VCCA Home Forums Thread Like Summary - 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

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.