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Bolt thread pitch
#1352186 Sun Mar 29 2020 02:18 AM
Joined: Jan 2008
Posts: 290
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bigedpa Offline OP
Shop Shark
Taking apart the frame on my 46 1/2 ton pickup I’ve noticed a lot of the bolts are fine thread. Is there a particular reason for this? A few of the parts include welded nuts so I guess I’ll have to case them down as my supply is all coarse thread. For the application where Nut and bolt are used does it make a difference if I replace with coarse threaded bolts?

Just wondering if there’s a critical reason for this.


"it's only old if you can't find a use for it; otherwise it's cool and i'll use it."
Re: Bolt thread pitch
bigedpa #1352204 Sun Mar 29 2020 04:17 AM
Joined: May 2015
Posts: 2,498
K
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
Many times, fine thread bolts are Grade 8 versus Grade 5, although Grade 5 are also available in fine thread.
Frame bolts would also likely be Grade 8 for strength in critical areas. I would be reluctant to replace a fine thread bolt with a Grade 5 coarse thread. Unless you can be sure, I would treat all coarse thread bolts as Grade 5.
Check to see if there are head markings on the bolts you remove to determine the the grade.

Attached Files
SAE Grade Markings.jpg (85.04 KB, 378 downloads)
Last edited by klhansen; Sun Mar 29 2020 04:22 AM.

Kevin
First car '29 Ford Special Coupe
#2 - '29 Ford pickup restored from the ground up.
Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com]
Busting rust since the mid-60's
Re: Bolt thread pitch
bigedpa #1352206 Sun Mar 29 2020 04:20 AM
Joined: Nov 2006
Posts: 238
J
J M Offline
Shop Shark
The science of nuts and bolts is a lot more technical then I originally thought. Probably the reason for the fine threaded bolts is that they can be tightened a lot snugger then a course threaded bolt.
Also they need to match up the right grade of bolt and nut. A mismatched bolt or nut will create a weak link.
I am sure others will come along with more information.
Good luck on your project.
Jim

Re: Bolt thread pitch
bigedpa #1352207 Sun Mar 29 2020 05:08 AM
Joined: May 2005
Posts: 7,263
B
General Purpose
Not sure what "frame" bolts you mean? Isn't your frame riveted? Shackle/spring eye bolts may be fine but what else? Possibly other suspension hardware like shock bolt? Steering adjustment? Can you describe these bolts? Usage, size/pitch?

Where nut an bolt are used, use coarse. Absolutely no reason for fine in those cases.

The normal NO. 10 screw is fine. 10-32.

Large majority of your bolts will be UNC.

Gr 5 is plenty strong for anything on your truck.

For our trucks: Coarse is plenty strong and torque-able. Fine is only necessary if forced by the design or available parts.


I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.


Re: Bolt thread pitch
bigedpa #1352218 Sun Mar 29 2020 12:48 PM
Joined: Feb 2019
Posts: 742
P
Jack of all Trades, Master of None
Fine threaded bolts are stronger and require more torque than course threaded bolts of the same size and grade. This is due to the larger minor diameter (deepest part) of the thread. I agree with bartamos that “Gr 5 is plenty strong enough” for our trucks but would replace like and find. If I found a fine threaded bolt, I would replace it with the same unless I found documentation saying otherwise.


Phil

1952 Chevrolet 3100
Project Journals
‘59 235 & hydraulic lifters
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Re: Bolt thread pitch
bigedpa #1352222 Sun Mar 29 2020 01:37 PM
Joined: Jun 2011
Posts: 3,536
E
Shop Shark
For bolts, NC are used when threaded into Ci and Al, NF is used in steel. Exceptions may exist.

Ed


'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
Re: Bolt thread pitch
bigedpa #1352228 Sun Mar 29 2020 02:01 PM
Joined: Jul 2000
Posts: 990
G
Shop Clutterist
The door hinge bolts are Grade 5 NF. Wish the GM engineers were around so we could ask why. ISTM, that NC would speed up the assembly line.


Gord
----
1954 1/2 ton 235 4 speed
Re: Bolt thread pitch
bigedpa #1352235 Sun Mar 29 2020 03:37 PM
Joined: Jun 2011
Posts: 3,536
E
Shop Shark
Lot of stress on door hinges, they thread into NF threads as part of hinge system.

Ed


'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
Re: Bolt thread pitch
bigedpa #1352266 Sun Mar 29 2020 07:33 PM
Joined: May 2005
Posts: 7,263
B
General Purpose
I have answered the OP's question. Most seem to agree. Now we can get into a long technical discussion if you want to but will be off topic. Like for instance......the door bolts are UNF to be able to quickly tighten after adjust. Not for strength. There are lots of greater stress places on the truck and they use UNC. There is no stress on door hinge bolts/threads when it's closed. When it's open, it is cantilevered, probably weighs 60-80-100 lbs and we lean on it. All those bolts are plenty strong enough for that, UNC or UNF.

Here is the reason we don't need to get into a strength discussion regarding our trucks. Gr 5 vs Gr 8, fine vs course and all that.
For example: using a 3/8 bolt. Using Shear as it is 60% of Tensile. using Gr 5 steel.
The course thread shear is 4985 Lbs
The fine thread shear is 5860 lbs
So 6 door hinge bolts will hold 30,000 lbs. 3 bolts is 15,000 lbs. So maybe the fine threads were actually for adjustment. Maybe the pin is weaker than the bolts!

After about 4 threads engagement, the stripout is as strong as the shear.

In a bolted assembly of two plates with a bolt and nut, the threads should be "below" the shear plane(the mating plane between the plates). So that the unthreaded shank passes thru the shear plane, if possible. But again, not necessary in our applications/stresses/vibrations/usage. Needs perspective.

Fine thread hardware are harder to find sometimes.

The extra strength, which depends on the application, is not needed in our trucks unless as stated in my answer to the OP and others who have responded.

If a person wants to use Gr 8 and UNF threads it's "fine", this is just a reference for those who don't.

Stay cooped up and wash your hands boys.


I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.


Re: Bolt thread pitch
bigedpa #1352282 Sun Mar 29 2020 08:57 PM
Joined: Aug 2019
Posts: 46
D
Wrench Fetcher
Fine threading also has a very shallow pitch (helical angle). This makes vibrations less effective at loosening fasteners over time. Fine thread fasteners are ideal for environments where vibration is considered an issue. Blue loctite helps too.
Another tidbit I learned from engineers, while working for a large heavy equipment manufacturer, is that a bolt needs to be two and a half times longer than its diameter for maximum holding. Typically considered in blind holes. Beyond that doesn’t add much.


1957 Chevy 3200
PS, A/C, Tilt column, Rebuilt 350, Rebuilt TH350, Reupholstered Bench Seat, sound proof/insulated
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