.020” Offset Grind?

Thinking about doing a .02 Offset grind on my crank, to make up the 1.98” compression height

Any drawbacks I’m missing? ... Seems pretty cut/dry, but don’t see it ever mentioned over lots of block decking

 

Pretty good idea

 

It can be done, the crank shop I deal with charges an extra $200 to add stroke, the machine shop charges about $150 to mill up to .030" off of the deck.

So the draw back would be the extra $50 bucks and they would need an extra .010" of rod journal to ensure it cleans up so you would have a crank that would need .030" under bearings.

 

Check around for crank stroking costs.
Many do it for far less than $200, esp. if 'only' .020".
Aside from that cost, getting into a chevy bearing size saves the $50 back, along with other saved costs of able to buy parts accordingly.
The only downside to a light crank stroke I see is that you might find out which shops to never deal with again on, which is a blessing at your expense.
It isn't hard, just takes a little care.

 

Good point, that price was for a .140" offset grind, .020" should cost way less. Only way to find out would be to call your local crank grinding shop.

 

Thanks!!

I’ve read thru Jim Weiss 470 info many times ... he does awesome, sharing some really great information ... I’m happy sharing what I know, on my 1/2dozen motors ... he’s doing that before lunch

With that said, I really don’t need a 482” stroke, for my 400-450hp

And, Jim did some pretty significant crank modifications, probably only 10% of crank shops could duplicate his work, and there aren’t many to begin with ... with his primary goals to use a commonly available rod, and better HighHP bearing options

My goals are primarily to close the quench gap, and better detonation control ... avoiding the over decking can of worms ... and, 1.98” pistons are much more commonly available, than 2.00” ones

Think I’m gonna try it. Figured I’d give shout, see if I was missing something stupid

 

The only minor caveat is if your crank has already been balanced for a fairly light assembly.
I've not checked the difference from only a +.020" stroke with the same weight before and after, but I don't think it's very much (crank takes a bit to come back, time not profitably spent to check).
I'd believe whatever a Wallace racing piston/con rod G force calculator suggests.

 

You also need a good crank,with standard journals,to start with.
I wouldn’t focus primarily on quench. One my engines has the piston .045” in the hole. I run an iron head on that,with 11:98:1 compression,and I drive it with 93 octane. I mix it 50/50 when going to the track. No issues.

 

If you do your math on your stack up. Figuring a stk stroke of 3.9/2 +6.6 rod length+ 1.980 compression height piston you get a stack up of 10.53. Now if you do what your thinking 3.92/2 + 6.6 +1.980 you get 10.54 so for your .02 offset grin your only gaining .01 up. Assuming a stk deck your still sitting about .03 down

If your worried about the quench and all that, then I'm sure you going to have the block deck to be true square and even. So if you already have to deck the block I would have then deck to your needed heigh over doing this.

Another down side is if you takeyour 20 offset right away, you are not really leaving yourself another bite at the apple if you have a bearing failure.

Im not sure this is the direction I would go to gain .01

 

10.530 is about the deck height were the intake does not fit anymore, so that leads you down the rabbit hole, and before you know it your sawing 60-90 off the intake flanges to fit the intake with decent gaskets. That gets expensive quick.

The simpliest and most cost effective solution here is to simply order the correct pistons, once the block deck is cleaned up and squared. As Ben said, the math does not work for what your trying to do, unless your block is cut to 10.550, then .010 in the hole is not the end of the world..

I have an Autotec 4032 Forged piston for $679 with pins and locks, that you can order with any CD you wish. Hate to see you cut a stock crank to .030 right off the bad, I would assume your going to have to take it .010 further than your offset grind, to clean up the entire journal.

Make sure you have your mits on a set of .030 rod bearings before you do this, while they are still in the catalogs, it is hit and miss to find a set on the shelf.

JW

 

https://www.summitracing.com/parts/cle-cb762p30

Summit "says" they have them?????? Maybe yes.

But I would just order piston and be done

 

That's right, a .020" offset will only take away half of that(.01o") at TDC.

What kind of pistons are you trying to use? A set a customizable AutoTec pistons would take care of your compression height issues, just order them with whatever extra compression height you want.

You can order them through JW now, he can help you figure out what compression distance you need.

 

Summit says they have things that aren't even slated for production runs at the manufacturer until there's enough orders to justify them. I'd make sure you actually have them...or cut to to the BBC size.

 

EDIT: haha, I get it ... .02” Offset, moves crank throw centerline .01”

Ok, that may be where my math is wrong ... but, still don’t get it ... help

.020” offset would be +.02” to crank throw? ... so, +.02 at TDC and +.02 at BDC ... ie, 3.94” stroke

How am I missing that?

 

It's simple.. offsetting the stroke moves the piston .010 farther down, and .010 farther up...

A .050 offset is what your after.. this conveniently moves the piston up .025 in the bore. And changes the rod journal to a 2.200 size, which is very common..(BBC size) so there is a world of rod bearings available... But that also requires a new rod, and is the basis for the 470ci kit you have read about.

 

It's .02 for the total throw, not each side of it..... your double dipping the number......half of the gain goes to the piston going up, half of it going down

 

There's a problem in the terminology. You could mean .020 offset, or .020 grind.

.020 offset requires .040 grind, plus some additional to assure that the crank journal gets ground 100% of the way around. .050 would be common in order to get to a readily-available bearing undersize.

.020 grind could theoretically produce a .010 offset, but almost certainly less than that in practice, again due to the need to "waste" some of the grinding in order to assure the journal is ground all 360 degrees (100%) around.