I know what offset grinding the means, but I’m having trouble understanding it. So you offset grind the crank and then you install longer rods ? I’m trying to figure how you gain stroke.
Rod length has nothing to do with stroke the crank provides. By offset grinding you just changing the center of the rod journal. Picture 2 circles that have the centers marked, one larger than the other.......over lay them so the bottoms where in the same spot.......by moving the circle center is how the stroke is gained
OK, the first thing you have to understand is that the new rods must have a smaller diameter rod journal than the original. I did this on my latest Rover V8 build. The original rod journal was 2.185 and the new Chevy rods were standard Chevy small journal of 2.000 By offset grinding the crank rod journals by approx 0.180 I increased the stroke by the same amount. The additional 0.005 was taken up in truing the journals. The stroke therefore went from 3.230 to 3.410
I think you mean you offset ground by 0.090", or about half of the difference in the rod journal change of 0.185" If you offset grind the rod journal by 0.180", you would increase the stroke by twice that amount, or 0.360". The offset would place the piston 0.180" higher at TDC, and also 0.180" lower at BDC.
@wkillgs That's not quite right. The crank journal centerline is moved by half the offset, in this case 0.090", but the amount of offset ground into the journal is the full 0.180". 2.180" = centerline of 1.090" 2.180" - 0.180" = 2.000" 2.000" = centerline of 1.000" So, the journal centerline is moved by 1.090" - 1.000" = 0.090 But the offset ground into the journal is 0.180"
Perhaps I don't understand your terminology of 'offset ground into the journal'. Offset generally means movement from the centerline. I'm thinking: moving centerline 0.090" = offset grind of 0.090". And you're reducing the diameter of the journal by a total of 0.185" to fit the chev rods..... but the offset from the original is still only 0.090". But you're saying moving centerline 0.090" is called an 0.180" offset grind? Maybe that's machinist slang? (I'm an Engineer not a machinist!) I agree moving the journal centerline by 0.090" will increase the stroke by 0.180".
Doesn’t all that grinding weakin the crankshaft ? I’ve always been told to stay away from cranks that were turned undersized to much.
Offset grinding takes a tiny amount of material but it’s not a large amount. You can also just order a billet crankshaft with whatever stroke you want. King made my crank, they were great to deal with
So if you use a rod that is same length that has a smaller rod journal end, then the stroke is increased because the rod journal centerline and crank journal centerline are further apart, right ? So do you need a piston with pin to crown height taller ?
The rod length has 100% nothing to do with stoke. 2" rod or 8" on the same crank journal with have the same stroke. Rod length with effect piston in the bore location. Certain combinations of rod and piston height will spend more time near tdc than others
In the days before cheap Communist crankshafts, it was common to knock .200 off the main journals of an OEM cast-iron Chevy 400 crank to get it to fit into a Chevy 350 block, making a "383" using appropriate pistons. Taking that much material off of a crank journal--rod or main--doesn't seem to be a problem in the real world if normal machine-shop practices are used, including leaving a nice fillet at the ends of the journals.
Yes. It all depends on how deep the original piston sat in the cylinder in relation to the new deck height / gasket combo. But yes in many installs, the pin location does indeed need to move closer to the top (crown) of the piston to due to deck height / gasket combo limitations. I am thinking that this is what you are referring to as "taller", as in being located higher up in the cylinder toward the top of the deck due to the longer stroke. Also, the distance from the CL of the pin to the top (crown) of the piston is referred to a the "compression height" of the piston. As the pin moves closer to the crown of the piston, the "compression height" dimension actually decreases. Confusing, huh.... Tommy
Depends. The smaller rod journal end of the new connecting rod means the rod journal on the crank has to be ground undersize to match. You can offset grind to increase stroke, you can offset grind to decrease stroke, and you can grind undersize without offset to maintain stroke. Tiny amounts of offset might be used to correct an improperly-ground stroke. In high-volume or careless production, a crank that's supposed to have a 4" stroke, with 4 rod journals 90 degrees apart, may have the journals ground 89 or 91 degrees apart, or with .005 too little stroke, for example. Grinding the shaft "true" can fix these minor problems. When used with increased stroke, yes--depending on "how much" stroke increase you have, and where the piston was "in the hole" before. Example: Piston sits .050 in the hole. You're using a typical ~.040 head gasket, so having the piston at "zero deck" gives you appropriate squish/quench. You could move the pin .050 lower in the new pistons to put the piston at "zero deck". You could cut the deck .050 to put the original piston at zero deck. Or you could add .100 to the stroke, which puts the original piston .050 higher at TDC, achieving zero deck, and also puts the piston .050 lower at BDC which may or may not result in interference between piston skirt and crank counterweight.
If the stroke is increased then a shorter rod or a piston with a shorter deck height, or a combination of both, will be required.
Most stroker engines use a longer rod with a raised pin height. If they didn't, you pull the piston further out the bottom of the hole, which results in piston rock, skirt wear, and worse. Piston rocking also becomes a problem at BDC and TDC both with a lowered pin height on a short rod. Lowering pin height and using a short rod, although possible, has absolutely no desirable mechanical effects, and can lead to a whole mess of trouble for various reasons, which is why people use longer rods and raised pins when stroking an engine. Going from a 3.85" stroke to a 4.5" stroke on the engines I am most familiar with requires going from a 6.6035 rod length to a 6.800. Some (almost all that I've built) have the pin height bisecting the oil control rings, which requires a support ring to be added to the bottom of the oil control ring stack, so 4 rings in total on just the oil ring set.
