Engine id pad and undersizing question

[Fredrik Günther]

Hello so to my understanding the letter "b" it means its .10 undersized and is that a good or bad thing, im not an engine builder so im wondering why undersizing instead of oversizing what are the pros and cons of undersizing?
Its an engine from my 69 roadrunner i bough dont know much but it runs darn good

Regards fredrik

 

[69Bee]

Welcome from AZ...

 

[69 Sleeper Bee]

I think it is 0.010 and just means when the engine was assembled they had to go with undersized main and rod bearings to maintain proper clearances. I will let others expand on the pros and cons of it being that way from the factory.

 

[33 IMP]

If that is correct.....it's sure nothing to worry about, for a second! There are countless millions of cars running around, and well, with ground undersized crankshafts.
If it means anything, it means they checked clearances on your engine when it was assembled, and they made sure the clearances were right.

 

[RJRENTON]

If that is correct.....it's sure nothing to worry about, for a second! There are countless millions of cars running around, and well, with ground undersized crankshafts.
If it means anything, it means they checked clearances on your engine when it was assembled, and they made sure the clearances were right.

IMO...
During the manufacturing of the crankshaft, perhaps due to an error in grinding the journals diameter, the journal(s), either main and/or rod bearing journals, rather than scrapping the assembly, the crankshaft's journal(s) was ground undersize (smaller), requiring larger bearing inserts, by the amount the journal was ground undersize to maintain the correct bearing clearances. Which journal was undersize was identified by marking the counterweight with a mark signifying which journal is undersize. This is explained in the FSM as to the interpretation of the stamping marks.
BOB RENTON

 

[WileERobby]

View attachment 1161170

... but it runs darn good

Regards fredrik

That's all you need to know... but seriously, it's basically been answered. They adjust bearing clearances as necessary during initial build. Renton's reply was good, as always.

 

[66Satellite47]

FYI, all my drag race motors have had the cranks ground undersize. Just have to use the correct size bearings. Not sure what that symbol means though. The factory service manual tells what those mean.

 

[1968rt]

I cant find anything that says "B"

Year Codes for B & RB Engines

• A - 1965
• B - 1966
• C - 1967
• D - 1968
• E - 1969

• F - 1970
• G - 1971
• H - 1972
• J - 1973
• P - 1960

• R - 1961
• S - 1962
• T - 1963
• V - 1964

• 4T - 1974
• 5T - 1975
• 6T - 1976
• 7T - 1977
• 8T - 1978

Other Stampings
Additional stampings may be found on the ID pad.
They identify non standard engine components.

A - .020 oversize cylinder bores
Diamond - oversize tappets
E - Cast Crankshaft
HP - High Performance
L or LC - low compression
M - .001 undersize main bearing journals
Maltese Cross - .001" undersized bearing shells
Maltese Cross and an X - .010" undersized crankshaft
O.S. - .005 oversize valve stems
P - premium fuel
R - regular fuel
RX or MX - all main or rod journals .010" undersized
S - special engine
WT or TW - water test
X - .010" undersize crankshaft

 

[69 Sleeper Bee]

Here is what my book shows.

 

[1968rt]

Nice book, which one is it?

 

[69 Sleeper Bee]

Nice book, which one is it?

Here is the book I have. I did find out the reference drawing of the left & right rockers is backwards so as with anything do diligent research.

 

[Fredrik Günther]

Thanks everyone so nothing to worry about then and nothing to be sad about hehe.