Continuing Big Block Knock Saga

[John R.]

After tearing my 67 L36 (390) engine down it appears that the upper rod bearings have excessive wear on the outside edges with quite a bit of the copper worn away in a sliver moon shape on each side. This seems abnormal to me after a little over 3,000 miles on the engine rebuild. In addition, one cam lobe was almost completely worn down and the corresponding lifter had a sigificant amount of material ground away, but that was the subject of my last post on this subject. Any suggestions as to the wear on the rod bearings?

[Clem Z.]

Re: Continuing Big Block Knock Saga

if the block was bored using a deck mounted boring bar and the deck was not parallel to crank shaft center line the bores will not be perpendicular to the crank shaft. if you do not want your block decked and removing the stamp pad marks you need to find a shop that has a boring machine that locates the block from the main bearing saddles and has a seperate parallel plate that locates the boring bar. this way the the cyl bores will be perpenddicular to the crankshaft centerline. if the rods were reconditioned and not checked for alignment this could cause the wear on the inserts.

[John R.]

Re: Continuing Big Block Knock Saga

Clem, thanks for the fast response. Wouldn't the improper alignment of the bores and crank result in wear on both top and bottom rod bearings? The wear I see is all on the top inserts.

[Clem Z.]

Re: Continuing Big Block Knock Saga

the force on the piston is downward on compession and when the plug fires so most of the load is on the top bearing insert. have your rods checked for alignment

[Jon #40768]

Re: Continuing Big Block Knock Saga

Also check to see that the bearing champher is larger than that of the crank pin. When you set the rod and piston in place, make sure that the bearing is not touching the radius of the crank pin where the journal curves up. If the crank was ground and left to to large of a radius, the radius would hit the bearing. Also check the side clearance between the rods.

[Roger Legge]

Re: Continuing Big Block Knock Saga

This is what I was thinking, especially if the crank has been ground undersize. It's called the crank fillet or fillet radius. It's the radius where the rod journal curves up to the crank throw. This would show on the rod bearings on the edges that meet the crank throw. Another possibility is the bearings themselves do not have the correct champher on that edge.

Roger
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