I'm flipping through the new Hot Rod, and it speaks about offset grinding the crankshaft in order to gain an increase of stroke. I've also heard of 5.0 mustang guys doing this to stroke up to 347 from 302.


How exactly does it increase the stroke? The article spoke about how it changes the centerline, but I'm not totally familiar with the concept.

Anyone have some thoughts about this?

 

If the crankshaft throws are ground to a smaller diameter, and almost all the material is taken off one side, the center of the journal is moved outward. A smaller con rod or thicker (spacer) bearing shell set is then used to compensate for the smaller diameter of the journals.

To change the stroke of a 302 to create a 347, the rod journals (and thus the crank) would be severly weakened from the smaller diameters. The more acceptable method is to use an aftermarket crank, but Ford guys often don't do the most acceptable thing.

 

It's a common thing to do on an Olds engine also. You can offset grind the crank and use longer buick rods that have a smaller journal. The buick wrist pin holes need to be enlarged for the Olds wrist pins but it can be done.

You can easily take a 455 Olds to 496.

 

And those 347 Ford engines can really respond

Car Craft has a nice write up in its current issue on one.

(I must be getting old ... the "death to Ford" chants are ringing with fainter clarity these days. I just like a good running machine anymore. Though there is still nothing like the interchangeability of small block Chevy parts

 

Vader, when you say the crankshaft throws are ground to a smaller diameter, are you referring to the rod journals? That's my sticking point.

I'm still kinda baffled about this.

 

Yes, the rod journal is ground smaller, but not concentric to its original centerline. To help visualize this, & I hope I can explain it in words rather than a diagram, do this. Draw a circle with a horizontal line through its diameter. This is your main journal with an imaginary center line. Now draw another circle and horizontal line directly above it. This is your rod journal. The distance between the 2 horizontal lines is your crank throw distance. Two times the crank throw distance is the stroke of the crankshaft.
Now, in your top circle that represents the rod journal, draw a smaller circle inside your original circle so that the tops of both circles coincide at the 12 o'clock position. Now draw a horizontal line through the smaller circle at its diameter. See what happens now? The distance between the main centerline & the centerline of the smaller circle is greater than the distance between the main center line & the original rod journal centerline. Thus, the crank throw increased & the stroke increased. If this is too "wordy" to understand, maybe Vader can explain it more clearly, assuming he had his coffee before reading this.

 

look at this funny drawing it is suppoused to show how the rod jornal is ground away to bring the rod center out more.

well it wont let me!

 

Offset grinding was used back when cheap aftermarket cranks weren't available,don't bother. As for the Ford 302-347,I get irritated when I read all the hype as if it were some mystical all powerful enity. The only reason it ever gain any respect is when it was installed in a very light body,I would run a similar prep 302 sbc against it anyday. Do I hear an amend?