Grooving the main bearing for a drilled #4 crank main

[stownsen914]

About to assemble a 3.5L race engine 100x74.4 mm. The crank has a cross drilled #4 main to improve oiling for the center most rod bearing journals. The crank is not grooved at the cross drilling, so I had two main bearing shells grooved to use for the #4 mains. Pic below.

Now that I’m looking everything over, I’m wondering if the groove is adequate. The holes in the case that feed the main bearings are .150”. And the cross drilling is a little larger at .180”. The hole is the bearing is .110” and groove is about the same width, and about .020” deep.

Should the groove be wider or deeper? Clearly the groove will not accommodate the full supply of oil from the case galley, but it doesn’t need to, as much of the oil for that galley feed is used for its primary purpose of lubricating the #4 main bearing itself.

If anyone has dimensions to share for grooving the #4 main, it would be appreciated!

[TimT]

We have made a number of them but not in awhile..And I cant find any good pics, but the dimensions on your appear to be similar to the ones we make..



I've posted better pics previously... but cant locate them right now

[stownsen914]

Thanks. I found the other thread with your pics => https://forums.pelicanparts.com/911-engine-rebuilding-forum/153351-trick-extended-high-rmp-survival.html

Your groove does looks similar width but possibly deeper than mine. Possibly closer to a mm?

On a separate note, do you drill the oil hole in the bearing any larger? The hole in my case is .160" and the bearing oil hole is .110".

[MBruns]

We drill the feed hole in the case and bearings to 5mm and has worked well
Mike Bruns

['78 SC]

Originally Posted by MBruns

We drill the feed hole in the case and bearings to 5mm and has worked well
Mike Bruns

Mike,

Just to clarify, do you only drill the case and bearing or drill the case and bearing AND groove the bearing or journal? I ask because one crankshaft shop told me the groove is not essential and just drilling was sufficient.

TIA,
Steve

[MBruns]

You need to groove one or the other so you have oiling 360 degrees, I like to open the holes as well
Mike

[stownsen914]

Thank you Tim and Mike for chiming in on this thread.

Seems there are different approaches to having a groove (or not). When I tore down the motor I'm rebuilding (3.5L race engine), I found that it has a center drilled crank, but the crank isn't grooved nor were the bearings. Apparently some believe the straight shot of oil from the feed hole twice per revolution is sufficient. Some groove the crank. Some groove the bearings.

I'm still trying to figure out if the groove machined in my bearings is sufficient at .110" wide and .020" deep.

[gled49]

I drill out the center oil galley hole to about 5mm, you can see here, but I think your ok.

[JoeMag]

Just looked at one took out of my motor and it’s 0.120” wide 0.038” deep. Got it from Peter Dawes so I think “right”.

[stownsen914]

Thanks. That’s about the size I’m thinking I want, and visually the others’ pics also suggest a depth of ~1 mm. I’ll get mine cut a little more.

[nocarrier]

I do mine at about 1mm deep and a little er 3mm in width. I also drill the oil hole to match the larger hole drilled in the case.