3.0SC connecting rod dimension
 

What is the 3.0 connecting rod big end ID?

I have a problem assembling some newly refreshed rods. They were shaved and reground. Spec is supposed to be 56.000 - 56.019 via Wayne's book table. The rods are very loose on the crank when assembled with the Glyco/Federal Mogul bearings. They were so loose that the bearing would just barely stay in the rod and certainly did not achieve any type of crush when torqued with old rod bolts to check clearance.

This is the first time I have encountered very loose rods.

These rods were tight on the crank when disassembled. The crank was polished and still measures in spec. I suspect either the rod (or the rod measurement) or a bad set of bearings. I have not had a chance to measure the bearing shells nor have I tried the old bearings as a backup.

Hi Jamie,

56mm is what I have.

Thanks Aaron.

I am measuring 55.9x with my calipers as the big end ID. I think I need to dig out the bore gauge and check for sure but this is pretty close. Something is not right. Crank specs out. Rods basically spec out. Bearings... Don't spec out. Maybe that is my problem.

Pop the old bearings in just for fun.

Craig. working to do that now.

I just checked all six of mine... They just got back from the machine shop with new ARP bolts and were re-bored...

1. 56.02
2. 56.00
3. 56.00
4. 56.01
5. 55.99
6. 55.99

Where do you take your measurements w the bore gauge? On the diagonal? I have a used set of 3.0 rods that seem to measure up within spec even w arp bolts installed. Contemplating running them without having the shop re-size them.

Let us know what you find re the bearings.. There is a 996 GT3 part number for the rod bearings which I am sure you are aware of. I spent the extra $ on them just for insurance given all the bad things I have read here about the manufacturing of the stock ones

Just wanted to update.

The rods were recently resized and on the low side of spec meaning they are tight. The crank was just polished and is in spec. The rod bearings were what we thought was the source of the error. The Glyco bearings were remeasured and spec out with 0.0015" clearance which is tight compared to the 0.02mm (0.0011" converted) minimum clearance.

So why were the rods feeling so loose on the crank? There was one rod in the group where the side width was just under spec. I measured 21.65mm rather than the 21.70mm spec. This provided enough side to side rock that it felt like the rod was very loose on the crank during a completely dry build for clearance measurements.

This rod will be assigned to the end of the crank where oil pressure should be the highest. The other rods measure in spec for rod width and are considerably tighter in "feel" once torqued down.

I am moving on with the build to assemble the crank with the new rod bolts and confirmed bearing clearance.

Measure. Measure. Measure just to be sure.

Would that lower side clearance rod be better in the center? Would that cause lower pressure down the line if at the end? Don't know just thinking out loud on a keyboard or doesn't that clearance have much to do with building pressure??

Craig,

My rationale is that the tighter rod clearance might be better in the middle. The oil system will have lower pressure in the middle of the crank. The tighter clearance would be better with this drop in pressure through the 2/5 rod bearings.

Weren't there two versions of 3.0SC rods and cranks? According to PET the crank part number was altered sometime in 1981. Wonder what was that for?