Engine: Tapping threads flywheel bolts

[drrotor]

I guess I need someone to convince me why I shouldn’t tap fine threads into the back of my 245 crankshaft and install the flywheel with grade 8 bolts instead of nuts on the factory rear-facing bolts... always seemed like the silliest thing to me about Studebaker engines. Other manufacturers seem to have overcome the need to do it that way!
And convince me why I shouldn’t do it on the next V8 I build as well!

[StudeRich]

I guess Kenny, we'll have to get a Stress Engineer to calculate how much stress 6, 3/8" Screws can take on only 1/4'" of their threaded smaller end with a 40 Lb. ? Flywheel and 25 Lbs. of Pressure Plate and Clutch Disc hanging on, and spinning at 3500 to 4000 RPM.

[Studebakercenteroforegon]

Not to mention that that the profile of a thread is a stress riser. I wouldn’t do it just to save the price of a oil pan gasket set.

[64Avanti]

1/4" thread is not adequate for a 3/8" thread with grade 8 torque. You should have at least 3/8" thickness.

[DEEPNHOCK]

Race and performance engines (Studebaker included) do this all the time.
Just drill and tap for ARP crank bolts.
BTDT...

[jclary]

I guess I need someone to convince me why I shouldn’t tap fine threads into the back of my 245 crankshaft and install the flywheel with grade 8 bolts instead of nuts on the factory rear-facing bolts... always seemed like the silliest thing to me about Studebaker engines. Other manufacturers seem to have overcome the need to do it that way!
And convince me why I shouldn’t do it on the next V8 I build as well!

I know I have very questionable qualifications to comment on some of the technical discussions we engage in here. This is one of them. However, we know the engineers with Studebaker were very concerned with accurate dimensions and concentricity in this area. Thus, the requirement to dial-in the bellhousing to each engine. It could be that their reason to use the bolts/nuts was to keep the crankshaft flange from possible distortion. In the assembly, that flange is mainly "clamped" and not subjected to the torque stresses if it were to be threaded. As we know, any item requiring precision machining involves very detailed measuring and every aspect has to be closely monitored. "Setup" is crucial with attention paid to all axis, tool wear, and even then...these parameters usually have a plus and minus tolerance sign displayed with each spec. In the science of taps and dies, even the % of thread contact between the threaded bolt and the threads in a tapped hole is considered.

So, on a one-time modification, you do your best and it might work out just fine. For a production operation, There would have to be enough repetitive work to establish such things as how many holes a tap would be able to thread before it wore out of tolerance and needed replacing and the same for every step in the process.

I have observed your posts regarding your projects and I have enjoyed them all. If you decide to thread the crankshaft flange, make sure to let us know the details, how you set it up, the tools used, and how it works out. You might just start a trend.

[DEEPNHOCK]

You make a salient point.
Studebaker engineers had two goals. Low cost (product/labor) and getting the vehicle through the warranty period.
A slide in bolt in a drilled hole is less expensive than tapping all the holes.
Not a negative comment, but a manufacturing reality.
Are there better methods? Probably. But cost trumps a lot of alternatives.

I know I have very questionable qualifications to comment on some of the technical discussions we engage in here. This is one of them. However, we know the engineers with Studebaker were very concerned with accurate dimensions and concentricity in this area. Thus, the requirement to dial-in the bellhousing to each engine. It could be that their reason to use the bolts/nuts was to keep the crankshaft flange from possible distortion. In the assembly, that flange is mainly "clamped" and not subjected to the torque stresses if it were to be threaded. As we know, any item requiring precision machining involves very detailed measuring and every aspect has to be closely monitored. "Setup" is crucial with attention paid to all axis, tool wear, and even then...these parameters usually have a plus and minus tolerance sign displayed with each spec. In the science of taps and dies, even the % of thread contact between the threaded bolt and the threads in a tapped hole is considered.

So, on a one-time modification, you do your best and it might work out just fine. For a production operation, There would have to be enough repetitive work to establish such things as how many holes a tap would be able to thread before it wore out of tolerance and needed replacing and the same for every step in the process.

I have observed your posts regarding your projects and I have enjoyed them all. If you decide to thread the crankshaft flange, make sure to let us know the details, how you set it up, the tools used, and how it works out. You might just start a trend.

[Rerun]

It is important to note that tapping a hole is not a simple operation, particularly in this type of application. The methods used to drill and tap, as well as the selection of the tap, can have significant effects on the strength of the bolted joint. I would recommend consulting with a qualified machinist before starting. There are also a number of resources on line that explain the significance of thread classes, GH numbers, etc.

https://www.newmantools.com/taps/taptech.htm#cla

[Colgate Studebaker]

If you have the time to wait for his response, Jim Pepper through the Turning Wheels Co-operator would be the perfect person to ask advise from. He was a R&D tool and die machinist at Harley Davidson for most of his working career. His knowledge of which taps etc. to use for each application is beyond anything I've ever come across. Send him an E-mail and he'll most likely get back to you in a few days time. Good luck, Bill.

[64Avanti]

Race and performance engines (Studebaker included) do this all the time.
Just drill and tap for ARP crank bolts.
BTDT...

When I said don't do it I was assuming that you only had 1/4" of flange thickness. The V8 flange is about .40 thick and that is thick enough for a properly torqued 3/8" fastener. So you can do that. I don't know what the flange thickness is on the 6 cyl engine. If it is at least .375" thick then it will be ok. In our business we want to have engaged thread length equal to the diameter of the fastener. In this case that also means that if you chamfer the threaded hole you have lost some of the threads so be careful. If you use a supper strength fastener then at this minimum size you can not apply full torque for the high strength fastener. Grade 8 torque is OK.

[drrotor]

When I said don't do it I was assuming that you only had 1/4" of flange thickness. The V8 flange is about .40 thick and that is thick enough for a properly torqued 3/8" fastener. So you can do that. I don't know what the flange thickness is on the 6 cyl engine. If it is at least .375" thick then it will be ok. In our business we want to have engaged thread length equal to the diameter of the fastener. In this case that also means that if you chamfer the threaded hole you have lost some of the threads so be careful. If you use a supper strength fastener then at this minimum size you can not apply full torque for the high strength fastener. Grade 8 torque is OK.

Actually it would have to be a 7/16-20 tap, because the existing bolt hole is already 3/8... so you can’t tap 3/8 threads into a 3/8 hole!

[Rerun]

Actually it would have to be a 7/16-20 tap, because the existing bolt hole is already 3/8... so you can’t tap 3/8 threads into a 3/8 hole!

But, you can use a 3/8" helicoil to thread a 3/8" hole. For this application, I would prefer to tap to the next available bolt size. However, sometimes that isn't possible because of other constraints. That is where the helicoil becomes a life-saver.

[64Avanti]

Actually it would have to be a 7/16-20 tap, because the existing bolt hole is already 3/8... so you can’t tap 3/8 threads into a 3/8 hole!

Yes you are correct. for a 7/16 screw the V8 flange is a little thinner than what I would like to see but should be ok. Again don't put a large chamfer on the tapped hole to deburr it. The 3/8 fasteners on the V8 are torqued to 35 lb-ft max and provide 5600 lb of clamping force per fastener. If you torque the 7/16-20 fastener to 55 lb-ft then you will have about 7500 lb of clamp force per fastener. That is the limit for a grade 5 screw. A higher grade fastener can be used but you should not torque it to a grade 8 torque to protect the threads in the crankshaft. I would not go above 60 lb-ft. Make sure that you use a fastener that goes all the way through the threads. You need all of the threads you have.

Torque values are for clean dry threads. You should not use these torque values for lubricated threads.

[64Avanti]

This thread started out with a question about a 245 6cyl engine. I have no idea about the flange thickness on that engine, only the V8.