Steve Bryant

iTrader: (1)

I'm confused by the terms Offset Pin Pistons and On-Center Rods. I assume that both are used to optimize rod clearance on stroker engines, but I'm not sure about this. For instance, Eagle recently announced On-Center LS1 rods in 6.125" and 6.200" lengths. However, they don't explain exactly what they do. Some custom pistons are available with offset pins. Again what do they do?

Somebody help this boy . . . please!

Steve

Steve Bryant

iTrader: (1)

I have found one reference on motorcycle pistons/rods/cranks that describes the advantage of the offset piston pin pretty well http://www.motorcycleproject.com/mot...ws-offset.html. The offset reduces noise, rocking, piston slap and wear issues. It also gets into the thrust and non thrust sides of the piston and cylinder. I need to read this several more times to fully comprehend it.

Steve

racer7088

Steve,

Offset pins are for noise reduction in OEM pistons when they are cold. It causes the pistons to be able to rock over at TDC in a very nice and controlled manner this greatly reducing any cold piston noise when the clearances are still greater than normal running temp.

On-Center rods like the ones I got Eagle to start making and selling for our little LS1 are different from the normal SBC rods which are NOT on-center but are offset sideways at the big end since the cylinder bore off set is less that the width of the rod journals. The SBC cylinders are off set or set back from bank to band around .880 and yet the rod journal width is 1.900 and half of that is .950 so the difference in rod width and cylinder bore offset is .070 or .035-ish per rod.

On the LS1 the rods are centered on their big ends and have no chamfer on the sides since the LS1 cranks have a rolled fillet and are right in the middle of their respective .950 wide spaces on that same 1.900 wide rod journal (for both rods)! The cylinder off set was adjusted to that same .950 width so now these rods are centered at each end and the rod small end is centered in the piston instead of being .035 sideways like if you use an SBC rod in an LS1 or an LS1 rod in an SBC.

Steve Bryant

iTrader: (1)

Erik,
Thanks for an excellent explanation! So let's see if I understand your explanation. The small block Chevy had the cylinder bore centerlines offset .035" forward (or behind) the centerline of the big end of the rod as it was installed on its half of the rod journal. As things go, .035" offset isn't a big deal regarding the thrust axis centerline. However with a stroker piston in an LS1 where they've gotten creative with skirt design to clear the reluctor wheel and counterweights, the .035" is clearance that you really want on the small end of the rod.

Are most stroker pistons made with the wrist pin bore centerline offset to minimize rocking, etc. or is this something that only a few manufacturers do?

Thanks again, you are a great resource for this community.

Steve

racer7088

Steve,

The big ends on the rods are the ones that are actually offset but of course they are attached or connected to the rod journals on the crankshaft and can only move side to side with whatever side clearance has been engineered into them.

Of course this rod offset is from front to back of the engine or side to sode on the crank and is related to the cylinder bank offset and not towards or away from the thrust sides of the cylinder like the piston offset is.

You are right that not all piston makers put pin offset in their pistons. Wiseco and Diamond do and I think some Mahles. It is a nice thing on a street piston mostly. Most piston are not offset unless you option them that way as on a custom or they are oem. Any normal 2 valve piston that has 4 valve reliefs is an off set piston that can be used on either side.

Steve Bryant

iTrader: (1)

Erik,
I now have a better picture. The big ends are offset to compensate for the fact that the cylinder bores are .035" out of step in a fore-aft direction when compared to the crank journals (both the rod and main bearing journals). The piston pin is offset laterally (if you view the engine from the back or the front). The idea with the piston pin offset is to allow the piston to rock back and forth in a more gentle manner (as opposed to slapping back and forth) as the piston transitions through top and bottom dead center. Of course this problem is more pronounced when everything is cold and not up to operating temperature because the piston to cylinder wall clearance is much greater when cold. I would assume that the direction of offset is dependent on direction of crank rotation just like the thrust and non-thrust sides of the cylinders are rotation dependent.

Thanks again.

Steve

DavidNJ

Can the SBC rods be adapted to an LS1?

racer7088

Yes David they are all the time you just need enough pin boss width compared to your pin end width on the rod you are using.

racer7088

Steve,

Yes I would say you got it right.

95ttoplt1

iTrader: (3)

Dragging this back up, To Adapt a SBC rod to LS1, you would mill the small end of the rod to eliminate this offset?

briannutter

You could do that and get it to physically fit within the pin bosses. The problem is the piston is being cocked sideways in the bore when the beam isn't channeling it's force straight into the crank. This causes wear issues, increased friction, and ring seal isn't as good. People did it for years, but there correct beam offset is a nice thing to have.

wrea398

I just bought some Scat H-beam 6.125 rods. I called Scat about the offset issues and they told be all thier SBC rods and LS1 rods are TRUE CENTER and they do not make an offset rod for these engines. Also got an LS1tech member discount. $369.00 shipped from Scat.

racer7088

Those rods are not on center and you can look right at them and tell no problem. They usually will work but are offset halfway between a SBC and a zero offset true "on-center" rod. I have used a ton of them back in the day as they did work much better than the other SBC rods.

It is true that they are not offset as much as a regular SBC rod. They call it an on center rod because the bearings when chamfered are also a little to the inside and the bearing is pretty much centered under the rod but the whole deal is still not centered in the piston when installed of course.

Actual on center rods have the big and small ends and rod beam all exactly inline with each other. On an LS1 the cylinders are exactly centered over the top of the rod journals whereas on the SBC the cylinders are not offset as much as the rod journals. LS1 has rolled fillet crank from GM so they don't care about chamfering bearing and it is centered as well in the rod journal end of the rod

Bank to bank offset on the LS1 is .950 and on the SBC its about .880 which is why a SBC rod is not centered in the pin boss when used in an LS1 since they are each offset about .035 towards each other. This causes the rod to not line up exactly with the piston when you put the engine together and on a tight pin boss spacing deal they can bind up in the pin boss cocking the piston sideways.

The SCAT so far have been only about .015 offset so they usually still clear even on inboard pin boss stuff but not always.

racer7088

Damn forgot all of this has already been said above.