bolts do not have shoulders and I expect to return them.

Transfer shear forces in the shoulders and not in the threads? I never thought of it this way and I do not fully agree with this. I believe that the maximum shear force is applied when the clutch is let out and the car is starting up from standstill. When using a pressure plate with the flyweights the maximum forces that need to be considered are the forces trying to pull out the bolts. Why not maximize the threaded length? Why not use fine thread instead of NC thread? Why not use 3/8 in bolts instead of 5/16 in bolts?

I am working on a flywheel that I got from James Rogers that does not have the counter bore. Counterboring an existing hole should be easy. Does anyone know how to increase the depth of an existing hole?

Thanks,
Vic
 
Vince:

I take the drill it deeper in good humor. Is there any way to drill the hole deeper without ruining the already threaded portion of the hole?

The pressure plate bolts for GM & Chrysler products at ther speed shop are 3/8 inch. I guess that I could redrill the holes in the flywheel for 3/8 in bolts.

Vic
 
Vic in E-TN said:
Vince:

I take the drill it deeper in good humor. Is there any way to drill the hole deeper without ruining the already threaded portion of the hole?

The pressure plate bolts for GM & Chrysler products at ther speed shop are 3/8 inch. I guess that I could redrill the holes in the flywheel for 3/8 in bolts.

Vic
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First off, I would not redrill for 3/8" bolts. That would require you to redrill the mounting holes in the pressure plate and there is no way to do that accurately.

The only way to insure accuracy of the c'bore is to do it in the same setup that was used when the holes were originally drilled and tapped. That is not possible to do now that the holes are already drilled and tapped. Accuracy cannot be obtained by simply drilling the top portion (0.20" deep) of the hole with a 0.3125 diameter drill in a drill press. There is no way to guarantee that you have the existing hole located exactly beneath the drill.

There are now two different ways to rectify the problem.

The first and most accurate way is to fasten the flywheel to a milling machine table. The milling machine needs to have a two axis readout. Then redrill the hole pattern approximately 60 degrees from the current hole pattern by accurately locating the center of the flywheel and zero the "x" and "y" axis. Then use the six pair of locating coordinates to redrill and tap new holes. After the holes are drilled and tapped and before the flywheel is removed from the machine , use a "NEW" 0.3125 diameter end mill cutter and locate the end mill cutter above one of the holes. Go to a depth of 0.20" from the top of the flywheel. Repeat for the rest of the holes. You are now ready to assemble the pressure plate with the new bolts.

The second and less accurate way is to use standard 5/16-18 x 3/4" bolts and bolt the pressure plate to the flywheel. Then remove one bolt and locate the assembly on the drill press table so the 5/16 diameter drill is just starting in the flange of the pressure plate. Then fasten the flywheel to the table so it cannot move. Set a stop for the depth of the drill so you drill a bit more than 3/16" deep. after drilling to that depth, blow the chips out of the hole and use the new shouldered bolt in that hole and tighten it down. Proceed to the next hole and repeat this process for the rest of the holes. You will have the pressure plate located as accurate as possible under the circumstances.

Unless you are a machinist or have machine shop training, my instructions may be a bit confusing. In any event, good luck with your project. Don/WI
 
fordgarage said:
Personally, I would stay with the original 5/16 thread size bolts, not go bigger.

I don't know the size you would need offhand, but it would be the larger OD size to match the shoulder, and the smaller pilot size to match the tap hole size or minor thread diameter size ("F") of the threaded hole. There is a wide range of size combos available in stock, typically in 1/64 increments, and maybe letter and number sizes.
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The correct size to "match the shoulder" is 5/16" (0.3125) diameter as I stated in my thread above. The shoulder is simply the bolt diameter as the threads do not go all the way up to the head. As Vince says, the tap drill is an "F" drill which is 0.2570" diameter. If you can find a step drill with a main diameter of 0.3125" and a step of 0.2570" diameter, that would work also although you would need to take precautions to insure that the step diameter (0.3125) does not go deeper that 0.20" into the flywheel. Don/WI
 
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Dumb question; Now that we're redrilling and tapping holes into our (I assume (I know)) balanced flywheel are we going to throw the balance out or are we not talking enough to matter??
Paul in CT
 
Don/WI said:
...The second and less accurate way is to use standard 5/16-18 x 3/4" bolts and bolt the pressure plate to the flywheel. Then remove one bolt and locate the assembly on the drill press table so the 5/16 diameter drill is just starting in the flange of the pressure plate...Don/WI
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This SHOULD work well, but my experience lately is that the hole patterns in new pressure plates are only approximately correct. I've seen a couple that had one or two holes so far off as to make it very difficult to get the bolts started.

I believe Vince's step-drill method would be better, assuming the drill is available in the right size combination. Better yet would be a piloted counterbore such as
counterbore from MSC Industrial Supply
with the correct pilot.

Joe
 
1931 flamingo said:
Dumb question; Now that we're redrilling and tapping holes into our (I assume (I know)) balanced flywheel are we going to throw the balance out or are we not talking enough to matter??
Paul in CT
Click to expand...

If stops are set on the milling machine or drill press to insure uniform depth, there should be no measureable change in balance.

Joe
 
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