5.2 Build - main bearing size code identification

[engineermike]

I’m currently building a basically stock Predator short block. The ford procedure for selecting the main bearing thickness, or “grade”, is described here:

https://www.mustang6g.com/forums/threads/bbq-tick-test-request.117273/page-2#post-2467476

However, the code on the block is:

I have reason to believe the main bearing code is the first string of characters, but I also have reason to believe it’s the second string of characters. So which is it?

And before anyone suggests it….yes I’m dead-set on using oem bearings not aftermarket.

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[bauern]

2nd string.

 

[engineermike]

Thanks @bauern that’s my initial thought as well, But I’ll show you why I’m doubting it. This attached pic is my predator crankshaft. It just has the 6 letter, 1 number code like the first string on the block, not a 5 letter code.

 

[GregO]

But I’ll show you why I’m doubting it. This attached pic is my predator crankshaft. It just has the 6 letter, 1 number code like the first string on the block, not a 5 letter code.

If I was in your place I'd ignore the OEM pin stamped letters.
I'd map out each crank journal and main bearing bores 1 thru 5 then use the Ford bearing chart numeric diameter range assigned to the lettering system.
Painfully time consuming but this leaves no chance of misinterpreting an obscure factory lettering system.

My guess is the seven character nomenclature first 5 letters will probably fall inline with the Ford bearing chart size and what you end up mapping out with micrometers and bore gauges.

 

[engineermike]

@GregO , done.

 

[K4fxd]

I'd map out each crank journal and main bearing bores 1 thru 5 then use the Ford bearing chart numeric diameter range assigned to the lettering system.

I used to hate this crap from Ford when I was working on them for a living. When we called for parts we needed to tell them the model, engine, trans, color of car, month, day and time it was built. With all that info getting the correct part was still a coin flip.

Wondering if you should get the block decked and line bored.

 

[engineermike]

One issue I’m having is that the difference between G and K is .0002”…it’s pretty difficult to measure to this accuracy with an ordinary starrett mic, especially the main bores.

 

[K4fxd]

Measure enough to know which string to use then trust the stamped code for that journal.

 

[GregO]

One issue I’m having is that the difference between G and K is .0002”…it’s pretty difficult to measure to this accuracy with an ordinary starrett mic, especially the main bores.

Yeah, that’s optical comparator territory that far to the right of the decimal…. Yikes !

 

[engineermike]

Measure enough to know which string to use then trust the stamped code for that journal.

That’s the plan. And the absolute value isn’t as important as the relative value. E.g. if the difference between main 1 and 2 is about a mil, then it’s probably AUDDA.

 

[DougS550]

Yeah, that’s optical comparator territory that far to the right of the decimal…. Yikes !

I know when I was building motor cycle race engine, I would ID and OD with my Sunnen High Precision Inside/outside Bore gages, the Engine block crank main bearing caps with new bearings installed and caps torqued, the Crank main bearing Journals OD’s, the crank ROD Journals OD’s, the ROD Caps ID’s with new bearings and torqued, and go from there. I got every respective bearing clearance as close as possible from one another.

Plastigauge are OK for stock engines but, building a close tolerance performance engine required all clearances to be as exact as possible from one Crank Main bearing clearance to the next, from one ROD/Crank journal bearing clearance to the next. I even ID and OD my Cylinders and Pistons to select which piston would go into which cylinder for the best clearance/gap. Then I would start on the rings and do the same. If you take your time with your engine build/blue printing, it will last under heavy loads for quite some time. I would have a book with every single measurement including my cams to cam cap clearances for record. Man was that fun, Nada, but a necessity when building race/performance engines.

PS. This Sunnen Gage is like the one I just sold last year to a machinist for $200, Dam I got screwed. Good Luck

 

[engineermike]

Here are the conversions using both strings, just to show the accuracy level we're talking about:

 

[GregO]

Here are the conversions using both strings, just to show the accuracy level we're talking about:

It’s enough to cause anxiety when thinking back to the days of taking crocus cloth and a flat boot lace to the crank journals on a tractor rebuild !

 

[GregO]

I know when I was building motor cycle race engine, I would ID and OD with my Sunnen High Precision Inside/outside Bore gages, the Engine block crank main bearing caps with new bearings installed and caps torqued, the Crank main bearing Journals OD’s, the crank ROD Journals OD’s, the ROD Caps ID’s with new bearings and torqued, and go from there.

Sounds like you may have worked at Star Racing or Paul Gast at the ProStock bike level.

 

[engineermike]

It’s nerve wracking but I measured each main in 3 locations and averaged them:

1. 2.8512
2. 2.8515
3. 2.8516
4. 2.8515
5. 2.8510

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