VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Torque for rod and main bearing bolts

My friend is going to drop the pan and check clearances on the rod bearings and mains. He is asking what torque would be suggested when he reassembles it.

When I rebuilt Lurch's engine in 2017, I did not tighten the main bearing bolts or the rod bearing nuts to a specific torque spec. I tightened them just snug enough so that the brass shims were completely flat.

The nuts that go onto the bearing bolts are castle nuts and there's a hole in the bolt for a cotter pin to hold the nut in place. The bolts that hold the main bearings in place have a locking plate under the bolt head that you bend a tab up against a bolt-head flat to keep it from rotating loose.

For the full story of my engine rebuild, click on this link: Resurrecting a '28 4 Banger

Cheers, Dean

Dean's approach is the way the engine was assembled at the factory and in service departments. Few if any mechanics owned a torque wrench.

If you want a torque value to give you some comfort use the values for Grade 2 bolts. The use of heat-treated (higher strength) hardware was almost non-existent during the time period your vehicle was built.

Whatever you do, do not think that you will make the engine and vehicle better by using Grade 5 or 8 hardware and torque to those higher values. The increased clamp load will compress/deform parts and strip threads in tapped holes.

Actually...

A bearing cap bolt that is only snugged up and not tightened to torque spec is in danger of fatiquing and failing catastrophically.
The explanation is an engineering concept and somewhat involved but I'll try.

All metals deform (stretch or compress) when subjected to a force.
When a rod bolt is properly torqued, a force is applied which stretches the rod bolt and compresses the rod and cap.
That means there is of course a preload on the rod bolt.
When an additional force is applied downward on the rod cap (and pulling on the rod bolts) due to rotational forces, this DOES stretch the rod bolt an additional amount, but this actually allows the rod and cap to DECOMPRESS which REDUCES that part of the load on the rod bolt.
So the total amount of load increase on the rod bolt is significantly LESS than the load increase due to the rotational forces alone.

If the preload on the rod bolt is significantly less, it INCREASES the AMPLITUDE of the change of force on the rod bolt during the rotational cycle.
That of course means an increase in the actual amplitude of change in length (stretch) of the rod bolt.
And that of course means more fatique.

What that means is rod bolts should always be torqued to spec, and not under torqued.
This is why rods with castellated nuts often called for the nut being torqued to spec, and then TIGHTENED to the next cotter pin hole, Not loosened.

All that said. Rusty is right that if no spec is available for an engine this old, a Grade 2 spec should probably be used.
And I'd still recommend using a torque wrench. 🙂

Hope that helps
It's a little hard to get one's head around, but I have been assured that it is correct.

This is the information I use,

Connecting rod nuts - 35 ft.lbs. maximum

Head bolts(if they are original ones)no highter than
50 ft.lbs. maximum

Main bearing cap bolts(if original)no higher than 60-70 ft.lbs

*Note:all torque settings to be done with threads oiled.

The original head bolts & main bearing cap bolts are not a high tensile grade of steel,and over the years,the threads will stretch to the point where they will fail if over tightened.All the threads on the head bolts for my engine were all stretched from being tightened up to who knows what torque setting over the last 80 years,so I had the replaced with high tensile steel studs & nuts.Now I know I can pull the nuts down to 60 ft.lbs.
without worrying about snapping bolts.
The main bearing cap bolts are the same.These I replaced with grade 8 high tensile bolts,and they handle 80 ft.lbs. without a problem.You might want to also replace the bolt that clamps the piston pin into the connecting rod with high tensile bolts.I had one fail,and though the top of the connecting rod ended snapping off,the only real damage to the motor was the grooves the piston pin made in the front & rear of the bore.Easily fixed by a bore & sleeve job.

Thanks for the excellent information.