Do not use ungraded bolts

[oldspert]

I was installing my new pressure plate and torqueing to the recommended 20 psi when the head snapped off one of the ungraded bolts. Luckily I was able to retrieve the stump without having to drill it out. On a previous post I mentioned the bolts were ungraded and got replies that it made no difference for the pressure plate, Wrong...I will but a complete set of grade 5 or better bolts.
Ed

 

[v8fordman]

If I’m not mistaken, flywheel bolts originally had threads all the way to the bottom of the head. If you used bolts with a small shoulder, you could have created your broken bolt scenario. Just a thought.

 

[nkaminar]

Yes, ungraded bolts can be very weak. For those who are not familiar with how bolts are marked, the three radial lines on the head indicate grade 5. The number of lines is two less than the grade. So a grade 8 bolt will have 6 lines. And grade 2 would have no lines. But a bolt without lines can be ungraded. A chart showing the yield strengths can be found here: https://www.reddit.com/r/Cartalk/comments/a9l5ln/there_was_a_person_looking_for_replacement_bolts/

 

[Hitman]

Use original bolts if you can and leave the torque wrench in the tool box. Ford never used a torque wrench, so any recommendations for torque are just a guess. Try installing them with a 1/4” drive us one hand. That should be just fine. Do it several times and you’ll start to develop the feel for it.

 

[kurt v]

I'm pretty sure that the bolts have a short shoulder on them that goes into the fly wheel , that way the pressure won't against any threads.

 

[Jim Brierley]

I'm pretty sure that the bolts have a short shoulder on them that goes into the fly wheel , that way the pressure won't against any threads.

They do, and I recommend using them, get new ones, old bolts have often been over-torqued which is not a good thing.

 

[oldspert]

I'm pretty sure that the bolts have a short shoulder on them that goes into the fly wheel , that way the pressure won't against any threads.

That would work if the flywheel was counterbored for the shoulder to fit into. My flywheel is threaded all the way to the top of the surface. And it appears to be a new flywheel installed by a previous owner.
Ed

 

[ModelA29]

...... My flywheel is threaded all the way to the top of the surface.

The threads are in the crankshaft not the flywheel. They do go all the way to the surface and once the flywheel is installed there is room for the area of the bolt that is not threaded to not bottom out on the crank.

 

[JRrev]

I'm sure he is talking about the pressure plate to the flywheel, not the flywheel to the crankshaft.

 

[v8fordman]

Here are pictures of original bolts. All original Ford bolts were of superior quality and were thick headed and unmarked. Therefore, they do not have grade marks on them. I have been rebuilding Model A and flathead engine for 43 years and have never been afraid to reuse them even though it may be true that reused over torqued ones could break. In my experience, I’ve never had one to break and I’ve obviously taken my chances with old quality versus new unknowns which are mostly now made in China. From left to right…pressure plate to fly wheel (no shoulder), valve chamber (side cover) and oil pan with slight shoulder, v8 clutch to flywheel-significant shoulder.

 

[oldspert]

The threads are in the crankshaft not the flywheel. They do go all the way to the surface and once the flywheel is installed there is room for the area of the bolt that is not threaded to not bottom out on the crank.

Not about the flywheel to crank bolts.
Ed

Here are the bolts that broke.

 

[nkaminar]

Rolled threads. The bolt has to start out with a smaller diameter so that the treads can be rolled which makes them weaker. Rolled threads have to be made with low strength (low yield point) steels. The bolts were not heat treated after rolling. Do not use bolts that look like this.

 

[alexiskai]

Rolled threads. The bolt has to start out with a smaller diameter so that the treads can be rolled which makes them weaker. Rolled threads have to be made with low strength (low yield point) steels. The bolts were not heat treated after rolling. Do not use bolts that look like this.

ARP rolls the threads on all their fasteners, and they heat treat before rolling.

 

[40 Deluxe]

If I’m not mistaken, flywheel bolts originally had threads all the way to the bottom of the head. If you used bolts with a small shoulder, you could have created your broken bolt scenario. Just a thought.

I think you have this reversed. Pressure plate bolts always had a shoulder to precisely center the pressure plate and to transmit torque. I was always taught not to use bolts that were threaded all the way to the head.

 

[oldspert]

Pressure plate bolts always had a shoulder to precisely center the pressure plate

My flywheel has a depressed area around the outer edge that the pressure plate fits snugly into that centers the plate as the bolts are tightened. Is this normal? The V8 pressure plate does use the shouldered bolts because there is not any other way to precisely center the pressure plate.
Ed

 

[oldspert]

Rolled threads. The bolt has to start out with a smaller diameter so that the treads can be rolled which makes them weaker. Rolled threads have to be made with low strength (low yield point) steels. The bolts were not heat treated after rolling. Do not use bolts that look like this.

Just to refute the post above. From Machinery's Handbook

Ed