Crankshaft rod journal lightening

[palmettoz]

In my 3.1L motor that I am building with a V07 crank, I had it lightened by turning down the counterweights (knife-edged) and would like to take it one step further with milling a hole in each of the rod journals.

Just curious if anyone here has done this and if so what diameter hole they milled (or drilled). It appears to be possible to do but would require repluging the oil galley hole inside the newly drilled lightening hole.

Seems like this could remove quite a bit of material off of the crank.

__________________

Scott

www.cardomain.com/ride/3154164

3.1L 280Z in progress.

[Zforce]

Bad idea,might set up a stress riser and lead to a crack.

[Phred]

I used to cross drill L6 cranks, both vertically, and diagonally. They were very hard cranks. Carbide tooling was need to do the job. In addition, we re-heat treated ( Ion Nitriding ) the cranks after machining, or grinding journals undersize. Part of the heat treating process is normalizing, where the crank is heated, and "annealed". So, to make machining easier, I did a two part heat treat, I picked up the crank after it was normalized, ( made softer ) did all my machining, then had the final heat treat done. The heat treat will re-align the metalurgical grain, and remove any stress risers created by any machining process. For reference, if you can find a Lycoming, or Continental aircraft crankshaft to look at, ( 4 or 6 cyl. ) you will find the same rod journal lightening as you have described. The aircraft cranks are very strong, and reasonably light. They are also nitrided. Most Ford Cosworth racing cranks are lightend through the rod journals as well. But, it's a major machining excercise, and costly. So, consider the use and design of a stroker crank. The idea is to create more torque with increased stroke, and therefor you should be able to gear the car to make power without using higher rpm. The the loss of rotational mass so close to the shaft centerline, will probably not be of much benefit. You could probably make the same rotational improvement by simply using a very light flywheel. I don't want to discourage you, but I have "been there, done that". If you have deep pockets, and it's an all out race car, go for it. But don't expect to see any improvements if it's a street car. Good luck.

Phred

[Zforce]

Phred,you the man.

[palmettoz]

I used to cross drill L6 cranks, both vertically, and diagonally. They were very hard cranks. Carbide tooling was need to do the job. In addition, we re-heat treated ( Ion Nitriding ) the cranks after machining, or grinding journals undersize. Part of the heat treating process is normalizing, where the crank is heated, and "annealed". So, to make machining easier, I did a two part heat treat, I picked up the crank after it was normalized, ( made softer ) did all my machining, then had the final heat treat done. The heat treat will re-align the metalurgical grain, and remove any stress risers created by any machining process. For reference, if you can find a Lycoming, or Continental aircraft crankshaft to look at, ( 4 or 6 cyl. ) you will find the same rod journal lightening as you have described. The aircraft cranks are very strong, and reasonably light. They are also nitrided. Most Ford Cosworth racing cranks are lightend through the rod journals as well. But, it's a major machining excercise, and costly. So, consider the use and design of a stroker crank. The idea is to create more torque with increased stroke, and therefor you should be able to gear the car to make power without using higher rpm. The the loss of rotational mass so close to the shaft centerline, will probably not be of much benefit. You could probably make the same rotational improvement by simply using a very light flywheel. I don't want to discourage you, but I have "been there, done that". If you have deep pockets, and it's an all out race car, go for it. But don't expect to see any improvements if it's a street car. Good luck.

Phred

Thanks Phred-

Great input, I was planning on doing this myself on a mill, but I think it is overkill at this point. I don't plan on taking the motor over 7500 rpm and already have an aluminum Centerforce flywheel as well. Just looking to reduce the rotational mass as much as possible. My cam will be a 306 duration with .530" lift, so trying to make some power up at top end. Thanks for the input.

[preith]

The same thought of drilling rod journals crossed my mind after seeing just about EVERY aftermarket 'upgrade' 4340 v8 crank sold by the typical producers (scat, crower, etc) had this. I wouldn't attempt it with an OEM crank but if going with a custom one-off one before heat treating...

I see you asked the same question on HybridZ. I was not surprised by the advice against it, but was surprised nobody mentioned this is a quite common practice in the v8 crowd. Granted, it's comparing apples to oranges in the crank department, but I'd think the harmonics would be worse with the v8's and they're getting away with it.

[JonnyRock]

Just out of curiosity:

I've heard that these L6 cranks are pretty beefy. Does anyone know how much a stock 240 crank weighs?

[Walter Moore]

I never got around to weighing the crank, but I would guess at somewhere around 50 lbs. I was shocked when I picked it up the first time. It was way heaver than the fully assembled cylinder head. (But that is mostly aluminum...)

[Zforce]

I just weighed my old 240 crank,48 lbs