4g63 main bearing out of round

Hey everyone,

I’m working on a my 4g63 where I’ve encountered an issue with the roundness of the main bearing housing. Without the bearings installed, I measured the housing to be out of round by 0.01mm. However, when I installed the bearings, the roundness increased to 0.06mm.

I'm trying to determine if 0.01mm is an acceptable tolerance, or if it indicates a larger issue with my block.

Here's a link to me measuring:

https://youtube.com/shorts/kWcsjBfE9to?si=MShOgSAykBKFzV3r

Specifically, I’m wondering:

1. Why the huge change with bearings installed?

Even if 0.01mm is an unacceptable amount why do i get such a significant difference with them installed.

2. Could this be due to my tools?

I’ve been using a standard mitutoyo micrometer and a basic dial indicator, but im confirming my measurements with another manual gauge.

3.Could it be human error?

I’m trying to be as precise as possible, but maybe there's something I’m missing when measuring with bearings installed.

I’m looking for advice on whether I should trust these measurements, or if further machining is necessary. Any insights on what’s considered standard for main bearing roundness and whether this could lead to issues down the line would be greatly appreciated!

Thanks in advance

First thing you do with micrometer is checking it against a known size calibre and see if there is a difference between real size and the size shown.

Secondly, 4g63 bearings shape isn't round. It's bigger vertically and smaller horizontally- the difference I've seen can be as much as 0.07 mm.

That's really interesting. I dont think there was any mention of that in the course.

When you say vertical im assuming head to sump?

I'll reinstall the bearings in that case and check clearances again. I'll check vertical, and at a 45 degree angle off the centre?

Will I only be calculating my tolerance by (vertical size of bearing - journal size)?

Thanks again

Well, it depends on bearings brand- I tried all there are for 4g63 and it can be anywhere from 0.01 up to 0.07 .

Yes, vertically is from head to sump.

The most important size is from top to the bottom providing that side to side clearance isn't too small to make journal jamed. 45 degrees anlgle should be close to top-to-bottom size clearance.

Here is some visual information on how it's all positioned.

https://www.google.com/search?sca_esv=3c0fea230776b8f8&sxsrf=ADLYWIJ08-3A0JenMmEbMm1wfR3FJVuErQ:1735431905026&q=journal+bearing+failure+engine+bearings+geometry&uds=ADvngMjcH0KdF7qGWtwTBrP0nt7dNFQNRLL2sQMHPlv5DqjP-XwW4sZRJ_e8A_lwV_Gk95XfvEMXm5Vz5RZCSqeg_GNH63eCcch_RaVUm_mPevwc9t1Volt49Cv3vRu0gPXhdRB5-M8NU3AUKFqPuqqwKgTdlPuIaiaaDu1zpXy17knmeG83bGE&udm=2&sa=X&ved=2ahUKEwiZnezR28uKAxWCe_UHHRDOC4UQxKsJegQIChAB&ictx=0&biw=352&bih=692&dpr=3.45

Really good information, thanks.

I ended up measuring around 30 degrees from the center. My recordings are:

Bores 1,2,5 = L 56.77 , C56.76, R56.77

Bores 3,4 = L 56.76, C56.76, R 56.76

With my main journal sizes being:

1 = 56.745

2,3,4,5 = 56.74

Clearance = 1= 0.015mm and 2,3,4,5 =0.02mm

Recommended is 0.03mm - 0.05mm so I'm slightly tighter

For performance engine I would go with 0.05-0.06 mm for both- main and conrods... For a stock or boost up engine 0.03-0.05 mm...

I must be working something out incorrectly.

Also do you mean to be in the tenths of a mm not the hundredths of a mm?

The manual says crankshaft pin oil : 0.03mm to 0.05mm with a 0.1mm limit

And

A big end clearance of 0.1mm to 0.25mm with a limit of 0.4mm.

Does the limit mean ±0.1mm from the recommended clearance? So the maximum range for main bearings will be -0.13mm to +0.15mm?

I'll try again when I'm back from holiday in a week!

Sorry for frustrating you. I missed zeros in both - now corrected.

Not at all, you've actually been a huge help with my build. Thank you I'll check this out in a week and let you know what I find.