I'm sorry I don't get it, I just looked at my genuine porsche rods and new Chevy "honda style" rods and when I measure the offset "the place where they will sit in the piston pin" mine if they are different to the Porsche rods, it would be out by 0.15 mm max. "My hand held tools don't allow for greater accuracy at this stage but it would be very close" Now maybe the Nascar rods have different offsets as they do not come out of a standard small block chevy, they come out of the Chevy SB2.2.

If this potential 0.15 mm is what we are talking about I can't see how thousands of pounds are transmitted because of this, which what I describe as minor variance? Remember there is that much side clearance minimum, the Porsche spec is 0.1 mm to 0.4 mm my clearance specified by Moldex not me was 0.5 mm. I take it you guys have turned you rods around? If you haven't done this you will be miles out and I understand what Greg is on about. That is the wide fillets of the rods to the inside. You then need to have chamfer added to what was the inside of the rod so that it doesn't wear on the radius of the crank journal.

Maybe somebody can enlighten me?

Greg

 

Hey Louie,

Just FYI, here's a (crappy phone) pic of one of my Oliver rods next to the Porsche Unit for comparison. I was hoping to get a stock SBC rod in the mix, but the motor at the shop hasn't been torn down yet. I got my rods from Motorsport SLC, so Garrity could chime in as to whether these differ from stock SBC rods. I'm assuming that they don't, and are just SBC rods with a reamed and bushed small end.

 

From the angle it's hard to see, but the small end opening is offset in the small end compared to the Porsche rod which has the wrist pin hole dead center in the casting. My wrist pin hole is north of dead center. From a prior thread someone put forward that this design minimizes piston slap.

 

Hi Adam,
As luck would have it, I took some pics of mine too. The small end of the Oliver rod has the bushing in the center just like the 928 rod. I thought Greg was meaning misalignment of the small end longitudinally on the pin. I mean the rod small end would be over to one side of the pin next to the piston. Maybe because the Chevy size big ends are not as wide as the 928 rods and when the narrower rods are on the (narrower) crank throw they wouldn't quite line up with the pistons. I have a pic of the underside of one of my pistons with the engine assembled and the rod small end is right in the middle of the pin. I guess the bottom line is I don't know what the problem really is and don't know enough to know what I don't know.
https://rennlist.com/forums/attachme...1&d=1231117398
https://rennlist.com/forums/attachme...1&d=1231117398

 

Greg
I'd like to see the data from Carrillo? Could you email it to me?

 

It's interesting that your wrist pins are centered. I remember when I posted pics of my rods awhile back Marc Thomas inquired as to why mine were off-centered. It makes sense now why he asked, he was curious since the ones in your motor are centered just like the stockers. I mounted some of my pistons into the stroker block today to measure for head gaskets. I can take a pic of the underside so you can see the rod/pin/piston orientation.
I agree though, if my motor is the poster child for the side loading issue that Greg has designed a fix for, I don't think I want to know about it. Let's face it, its a Porsche, it only makes sense that the motor will do everything possible to force itself into a horizontally opposed engine configuration. LOL.

 

I can't get my head around why it would matter whether the rod small end had the bore centered in the forging or not, other than strength. I can see why the piston pin would have offset for thrust, but not the rod. Did you ever notice how the 16v 928 motors really look like they are opposed and not V style? Marketing to the VeeWee folks?

 

Maybe he means the big end and how its machined to sit on the journal, and how that affects where the rod sits on the pin when in situ?

 

That's probably it. Have the rod beam not quite centered over the big end journal. However, mine seems aligned fine as it is.

 

I must be missing something here. The pin-to-big end spacing seems like it would be critical of course. If the pin is not aligned with the center of the beam, the rod beam would just live a little offset from the thrust. I guess I'm not able to visualise how this would affect side loading on the piston. Greg or anybody, straighten me out please.

 

When the Carrillo Chevy rods sit on the crank journals, the wrist pin is not centered under the piston...there is offset of .042". I never tried to turn the Chevy rods around...that would make a "double champher" and part of the rod bearing would sit in the unused inboard champher, it would seem.

I had a set of "Oliver Chevy" rods....on a Scat Crank that came from Marc Thomas, originally. These seemed to center better than the Carrillo rods. Perhaps Marc ordered these rods with a different offset or the cranks were made with the journals in a slightly different location to compensate. Perhaps he might shed some light on this. It would seem that both Carrillo and Olliver should make their Chevy rods with the same offset, since this is a bore spacing measurement and should not vary.

 

I can provide pictures of the offset issue.

 

This is part of a picture I had of the bottom of the piston with the rod alignment. It appears very close to center to me.
https://rennlist.com/forums/attachme...1&d=1231131963

 

Louis pic in #138 show how it should be, I think. I'll take some pics of stock and two different Chevy rods in near future which show how wrong it can be.

 

the picture that Louie has posted gives you an idea of where the rod /piston relationship should be--

the small end of the rod should be centered on the pin in the center of the piston. if not, the piston may exhibit wear (on one side) above the pin and (on the other side) below the pin, depending which side of the piston the rod offset is, assuming the piston was properly manufactured (with rod centering in mind).

Another issue is where the pin hole/crank hole is cut into the rod itself. If the hole (s) is not centered on the rod (offset when looking at the open hole in relation to beam center), it will make one side of the rod weaker and side load the piston on the sides perpendicular to the length of the piston pin.

the point to all of this, is that your crank supplier must know the measurements of your rods (the thickness of the big end of the rod times two, and the spacings between the vertical centers of the beam) and your bore spacings on each side and the bore spacings between opposing sides, to properly position the rods on the crank so that they are on bore center. Similarly, the vertical centers of the rod beams (when places together in pairs) must also be the exact measurement of the opposing bore centers. all of these measurements must match.

if not, your pistons may have a side load issue and contribute to excessive cylinder/piston skirt wear.

O O O O
l
lO O O O
l l
^ this is the bore to bore measurement you need (bore center to bore center of opposing cylinders)

H H
l l
l l
l l
lllHlllHlll
^--^ this is the rod vertical beam center measurements between two paired rods on a crank throw that you need

(OK, this is weird, I typed these drawings and they didn't come out right when I submitted the post, nor will they edit correctly. hopefully, you get the idea.)

These two measurements must match.

Of course, none of this takes into account flame travel across the piston top and its forces that are unevenly distributed on the piston, which does cause some side loading and wear.

It's all in the math and the measurements-----







--Russ