Poll: Main bearings...keep or toss? (pictures)

[HondaDustR]

Just pulled the mains out of my motor for rebuild and can't decide whether I should just break down and replace them. I haven't plastigaged them yet. I can't really feel most of the scratches, but a few are bad enough to catch a fingernail and there's some discoloration that might indicate it's getting close to the copper. The crank looks good and only has a couple very minor scratches, but I will see if the machine shop thinks it needs to be polished.

#1 front of motor upper



#1 lower



#2 upper



#2 lower



#3 upper



#3 lower



#4 upper



#4 lower



#5 upper (rear)



#5 lower

[924RACR]

First off - good pics!!

I'd hang on to 'em, but put in fresh new parts. Since you can get 'em, and you're in there - no real excuse.

You can't buy 924 main bearings anymore, but apparently it is possible to have good used ones re-babbited, or whatever they call it - lay fresh babbit metal (the soft grey bearing material) on the backing and then reuse. If only I hadn't thrown away all those used bearings...

They don't really look bad at all, except for the #5 lower seems like it's hitting copper or further. #1 lower is getting there, too. Thrust bearing surface would be another interesting one to check, but since you should replace, not so critical to check.

HTH...

[bazar01]

I would replace those bearings.
Also have the crank polished since there are a lot of scratches on the bearing surfaces, the crank journals will exhibit the same.

When I did my rod bearings, I checked the mains and plastigaged them and checked for axial play on the thrust brg surface. All good and smooth.

[Slam]

They'd scare me enough. I'd replace them. 944 bearings are available and relatively cheap, and, after all, you're in there. Can you live with wondering how the old ones are doing every time you turn the key?

[porsche4life]

You've got em out I wouldn't fuch with fate by putting those back in.

[running_cold924]

that last one scares me.....if your building an engine (TO KEEP) i'd do it right. some stuff you just can't skimp on...

[HondaDustR]

Originally Posted by running_cold924

that last one scares me.....if your building an engine (TO KEEP) i'd do it right. some stuff you just can't skimp on...

Yeah, it had me concerned, too. That weird spot is actually slightly rough feeling.

I think I'll just replace the bearings. everything else on the motor will be refreshed. It would be a shame to incomplete the complete rebuild and risk marginal mains. I want at least another 100,000+ miles out of this motor and don't want to be cut short because of something stupid like this that needs every single piece of the motor to be disassembled in order to replace them. The crank is in good shape, but there are a few small scratches near the oil feed holes, so I'm going to talk to my machine shop about polishing them. The thrust clearance came up on the feeler gage at only 0.15mm, so I was tempted to just leave them undisturbed, but I'm glad I needed to pull it all apart anyway to wash the cylinders.

I want to know how grit big enough to do this stuff gets there to begin with. The oil feeding these is straight out of the oil filter.

[Zero10]

Rear one is toast, for the most part the main bearings show little to no wear on those surfaces, its the thrust surface of the one main bearing (#3 in your pictures) that you need to worry about. That tends to be the only surface that is really worn out. In your case the rear one is done for, but the thrust surface is not visible in the pictures.

[HondaDustR]

Well, the thrust bearings were showing some copper, but the clearance was well within spec on the fealer gage. Doesn't mater, I've got new ones coming. I wonder if the rear one looks bad because the clutch or flywheel was out of balance?