Main Bearings

MichaelB · Jun 19, 2024

Engine is in Bits, Crank is out for balance.
Main bearings are C3 spec and by all appearances, appear to be fine.
Crank play about 22 thou. Unknown mileage on bearings.
So, Do I replace just because, or reinstall?
I ask because years ago I did a remodel project at a shop that makes Gears.
From NASA spec to JIm's Harley's gears and so on.
Owner is litterly a real Gear head with Hot Rods boats etc. Knows his way around.
Shows me around his shop and stops at some bearings.
Starts talking about bearings and how everyone is in a hurry to replace bearings.
Explains how in many cases the used bearing is better because it's been polished in use.
New bearings have micrscopic burrs, that get polished out in use.
You also have a known performer with the race and cage, anything new can fail.

So what to do?
A real conundrum..

ashman · Jun 19, 2024

If you have the motor apart then best to replace with new bearings and as you say no idea of milage on old bearings, but it's your bike, main bearings will last a long time as well high milage but if all apart replace.

Stephen Hill · Jun 19, 2024

I don't think mileage is the critical factor in bearings. The issue is wear, tolerances, surface finish, etc.
I'd be interested in hearing how folks determine the state of wear in crank bearings.

concours · Jun 19, 2024

MichaelB said:
Engine is in Bits, Crank is out for balance.
Main bearings are C3 spec and by all appearances, appear to be fine.
Crank play about 22 thou. Unknown mileage on bearings.
So, Do I replace just because, or reinstall?
I ask because years ago I did a remodel project at a shop that makes Gears.
From NASA spec to JIm's Harley's gears and so on.
Owner is litterly a real Gear head with Hot Rods boats etc. Knows his way around.
Shows me around his shop and stops at some bearings.
Starts talking about bearings and how everyone is in a hurry to replace bearings.
Explains how in many cases the used bearing is better because it's been polished in use.
New bearings have micrscopic burrs, that get polished out in use.
You also have a known performer with the race and cage, anything new can fail.

So what to do?
A real conundrum..

Good points.

Replace them in this case.

marshg246 · Jun 20, 2024

MichaelB said:
Engine is in Bits, Crank is out for balance.
Main bearings are C3 spec and by all appearances, appear to be fine.
Crank play about 22 thou. Unknown mileage on bearings.
So, Do I replace just because, or reinstall?
I ask because years ago I did a remodel project at a shop that makes Gears.
From NASA spec to JIm's Harley's gears and so on.
Owner is litterly a real Gear head with Hot Rods boats etc. Knows his way around.
Shows me around his shop and stops at some bearings.
Starts talking about bearings and how everyone is in a hurry to replace bearings.
Explains how in many cases the used bearing is better because it's been polished in use.
New bearings have micrscopic burrs, that get polished out in use.
You also have a known performer with the race and cage, anything new can fail.

So what to do?
A real conundrum..

For me: Split Cases = New Main Bearings and new Big End shells unless I know they have very little use and it's clear that the engine has been maintain.

Also, not to re-ignite the recent war, IMHO, C2 is not needed (and nearly impossible to find). C2 of yesteryear falls within the tolerance of C3 today. Of course, assuming quality bearings.

maaseyracer · Jun 21, 2024

Replace them, you can always micro finish or polish the new bearing race.

elefantrider · Jun 22, 2024

Inspect for wear, corrosion, roughness and re-use! Especially if C3 as they were already replaced since the bike was built.
What manufacturer and country of origin are your C3s? IMO, R&R bearings cause more damage than use itself.

concours · Jun 22, 2024

Supprting evidence for my recommendation.

milfordite · Jun 22, 2024

Well sure, if the bearings are toast, renew them! They will last a long time under normal riding conditions. My are still going strong at 70K miles. As the owner, your call of course.

Main Bearings

MichaelB

"Sons of Arthritus"

ashman

Stephen Hill

concours

marshg246

maaseyracer

elefantrider

concours

milfordite

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