When to Leave Well Enough Alone?

concours said:
That, is a wildly inaccurate statement.
Click to expand...
"Plastigauge will get you in the ballpark but isn’t as precise as using “proper” measuring tools. On paper, a .0002 to .0004-inch difference doesn’t sound like much, but that is a 9- to 18-percent difference in accuracy"

www.enginelabs.com

Video: David And Goliath — Plastigauge Vs. Dial Bore Gauge Tested

In one corner, we have precision-crafted measuring tools that carry a hefty pricetag. In the other corner, we have an incredibly simple, inexpensive option.
www.enginelabs.com www.enginelabs.com
 
TomU said:
"Plastigauge will get you in the ballpark but isn’t as precise as using “proper” measuring tools. On paper, a .0002 to .0004-inch difference doesn’t sound like much, but that is a 9- to 18-percent difference in accuracy"

www.enginelabs.com

Video: David And Goliath — Plastigauge Vs. Dial Bore Gauge Tested

In one corner, we have precision-crafted measuring tools that carry a hefty pricetag. In the other corner, we have an incredibly simple, inexpensive option.
www.enginelabs.com www.enginelabs.com
Click to expand...
I’ve worked in engine plants of several automotive OEMs.

I’ve seen some fabulous top end measuring equipment… I don’t recall ever seeing plastigauge being used…?
 
Fast Eddie said:
I’ve worked in engine plants of several automotive OEMs.

I’ve seen some fabulous top end measuring equipment… I don’t recall ever seeing plastigauge being used…?
Click to expand...
I've used Starretf, Fowler, Mitutoyo, Browne & Sharp, Sunnen measuring tools, but I've never heard of "Fabulous".🤡🍻
Do you mean CMM?
Laser?

Renishaw: Advanced laser tool setters for CNC machines

A range of non-contact laser tool setters and broken tool detection systems for CNC machining centres.
www.renishaw.com www.renishaw.com
Are you talking about big, expensive machines designed for high speed mass production?🤩
Please be more specific.


The point is, Plastigage isn't a stand alone, end-all, be-all replacement for a micrometer, a bore gage. But rather, a very accurate method of CONFIRMING ASSEMBLED CLEARANCE.
Repeatability.
And it works great.
It's simple.
It's inexpensive.
It's available everywhere.
It REPEATS.




I have the luxury of having access to just about any measuring tool there is, and still employ plasti-gage to ensure it all goes together as expected, AND to confirm repeatability of my work/measuring.
 
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concours said:
I've used Starretf, Fowler, Mitutoyo, Browne & Sharp, Sunnen measuring tools, but I've never heard of "Fabulous".🤡🍻
Do you mean CMM?
Laser?

Renishaw: Advanced laser tool setters for CNC machines

A range of non-contact laser tool setters and broken tool detection systems for CNC machining centres.
www.renishaw.com www.renishaw.com
Are you talking about big, expensive machines designed for high speed mass production?🤩
Please be more specific.


The point is, Plastigage isn't a stand alone, end-all, be-all replacement for a micrometer, a bore gage. But rather, a very accurate method of CONFIRMING ASSEMBLED CLEARANCE.
Repeatability.
And it works great.
It's simple.
It's inexpensive.
It's available everywhere.
It REPEATS.




I have the luxury of having access to just about any measuring tool there is, and still employ plasti-gage to ensure it all goes together as expected, AND to confirm repeatability of my work/measuring.
Click to expand...
Both high speed / mass production, usually termed ‘in line’ inspection. And audit / sample inspection, where parts are taken off of the assembly line so the time constraint aspect is removed. Digital hand held mic’s and verniers through to large CMMs.

I‘m not saying you, or anyone else shouldn’t use plastigauge, it’s just my reflection of my observations based on Tomu‘s point.
 
concours said:
The point is, Plastigage isn't a stand alone, end-all, be-all ............. but rather, a very accurate method of CONFIRMING ASSEMBLED CLEARANCE.
Click to expand...
Repeatability.

And it works great.
It's simple.
It's inexpensive.
It's available everywhere.
It REPEATS.


From experience I'd agree wholeheartedly but I also have to smile .....

When I grew up on the tools ( trade ) quite some decades back the greatest, simplest, most inexpensive, repeatable and available everywhere ASSEMBLY CLEARANCE CONFIRMATORY piece of equipment was the humble fag paper or cigarette paper to our US friends.
Assuming shell nip etc was correct and if the journal rotated freely we would then insert a fag paper and repeat - if rotation was impeded or stopped then the clearance was indeed correct.


How things have moved on ...... I've used most all of the modern techniques in industry and still find virtue in the old ways. ;)
 
You can only be as accurate as the tool allows you to be .............. AND the tool can only be as accurate as the person using it .

How many times have you witnessed someone using a torque wrench either bring the torque up rapidly and then after the " click " give it an extra tug for good measure .

Years ago , manufactures added a ratchet to the spindle of a micrometer to help ensure that the pressure applied to the spindle gave consistent and accurate readings .. WHY ?? Because the person using it did not have either the knowledge or experience to use the old style making it no better then plastigage.
 
What your THING is , is OVALITY .

If theyre ROUND , your starting to be O.K. .
A nasty bloke centuries ago would face / lap the rod - big end cap . To close the gap.
NASTY .

IF you journals are elliptical , its time to panic .

FORD shells come in a .001 underize .
Like 1.5 thou oversize side valve race pistons - the fit clead up honed new stress relied ( by use ) bores .

So there could ?? be .001 shells somewhere , maybe .

The OVALITY is the determining factor , regarding a regrind , if the journals are undamadged ,
like copper lead shells would do to it , More heat resistand but more abrasive to the crank .

In case anyones forgotten . :p
 
Well have a read


if you can't read all 8 pages then page 6

The wall is specifically machined thinner at points 3 to 6, how much thinner depends on
the application. This feature is called eccentricity owing to the fact that bearing shells
are bored eccentrically and it influences the oil film thickness and oil flow. The bore of a
bearing shell has a further machined operation called bore relief which is usually
visible close to the joint faces. This is additional clearance required to cope with
potential poor alignment of the housing cap known as joint face stagger. Without relief
the bearing shell could end up acting as an oil scraper. In many bearings, the amount of
relief is excessive creating a larger oil leakage path.
The overstand of the bearing is the dimension which defines its peripheral length which
in turn controls the interference fit of the bearing. Along with the steel backing the
overstand specification is crucial in minimising fretting damage and maximising the
efficiency of the heat transfer into the housing.

So shell bearings are measured at the crown/centre for thickness, this is the dimension that is now graded to 3 or 5 micron bands and colour coded, once off the crown the thickness starts to reduce until it gets to the ends. This was change in thickness was measured at the start of the batch in a quality control room before the green light is given for production to start. Then the operator bores to the crown thickness alone as the shop measuring equipment is not capable of measuring eccentricity. 9 times out of 10 any issue customers raised with the thickness was actually due to the crank machining when I was involved in making Shell bearings.
 
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I would not like to try to get big end clearance right by using internal and external micrometers. I usually just buy a new set of shells and use plastigauge. It I cannot achieve the right clearances, I have the crank reground., and buy another set of shells. Some guys do silly stuff such as removing metal off the caps on a sheet of emery paper on a pane of glass.
The crank-grinding shop which used to do my work, was run by two drunky guys. Whenever I went there, I had to find them in the pub next door. But when they ground a crank, if you told them a size - when you measured the crank afterwards, it would always be dead on size.
 
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When to Leave Well Enough Alone?
 
Dan1950 said:
I spent the afternoon polishing the rod journals of my crank. Worked my way through 400, 600, 800 and 1000 grit wet of dry paper. The journals look great. After the polishing they mic'ed out at 1.7495" on the drive side, 1.7496" on the timing side. .0005" and .0004" undersize respectively. The micrometer was calibrated just before the measurements were taken. I came up with exactly the same dimensions that a local machinist found when I first tore down the crank.

My concern is that if I turn someone loose on my crankshaft to grind for .010" undersize shells, I might end up with worse than I have now. I feel like a new set of inserts in the rods will result in a long service life.
Click to expand...
To snug up the bearing shell clearance simply smear a thin film of red loctite on the back sides of the shells. The loctite will harden and take up the loose clearance. I tried it on a race bike once. It worked and did not change. Then you scrape off the dried loctite next rebuild.
 

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