Regrind or not?

[limeyrob]

I've got a big bearing crank that I'm going to fit.  It was in the box of parts so I'm not sure of its history.  It looks like its - 010 on the TS Bush and big ends plus some wear.
Question is, regrind or just fit new bush and bearings.  Big end surface is smooth, TS bush looks like it may have picked up but its polished out.

Big ends TS (vert / horiz)  1.674 / 1.675
             DS                      1.673 / 1.673

TS bush   1.361 / 1.360

Thoughts?

PS I'd tell you about getting the sludge trap out (solid) but this is a family forum

[limeyrob]

OK a related question:
Its working on 1.25 thou / inch journal dia appropriate for this engine on 20-w50?
My calcs suggest best journal 3 thou running clearance with new shells, worst 5 thou. 
For TS bush its 5 1/2 thou

If 1.25 thou is appropriate then the clearance should be 2 thou for journals and 1.8 for TS bush.

That would make my clearances worst case 3 thou over on journals and 3 1/2 thou over on the bush.

But what I can't find is the usable wear limits.  I'm trying to get a feel for how this engine will run with these clearances and how the oil pressure will hold up.  I'm using to finding wear limits from cranks but have failed for this one.  I've used 1.25 thou / inch dia but I could have used 1 thou, it seems a bit arbitrary.  Thoughts?

I'm off to a local engineers that grind cranks, their prices are sensible and I need new shells and bush regardless so this is not about cost.  My concern is the point at which it is necessary (rather than nice to have) to take a crank down from -010 to -020.

[KiwiGF]

OK a related question:
Its working on 1.25 thou / inch journal dia appropriate for this engine on 20-w50?
My calcs suggest best journal 3 thou running clearance with new shells, worst 5 thou. 
For TS bush its 5 1/2 thou

If 1.25 thou is appropriate then the clearance should be 2 thou for journals and 1.8 for TS bush.

That would make my clearances worst case 3 thou over on journals and 3 1/2 thou over on the bush.

But what I can't find is the usable wear limits.  I'm trying to get a feel for how this engine will run with these clearances and how the oil pressure will hold up.  I'm using to finding wear limits from cranks but have failed for this one.  I've used 1.25 thou / inch dia but I could have used 1 thou, it seems a bit arbitrary.  Thoughts?

I'm off to a local engineers that grind cranks, their prices are sensible and I need new shells and bush regardless so this is not about cost.  My concern is the point at which it is necessary (rather than nice to have) to take a crank down from -010 to -020.

From memory the big end clearance on my reground crank was set at between 0015, as the cranks wear the journals go oval and an often quoted limit is 002 for ovality, rather than clearance, so you could work out that a clearance of 0035 is the limit, I’m sure there are engines still going strong with more 😏

[muskrat]

G'day Rob.
A freshly ground crank should have 0.0015" clearance with new shells/bush. 0.001" per inch of journal is standard practice.
After 40 years my A7 had 0.008" at the bush and 0.003" at the bigends. For the last 5 or so years I used 40-70 oil and was still running well.
Cheers

[limeyrob]

Very helpful, the ovality is around a thou on both journals and bush so its still a possible runner as is.  The decider could be the surface on the TS main as it needs a polish, I'll see how it comes up.

A curve ball is that at some time in the past the drive main has spun and worn 10 thou into the face.  Bearing fit is good, would need loctite but no play.  Old bearing (just) needed an extractor so this looks like a historic problem.  My plan is to put in another 10 thou shim then set the end float, and really wind up the drive nut.

Aside - it looks like someone took one sludge trap plug out, cleaned it and never realized there was a tube to remove as well as this was solid. So the tube was clean but under it was a solid mass of sludge, hard as rock.  Had to chisel the tube out.