Main Bearing Clearances

[tinkerputzer]

I'm in the process of slowly assembling my 3.1L stroker which will be using a green dot s52 crank and a red dot eta block. I checked the main bearing clearances using a .0001 bore gauge and here are the results.

1) .0024
2) .0024
3) .0024
4) .0025
5) .0025
6) .0027 (thrust bearing)
7) .0026

Bently specs are .0012-.0028. So as far as that goes they're with in limits. I'm wondering if I should be concerned about 6 and 7? For a DD/street engine that will see redline on a daily basis is this going to be a problem? Also, should I be concerned about the slight markings in the bearing surface coating that results from checking the clearances with a bore gauge.

[The Humjet]

The markings on the bearings left by the bore gauge isn't a problem. I too was concerned when I first checked oil clearances its what bore gauges do its the same thing when you check piston to wall clearances it leaves a light mark on a freshly honed block, its not going to affect anything
As for the bearing clearance you are within tolerance, my boss once said its fine to have a bit more clearance on the thrust bearing. You should be fine but I'm still not sure if running on the loose side is ideal.
I hope some of the other more experienced members like MR 325 could chime in, I'm still young and learning and I've never worked on an m20 before. So I could be a bit wrong.

[tinkerputzer]

I wonder if it is intentional for the thrust bearing to have a bit more clearance then the rest.

[jlevie]

The "book" procedure is to pick the bearing "color" from the table in the TIS based on the journal diameter. That can, even on a new crank, mean using more than one "color" bearing. I've never heard of anyone saying that the thrust journal is intentionally given more clearance. But that end of the crank can see more wear.

[The Humjet]

I'll have to ask my boss again about having more clearance on the thrust. I didn't ask why it's like that or if its deliberate by the bearing manufacturers. Maybe I didn't hear him right.

[tinkerputzer]

The "book" procedure is to pick the bearing "color" from the table in the TIS based on the journal diameter. That can, even on a new crank, mean using more than one "color" bearing.

I guess that is the only solution here if I wanted to tighten things up.

I've never heard of anyone saying that the thrust journal is intentionally given more clearance.

In your experience would you say these clearances are acceptable for DD/street driving?

But that end of the crank can see more wear.

That makes sense since it is farther away from the source of oil pressure. I don't think that is the issue here though. I'll post up the jounal diameters tonight. I'm also going to measure the bearing shell thickness to find out the difference between the thrust bearing and the rest.

[AndrewBird]

You might want to re-check them with plastigauge. This will give a more accurate measurement as it measures the clearance with everything assembled. There is also no room for interpretation of the measurement like there is with a bore gauge.

[jlevie]

Actually, a bore gage and micrometer (used correctly) is more accurate than plastigauge.

[AndrewBird]

Actually, a bore gage and micrometer (used correctly) is more accurate than plastigauge.

Here is the problem. When you start measuring things down the .0001 range, different people will use/read the gauge differently. Pressure applied to the tool has a BIG impact on the reading, which is why many micrometers have ratchets, so different people will tighten it down the same amount. I guarantee if someone else measured his clearances, they would get different numbers. Happens ALL the time at work. But this is why tolerance are always a range, not just a number.

Either way, it is always good to get a second opinion. And once again, plastigauge checks the clearances with everything assembled, which will also make a difference.

[jlevie]

I'll agree that plastigauge is a good sanity check, but I don't trust it for actual clearance. I do clearances "by the book" so the only measurement I need is journal diameter. With high quality micrometers and knowing what you are doing it isn't difficult to get repeatable numbers.

[tinkerputzer]

Measured everything again tonight and all the clearances were .0001-.0002 tighter across the board. I guess some slight variation is to be expected when dealing with .0001 increments. After all .0001" is 1/30th of a human hair. I suppose if you come within .0001-.0002 everytime you measure that is pretty darn accurate. With that in mind I'm going to take these clearances and run with them. Will definitely check everything with plastigage during the final assembly process just to be safe.

[TobyB]

Will definitely check everything with plastigage during the final assembly process just to be safe.

THIS is what plastigauge is good for. It's more a reality check than precision tool.

But it's caught mistakes for me before, so I use it.

t

[$hane]

When was the last time you had your bore gauge calibrated? We send all of our inspection equipment for calibration twice a year.

[tinkerputzer]

Valid question but i think they're pretty accurate. Say i set the mics to x dimension and then zero the bore gauge on x dimension. When i vary x dimension by .0001 on the mics it shows up on the bore gauge as exactly .0001.

[The Humjet]

Back to work today after long weekend. Spoke to my boss about running more clearance on the thrust, it's because more oil is needed to lubricate the bearing and the thrust part.