Main bearing bolts

[Synchro909]

As anybody who has had to work on their Model A on the side of the road knows, those bolts that hold the main bearing caps for the front and middle bearings can be a pain in the @$$.
Getting to them at any time is a hassle, especially the one in the valve chamber which is even worse on a RHD car.
Has anybody replaced them with studs. Here's how I see it working. The existing holes in the block for the bolts is 0.500" (maybe plus a little). The bolts are 1/2" with a UNF thread. It so happens that the diameter of the pilot hole for 9/16 UNC is 0.484", only 0.016" smaller than the existing holes. To tap them 9/16 UNC, the top 0.008+ would be missing off the tread. I can't see that being fatal especially if you tap the holes a little deeper to ensure strength. If you want to go the extra, drill the holes out to 0.547 (35/64") and use 5/8" UNC threads. I think I'd opt for 9/16 and tap the holes about 1" to make up for the small amount lost off the thread. I'd make stepped studs 9/16" UNC at one end and 1/2" UNF at the other to take the original nuts. To stop the studs coming loose, I'd drill a hole through them near the UNF end and make them long enough to put a pin through them both just like the one that holds the bolts for the front A frame ball socket or take a wire. There might be another hole to secure the nuts with a split pin just like now. Another option could be to use 13 mm metric coarse in the block. The pilot hole for that is 0.510" just about perfect!
I'm sure this has at least been thought of before but has anybody any knowledge of it or even a comment.

[Lawrie]

Arnold,what are you planing on doing on the side of the road that need attention to these bolts. a 33 engine would negate all of your worries.
Lawrie

[Synchro909]

Originally Posted by Lawrie

Arnold,what are you planing on doing on the side of the road that need attention to these bolts. a 33 engine would negate all of your worries.
Lawrie

Lawrie, I'm not planning anything, especially wrestling with those bleeding things!

[Synchro909]

Originally Posted by tbirdtbird

john at pete's auto machine, NM, has been using 1/2" helicoils and 1/2 NC grade 8 bolts for years with no trouble. use hardened washer. no need to pin them in any way. torque to 100, they ain't movin'

Helicoils! - now there's another option but I think I would prefer a fine tread at the other end and less torque. 45 ftlb is easier to apply.
What you say about them not moving is very interesting though.

[RUNNERBUN]

Originally Posted by Lawrie

Arnold,what are you planing on doing on the side of the road that need attention to these bolts. a 33 engine would negate all of your worries.
Lawrie

Hi Lawrie, sounds like a sales pitch, do you have a 33 engine you would like to unload.

[Synchro909]

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Originally Posted by RUNNERBUN

Hi Lawrie, sounds like a sales pitch, do you have a 33 engine you would like to unload.

I wondered the same thing. Knowing Lawrie, it would have 8 cylinders. $ would do e from '33

[Lawrie]

Noooo,
I only have a few left.

Lawrie

[JBohannon]

This post does bring up an interesting question. Why did Henry Ford do the main bearing bolts the way he did and make it so hard to work on. My only guess is he did not think the block metal would hold the thread well enough. Just Curious. Joe Bohannon

[George Miller]

Another way is to use studs and lock tight a nut on the top side. Make sure you use the proper torque. the nut will not come loose if you use proper torque.
How many modern cars have you seen where the main bearings come loose.

With proper torque the bolt is streched and held in retention.

[Tom Wesenberg]

Originally Posted by George Miller

Another way is to use studs and lock tight a nut on the top side. Make sure you use the proper torque. the nut will not come loose if you use proper torque.
How many modern cars have you seen where the main bearings come loose.

With proper torque the bolt is streched and held in retention.

That's the way I'd do it if I wanted to change. I'd use a cotter pin on the top and weld a tab to the side of the nut to keep it from spinning as you remove or install the lower nut.

[Jim Brierley]

I make 9/16" studs with approx 1" of thread. The stud continues all the way to the top at 1/2" size and I use 1/2" nuts up there. On the bottom I thread the stud to 1/2". I do not use castellated nuts as they have very few complete threads on them. I don't use Locktite either. I torque top and bottom nuts to 75 lbs, 90 lbs on my race engines. This solves the problem that Syncro talks about and may even strengthen the block a little? Such studs are pictured in mt Banger book. I use ARP nuts and washers. I ground a 3/4" socket so I can torque that troublesome top nut. 5/8" studs protrude a little into the babbitt.

[BILL WILLIAMSON]

If Vermins' mains went out, I'd probably sell him! I hope I DIE, before he DIES!
Bill W.

[ian Simpson]

Originally Posted by BILL WILLIAMSON

If Vermins' mains went out, I'd probably sell him! I hope I DIE, before he DIES!
Bill W.

Bill, an onery old phart like you will not die, you will simply fade away.

[MikeK]

Originally Posted by JBohannon

This post does bring up an interesting question. Why did Henry Ford do the main bearing bolts the way he did and make it so hard to work on. My only guess is he did not think the block metal would hold the thread well enough. Just Curious. Joe Bohannon

I believe that engineering decision was based on the type of bearing adjustment (multiple shims for babbitt).

All metals/alloys have a certain modulus of elasticity. That is, they can be stretched and will pull back, like a spring, returning to their original length repeatedly so long as you do not exceed that elastic limit. For a given bolt size longer lengths will act like a longer spring at the same torque.

A shim pack will go through some settling (compression) over time, just as a head gasket will. If you have a very long bolt/stud then the settling of the shim pack a thousanth or so will be of little concern as long as the bolt stretch which maintains the clamping force is somewhere in the double digits thousanths stretch.

Now, if you replace those long bolts with a stud directly in the block you have considerably shortened the stretch at the same torque. The front and center mains of an A have the spotface for the bolt rather close to the block. Using a stud directly into the block at that point will give less stretch than the shim pack may settle, causing the caps to become loose and walk. In an insert bearing application there is no shim pack so an extremely short stretch is not a concern.

Another aspect of the entire bolt/stud threaded fastener issue is tensile strength of the alloy. I laugh my a.. off when ever I hear about someone using very high alloy fasteners in a place where standard grade 5 fasteners are more than adequate. (Henry's bolts/studs are approx grade 5). At a given torque and length the stretch of a fastener decreases as you increase the tensile of the alloy. In many apps that is not a good thing. You give up the ability to maintain clamping force as the parts, separated by shims, gaskets, or multiple mating faces settle into each other.

[Synchro909]

Very informative, thanks Mike. I will be using inserts so from what you said, the studs will be fine.

[BILL WILLIAMSON]

Originally Posted by ian Simpson

Bill, an onery old phart like you will not die, you will simply fade away.

Yes, Ian,
And after 2 weeks, "THEY" will have all my stuff converted to PRE-PAID GIFT CARDS & when questioned, will say, "BILL WHO"????
Bill Who?