M20B27 Crank Shaft Bearings Selection

[Stapes]

I need some experienced advice on what approach should I take in selecting the bearings for my crank shaft.

Some background first.
Took out the engine (240K milies) and removed the crankshaft . It had what it looks like a green paint mark, but it could be blue or yellow but turned different color because of the oil. Anyway, I sent my short block and head to a local machine shop that does a lot of work for Pro 3 racers. The owner was great at explaining to me what he will be doing and gave me the crank measurements. 1.7706" for the rods and 2.3622" for the mains.

I could have him sell me the bearings but I'm new on all this and want to experience the build as much as I can, so I want to select the crank shaft bearings and finish the rest of the engine rebuild myself.

The first thing I was surprised was to hear that all main bearings measured the same and all rods measured the same. It makes sense considering the condition of the original bearings, very uniform and little wear given the miles on the engine. So I want to take the machine shop's word for this.

So what's the problem?
When I looked at the Bentley manual I can't find which bearings I should order as I feel like none of the measurements I was given fall within the options.

a. Should I question the measurements?
b. If not, what size of bearing I should get?

I'm definitely planning on plastigauge the crankshaft to ensure proper gaps. (.0001 per inch of journal). I'm not sure I should get a micro form HF as I doubt their manufacturing and proper calibration.

Thanks

[TobyB]

Umm, based on the Bentley, which has been wrong once or twice, your crank is not worn.

Looking at the bearings, it should be a little worn, or scored a bit.

Usual practice would be to polish it if it's within spec (the measurement for the mains looks big, the rods look a bit small, but they're all within the realm of believability)
throw the standard bearings in there, and be done with it. You could remeasure the mains
after polishing, since there are 'classes' available, but your main measurement is coming
out high to start with...

This is why I own micrometers, inside and out, and have slowly learned how to use them.

I think you can order the standard bearing and be done.

t

[Madhatter]

It's easy to linish the bearing surfaces to match oversized shells. You don't select bearing shells before machining, it's done afterwards. What dictates how much machining is required is the condition (out of round) and diameter of the journal. It might just need a linish, it might need grinding. Either way you complete this process to return all surfaces to serviceable condition THEN you select your corresponding shells.

Have the shop measure all the bearing surfaces to find the smallest journal, then linish this down to a new clean surface, then check the diameter to see if it corresponds with shells on offer.

Decide if you are now undersized on the journal and need a oversized shell, or measurement corresponds with allowance for standard shells. Machine as needed to reach required spec of standard or oversized shells.

With shell sizes selected, have them machine the remaining journals to match.

Fit bearing shells, crank and caps, then plastigauge to double check the clearance.

All checks are done with micrometer or caliper measurements. If you aren't used to using calipers and transferring the measurement to verniers, get a GOOD micrometer. It's virtually impossible to place too much tension on the dial as it's a ratchet.

You only use the plastigauge to double check your measurements are right. Don't rely on it by itself.

Not much you can do to be involved in the machining process, other than taking measurements of the journals before/after polishing/machining. Speak to the shop if you want to see this step, otherwise the above is how you do it properly. The building is still all on you.

[Stapes]

Follow-up

Thank you for the great feedback. So I got everything back from the machine shop and decided to buy a set of Mitutoyo Micros and a Fawler bore gauge to measure the engine. Although I still have to confirm the piston to cylinder clearance, so far everything but the oil clearance for the main journals are within spec. here's my issue with the main journals and I'm hoping to get some feedback.

Please bear with me as I was completely new to this, a "bit" of OCD, and my approach to this engine re-build has been the result of the typical "while I'm there" syndrome. So lots of improvisation and no preparation/planning.

Measuring oil clearance on my crank shaft (blue) main journals using two methods returned inconsistent measurements.

My main journals measure 2.3611". Using off the shelf standard KolbenSchmidt shells (PN 87 491 601) I got oil clearance in the range of 0.0027 - 0.0032" with the bore gauge, while 0.002" using plastigauge.

I've been thinking that perhaps this over the 0.0028" oil clearance is because I need to use one of those colored bearings instead. But since all of them std are for 60mm I don't know if I should go this route.

Also, on one of the crankshaft weights there is a KS2 mark with 5 dots. I wonder if that has any reference to the double/triple classification that Bentley talks about.

here's a picture of the dots on the shaft.


Thanks for your feedback

[TobyB]

linish

US= polish.

Ah, hey, I can see your pic here- those are casting marks, done before machining,
and I really doubt they relate to bearings. The dots are odd, aren't they?
Often, things like that indicate dates, or specific molds, or phase of the moon...

If you can find a .001" undersize bearing, that'd be my 2c. But at .003, I think you'll be
ok on the mains.

fwiw.

t

[KillaCam]

bump this, Im in the same situation, idk what i should do with the main bearings

[TobyB]

Use some. They give you a nice, smooth feeling.

And if you're at .003", like Stapes, see if you can find the 'service undersize'
that's just a bit (.001" or so) undersize. They're designed for just this scenario.

Otherwise, LS1.

t