Rod Bearings - Opinions Needed

['76 911S 3.0]

I recently reassembled the connecting rods on my crankshaft and have some interesting characteristics of the rods while on the crank.

Engine is a '79 3.0L, crank was micropolished, magafluxed, and measured all STD on all journal surfaces as well as mains.

Rods measured in spec (resized ~3k miles ago with ARP bolts). Clevite STD coated rod bearings, ARP bolts stretched to spec. No play in the fore/aft direction, no play in the side to side direction. However, the rod seems to rock/pivot when applying a force on the small end. I have read about the side clearance seeming to cause this phenomenon and that it can just be that. In the video you can clearly see the rod pivot on the journal at about the 12 second mark.

The other issue is the bearing alignment in the rod itself. The bearings do not line up with one another across the shells at the parting lines. I have attached a photo (not mine) but same principle.




Any thoughts/suggestions are appreciated.

Link to video:

https://youtu.be/6uu5wA4ucHA

[stownsen914]

I'll let others comment on the rocking/pivoting, but re: the bearing misalignment, there was another thread on that exact topic a few months ago. I think he may have found that even some of the more highly regarded bearings (may have been GT3 or Clevite?) showed the same result, but do a search to confirm.

[RedCoupe]

If This were my engine, I would check the bearing clearances with an inside mic or plastigage (or both). In the picture, there is a shiny spot on the parting line of the big end. If this is a little burr, this could cause increased clearance. When I have experienced clearances greater than expected, I have polished the mating surfaces with very fine wet-or-dry paper using oil on a surface plate or thick glass. This both cleans the surface of any imbedded debris as well as identifies any high spots such as burrs on the edges.

[KTL]

jonny042 went into a lot of detail about this bearing misalignment

Project Heavy Metal - an all steel classic, remastered

(you want to look at post #504 in the above thread if you're using the mobile web page version and it takes you to the start of the super long build thread)

jonny042 also chimed in on this thread where I screwed up like a boob, misread my dial calipers

Rod Bearing alignment of edges off

['76 911S 3.0]

Originally Posted by KTL

jonny042 went into a lot of detail about this bearing misalignment

Project Heavy Metal - an all steel classic, remastered

(you want to look at post #504 in the above thread if you're using the mobile web page version and it takes you to the start of the super long build thread)

jonny042 also chimed in on this thread where I screwed up like a boob, misread my dial calipers

Rod Bearing alignment of edges off

I see in Jonny's thread on page 33 he filled the tangs down 0.75mm to get them aligned correctly in the bore. I may follow suit on this.

[Eagledriver]

I have noticed this type of motion before on my engines. Bearings have a couple thousandths clearance. This results in more angular motion than you would expect. Might check one with plastiguage just for piece of mind.

-Andy

[Walt Fricke]

2 thou total clearance, 1 thou each side of the rod journal, should allow rocking of maybe 2 thou (one thou up, one down)? How much movement from one side to the other would that be 74 or so mm out?

[Dpmulvan]

I would send them back it’s bad enough we get hosed on main bearings and rod bearings paying 10 times more than most other vehicles, they should be perfect. Wouldn’t hurt to own a dial bore gauge and some decent micrometers.
Spinning a main or rod bearing will likely destroy your case, maybe bite the bullet and spend the extra for Oem.

[Eagledriver]

Originally Posted by Walt Fricke

2 thou total clearance, 1 thou each side of the rod journal, should allow rocking of maybe 2 thou (one thou up, one down)? How much movement from one side to the other would that be 74 or so mm out?

Geometry does funny things. Imagine the first tiny angular movement, the edges of the bearing move up and down respectively but they don’t move inward toward the journal by a significant amount. In effect they start out moving vertically almost exclusively. Once the angle starts to increase the edges of the bearing begin to move toward the journal at an increasing rate but still much less than they are moving vertically.

If I weren’t so lazy I would calculate the motion. It’s fairly simple just drawing out the triangles and using sin or cosine. You know what a sine wave looks like. What I’m describing is the area near the top or bottom of the sine wave where the graph moves horizontally alot and vertically very little.

-Andy