machining rod journals

[CleverHans]

Is there any risk to machining the rod journals on a crank (forged steel) to suit 2" (50.8mm) small journal chev rods. The standard rods have BE which are 51.9938mm.

I'd be looking at nitriding or cryo treating the crank. COmments on the latter is welcome.

 

[Tmoose]

So you would effectively be machining the cranks's con rod throws 1.2 mm (~0.05 inch) undersize?

Might not be a big deal if :
- proper radius is maintained
- Crank is up to the task at standard size
- crank is magnafluxed to assure it is free of surface cracks
- Harmonic damper is installed if necessary
- nitriding is a nice addition for scuff resistance and increased fatigue strength

 

[patprimmer]

You will reduce the journal overlap and therefore weaken the crank, but it all depends on how much overlap you have to start with.

Increasing radii at the journal to crank arm interface and nitriding will both help considerably to improve durability.

You could also off centre grind it to increase the stroke at the same time.

Grinding may, with some luck, remove any small cracks that may have been the start of a failure.

Cryo treatment is gaining popularity, but I am yet to see compelling scientific data to support it's claimed virtues.



Regards

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[rpmag]

Such a conversion (chev rods) is popular with the venerable Fiat TC engine, as is destroking by offset grinding the crank. Strokes have been reduced by up to 3mm , however you must crack test prior to spending the money, but then it is only one part of the crank prep for a race engine.

 

[larrycoyle]

Actually this is something we have been doing on a regular basis since back in the 1970's. Just make sure whoever grinds your crank maintains a proper radius.

Larry

Larry Coyle
Cylinder Head Engineering, LLC

http://www.cylinderheadengineering.com