Visit The Dirt Forum for More Information

Author Topic:   off set grinding a crank benefits?
eastsidemob
Dirt Full Roller

Total posts: 27
posted October 08, 2002 11:48 PM
what are the benefits of offset grinding a crank just wondering been hearing about guys having this done and just wondering what it does to the engine and ot the crank

c21
Dirt Freak

Total posts: 346
posted October 09, 2002 10:45 AM
The benifit is longer stroke which translates into increased displacement (and more torque in most cases).
as an example:
The simplest offset grind would be to grind the rod journals down to the size of the smallest undersize rod bearing available (lets say that is .030" under). In the process of grinding you would remove .030" off of the side of the rod journal closest to the crank centerline and zero off of the side of the rod journal farthest from the crank centerline .... this moves the centerline of the rod journal .015" from the centerline of the crank ..... which translates into the rod moving .015" farther at the top and at the bottom of it's stroke .... or a total change in stroke of .030".

Of course that is a pretty small amount of change and possibly not worth the bother, however, it is more common to turn the rod journals down to a smaller rod journal size (example: turn a sbc journal down from 2.100" to 2.000")and make a much larger change in stroke.

A common offset grind would be to convert a 3.480 stroke crank of a chevy 350 into a 3.562 (3 9/16", often called a "9/16"motor)stroke ..... creating a 363" motor (with a .030" over bore). This offset grind requires the 2.000" small journal rods (or the use of Lunati bearing spacers if you want to use 2.100" journal rods).


clear as mud?

c21

jammin
Dirt Administrator

Total posts: 5556
posted October 09, 2002 10:47 AM UIN: 16262997
400's done the same way to create a 421.

jammin

ryan
Dirt Freak

Total posts: 430
posted October 09, 2002 04:58 PM
Drawbacks?? Average Cost??

Monster
Dirt Maniac

Total posts: 135
posted October 09, 2002 06:49 PM
A few years ago, when I was building lots of Ford 5 liter engines for the biggy shorts Mustang crowd, an offset ground stock crank was one of my 'secrets'to a very strong running engine. If you are not a real good customer of the crank guy, he's probably gonna tack another $150 or so onto your bill. Drawbacks? Overlap is reduced, making the crank weaker, if all else is equal. However, if the crank guy knows his stuff, he increases the fillet radius, making the crank at least as strong, and probably stronger, than it was before you started grinding. Ford 302s can go to 316 or so, 351C's to 370(Screamers!). As far as I know, the only offset ground crank that I had done that failed was a SBC LG 327 forging that I had welded into a 3.39 stroke. Cost $1750, and cracked 3rd night out. OUCH! Luckily(I guess), it was in my own car.

ryan
Dirt Freak

Total posts: 430
posted October 10, 2002 04:48 PM
Thanks Monster!

Back to the Archives