Main Bearing sizes and plastigauge

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Main Bearing sizes and plastigauge

Postby TorqueHorse » Tue 03.05.2022, 04:49

So I've got some of the color coded main bearing shells for the crankshaft, but I'm having a hard time with the plastigauge. I'm trying to replace my #5 main bearing and I've put in a blue bearing, tested it; and then a black bearing and tested it, yet the clearance from the plastigauge looks the same for both. I was expecting the black one to have more clearance than the blue ones...
But regardless, this is too little clearance if I am reading this right. Should be .02mm min, but this looks like ~.015mm right?
Image
'90 FED M100 -mid resto
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Re: Main Bearing sizes and plastigauge

Postby Simon_P » Sun 08.05.2022, 23:01

The first thing to do is take measurements. Without knowing the measurements and markings the discussion has no sense.

Plastigauge comes in different sizes and the colour of the stick should be the same colour as the scale. Yours looks grey but the scale is green

Once you have the measurements and markings look them up in the table. Ignore the "worn" comment in the crank web mark column it is misleading - go by the measurements.

The chances are that if fully synthetic oil has been used there will be almost no wear. So it also helps to ignore the comment about crank marks only being relevant when new. It is a good cross-check if you can't measure accurately.

The plastigauge in the picture has been completely squashed assuming that you have the right range of plastigauge there is insufficient clearance.

Why did you choose blue to start?

To make any sense of this please post the plastigauge range, block marking, crank marks and journal diameter.
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Re: Main Bearing sizes and plastigauge

Postby TorqueHorse » Mon 09.05.2022, 16:08

Thanks Simon.
The plastigage used was MPG-1; "green" .025to .076 mm clearance. (color diff showing in photo is just the lighting)
The block mark was a "2"
The crank mark was a "- -"
The Journal measurement that I got was "51.89" (but I've since come to suspect my tools suck and are not too accurate)

So the reason I started with Blue was because looking at the chart on page 55 from section EC of the workshop manual, given that measurement of the journal and the cylinder block mark, it seemed my journal was "worn" and therefore Blue would be appropriate.
and in my brief youtube education of how to read plastigage, everything was focused on having too much of a gap, that I didn't pay attention to the minimum clearance as well when I first installed it. :oops: :(
So when I pulled the engine again, after running that Blue for just an hour, I could see evidence of wear already and I thought "ah, the journal wasn't really worn...I should just go off the markings in the table", which being "2" and "- -" point to Black as being appropriate. But alas, that doesn't seem to be the case either, given the image above.

So in the end I've just put my original bearing back in :roll:
'90 FED M100 -mid resto
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Re: Main Bearing sizes and plastigauge

Postby Simon_P » Tue 10.05.2022, 22:19

After the fact for you and it sounds like you have it fixed. But for anyone else that may read...

The crank mark was a "- -"
Each web is marked for the adjacent journal, not just the crank.
Plastigauge here is red for that grade hence the question.

2 and - - could be as low as. 0.22 and the plastigauge starts at 0.25 as you can see the scale gets wider as as the clearance reduces so technically it is off the scale. If plastigauge were digital it would say out of range.
TorqueHorse wrote:The Journal measurement that I got was "51.89" (but I've since come to suspect my tools suck and are not too accurate)
I had a similar issue hence earlier comments. I concluded that the issue was with the user though.

The different bearing grades have different part numbers so you can cross reference what you took out.
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