LEGENDS > Engine Care & Tuning

Rod Bearing Selection

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Tom Cole:
The rods in my engine no longer have the code 3 or 4 on them.  I have some new, "prepped rods" that I want to use, but they do not have codes on them either.  The codes on the crank for the rod journals are all 2's

The rod bores for the rods I took out of the engine each have a bore size of 1.615" (without bearings) and they have the "black" bearings which makes them #4 rods.   However, they are not round and are 1.616" at their largest point.

The bores of the new rods all measure 1.6145" and are round.

I'm thinking that the used rods could very well have opened up a half thou, but since the bearings are not cheap, I don't want to order the wrong bearings.  And spec clearances only give me a half thousandth margin.

So here is my question.  Does anyone know what the bore diameter of a #3 rod and a #4 rod should be? 

Tom Cole:
I can't believe I've stumped y'all!   ;D

I'm going to order a few different sized sets and measure them.  Maybe I can come up with some useful info for some of you.

gramps05:
Went through all my books and information I have and no where is that info to be found. I myself will be waiting to see what you come up with so I can add it to my notes.

Tom Cole:
The only measurement I know right now is oil clearance listed in my Hayes manual .  And that even has a typo...it says .017 to .040 mm (.0017" to .0016")  .017mm is .0007", not .0017"

So connecting rod oil clearance should be .0007" to .0016"  That is seriously tight clearance.  But they recommend checking the measurement with plastigauge, which is crude.  I'd rather measure with a mic and a dial-bore gauge.

I am getting the codes to a brand new crank and its' corresponding measurements.  That will help with deciphering that side of the code.

I have learned the difference between 1200 and 1250 rods.  The 1250's have more meat around the wrist pin.  Rules say that you can't use a 1250 rod in a 1200 engine... ???

Tom Cole:
From a new crank, the main journals measure 1.417 across all 5, and the rod journals measure 1.496 across all 4.

The code on the crank is  2 1 2 1 2 1 1 1 1

The first 5 digits left to right represent the main journals 1-5 and the last four digits represent the rod journals 1-4

There is obviously a problem with the man's measurements (probably not precise enough) since the first 5 code numbers are not all the same while the measurements he took across all five journals is 1.417.

Still, it is more information...I will have mine ready to measure tomorrow and I'll post my findings.

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