atb

If I were a betting man, my vote would go here. This sounds like the kind of design work that Marc Thomas would do.
It would be interesting to compare a Moldex Porsche rod-journaled crank to a Scat or Moldex SBC rod-journaled crank.
How's that for a subtle hint?

slate blue

Ok, so I went back to my storage locker and checked the rods, the bearings sit directly under the beam i.e they are centralized in relation to the beam, so if you turn the rods around they the bearing in relation to the rod doesn't change its position, that aspect sorted. However the chamfer that is required for clearance against the crankshaft's big end radius needs checking,( I can't get access to the crank quickly ) there is a small amount of material available to be cut away, hopefully that will be enough. As to the double chamfer, well, can't see any problems there as there is about 3 mm of material on the sides of the rods for them to run against.
I don't have my photo log in details with me so here is a pic of the sides of the rods from an old post. https://rennlist.com/forums/928-foru...pers-pics.html

I suppose given my time again, I could have had the rods slightly cut down, by the 0.15 mm that they are out and had the side clearance of the crankshaft adjusted, live and learn, it certainly is a learning episode when you try to build a top Porsche engine.

Greg

By GregBBRD

terry gt

Hi, Re rod off center on the wrist pin .I have one of the original DEVEK / SCAT cranks ( one of the first four) the motor used 6" manely S/B rods with weisco pistons / gapless rings . The motor failed , it started with .004 bore to piston clearance and wore out from there. Its a 16V 97.5mm bore .I still have the pistons and pins so I measured where the rod was (polished area) the rods were .054" or 1.83mm off center ! THe second rebuild is failing 3 cyl dead, So I purchased from 928 INT a block /968 pistons and oliver rods to go the 6.5L 32 valve route Terry

GregBBRD

All:

I reviewed this issue.....it has been many months since I've thought about this problem. The location of the crank journal is not the issue. The offset of the rod is the problem. As you look at the bottom of the piston, straight on down the rod, the rod ends up closer to the left pin boss than the right pin boss, regardless of which side of the engine it is on....if you are running a Chevy rod in a 928 engine. If the crank journal was in the wrong place, the problem would be on all the front or rear sides.

If you measure where the bores are on a small block Chevy (from side to side) it is a different "offset" dimension than a 928. They build the rod so that it sits exactly in the middle of the two pin bosses on each piston. When the cylinders are offset a different amount, the rod, naturally needs to be offset differently.

Imagine, for a moment an imaginary V-8 engine, where the centerline of the right front cylinder is an inch farther forward than the left front cylinder. Now, imagine that the rod, for this engine, is exactly 1 inch wide (at the crank end.) If the two rods are placed side by side (bolted onto one throw on the crank....which is 2" wide, in this example), the centerline of the rod beam (in order to be centered on the piston), will be exactly in the center of this rod [1/2" on each rod (to center line), which totals the 1" offset.]

Now, keep everything the same, but move the bore centerlines apart 1.200". For the rod beam to now be exactly centered, they need to be 1.200" apart. That means, on this imaginary V-8, that the rod beam centerline needs to be .600 from the rod edge that touches the other rod. The centerline will be .400 from the edge with the champher. (The beam of the rod will need to be offset .100", from centerline of the cap.)

Whenever the cylinder offset changes the rod offset needs to change. The Chevy offset dimension is different than a 928. The rod, therefore, needs to be different.

Louis:

That picture is not looking straight down the rod beam, but it does look like the rod is closer to the left pin boss than to the right...which is the issue I'm talking about.

Terry:

That's the exact problem. This allows the pistons to rock in the bores, so that it wears the bores out. This is especially an issue with the "soft" Alusil bores....like you are using.

If the rod is strong enough (like any of the H beam designs), the rod may not break....but the side thrusting then is transfered to the bearings and the cylinders. If the rod is weak (like an A beam rod design) the rod flexes and can break.

If you are planning on running an 928 engine and you want it to live for many miles, you need to get a rod with the correct offset.

Lizard928

Greg,

Please answer the question previously asked, how much $$$ is one looking at for the proper rods you have had made. I know Terry will need to know that information.

GregBBRD

Colin:

I'm not selling here, just providing information that many people will find interesting. If someone wants to know more, they can contact me, privately.

Lizard928

Understood Greg, thanks for making that clear.

slate blue

Greg when you say that some rods are weak and others strong mainly from which design the rods are, i.e either A or H, I think that it can be more complicated than that. I can understand what you say in relation to Carrillo as their top rods are the H beam, the budget range is the A beam, maybe cheaper materials, less complicated design, leading to less expensive production, i.e easier machining, I am just guessing here but I think that it may be a logical cost cutting measure.

Now personally I like the Pankl rods as they are similar to the Carrillo but stronger looking above the upper bearing. However I have Lentz rods, there is two main types of rods used in Nascar, Carrillo and Lentz, as mentioned Pankl is also used but to a lesser degree. The Carrillos are of course H design and machined from billet and the Lentz are forged and then machined given both benefits from forging and machining, that is the removal of surface faults. I don't know what material the Lentz rod is made but it is extremely strong and both Lentz and Carrillo have told me that the rods are completely reusable. Teams change them because they can, is basically their answer to their durability.

The Nascar rods have a greater force applied to them than a F1 engine, so given both are of different design and run highly successfully dividing the toughest race market I think that it is more a material and a processing issue than a pure design thing.

So with my offset issue, the 0.15 mm problem, I have new oversize pins, so that my pistons and rods can be honed to exactly to the specification, which is 7 tenths of a thou. The pin while being tapered is is quite thick walled and as such should not have any ovality/pin bending issues. I would have thought out of tolerance in relation to centralization issue that is smaller than the big end clearance should not be a problem, I should have addressed this but it is too late. Also my pistons are run to a tight tolerance, being 4032, now I wonder if some of the engine failures also used 2618 pistons which I don't think would work well with Alusil, as this material is very aggressive to non coated alloy surfaces. Thoughts? Problems for seen with my build?

Cheers Greg

GregBBRD

This entire subject gets a bit confusing, when dealing with other rods. The "Chevy" offset measurement is a given dimension. All Chevy based rods should be built the same way...no matter who makes them. For your rods to only be off by .15mm (.006") is very confusing...did you indeed turn them around?

Anyway, no matter how you got there, I'd think that .15mm is very, very close to correct. I would, personally, not think that you would have any large issues....that being said, keep reading.

The Carrillo "A" beam rod was originally built to fill a gap between stock rods and their "H" beam rod. Now they offer "Super A Beam" rods. etc. I've read several "threads" about the "A" beam rods failing. My guess would be that the original "A" beam rod will completely disappear and be replaced by a stronger piece, eventually.

The "A" beam rod seems very flexible. If you clamp the pin end on a very solid table, push on the cap end, you can actually flex the entire shaft, quite easily, with only your thumb. This is not true with any other rod I've tried this with...including the stock "thin" GTS rods. Carrillo's "H" beam rod doesn't seem to flex any.

I'd guess that my rod breakage problem might be isolated to this rod only...and probably only in a uber high compression, high rpm race situation. Bearing wear and cylinder wear would be a different issue, completely.

Regardless if the rod fails or not, the engineering was very, very interesting. Turns out, regardless if the rod flexes or not, the force on the piston, rod, and the bearing are exactly the same as if it could flex the full amount (which is obviously limited by the clearance between the piston and the cylinder. Also, the forces get very high, very quickly. I know that they did calculations all the way from .055" to .020" (offset on pin) and found that the force barely changed at all. I do not recall them doing calculations as low as .006", as in your case. At any rate, it is amazing that Alusil based engines run, at all, with the Chevy rod. Bore deterioration has to be very, very quick. Rod bearing issues have to be significant.

I think both we and Carrillo learned a bunch from this engineering. They were actually very surprised at how high the forces were, once there was an offset issue. I think it reinforced the need to make things fit, properly, for every engine. I seriously doubt that any saleman, at Carrillo would dare sell you a Chevy rod, today, for a 928 application. BTW...this was a very friendly exercise. Carrillo and I were never at odds over this issue, so our interactions were always quite good. We both desired to only find out what the failure was caused by, what forces were involved, and how we could solve the issue. Carrillo, at no time, accepted any responsibility for the failure.

Hope this helps.

slate blue

Greg thank you for your response, firstly my rods are Nascar, the engine while being a small block Chevy is the SB 2.2 which is pure race engine and I suspect has different bore spacing. I know for a fact that the new Chevy Nascar engine has a different bore spacing, it is from memory 4.5" That engine is the RO7. The only way my build will work is with the rods turned around, as mentioned before I always thought that the standard rods would be off a bit but never knew anything more than what I possessed in my hands, that being the Nascar rods and thinking that 0.15 was not that much although it is somewhat of a concern that the pressures didn't change over the distance tested.

Just thinking out loud here, I would have thought we are talking about a leverage thing here, which then confuses me, why if the point of leverage is changing why isn't the pressures?

Also I have thought of a solution to my problem, I can have the engine offset bored to take away that 0.15 mm problem, as my pistons are only 103.124 diameter this gives me some scope. Luckily it is in the direction that unshrouds the intake valves. In some ways I am glad I haven't got this engine built as yet as it certainly has prevented some mistakes or at least not optimizing the build.

Greg

largecar379

Greg and Greg----

the measurement you need is beam center to center, when you put two rods together (as they would be installed on the crank throw).

You can check this with a pair of 928 rods, and then compare to whatever you are replacing them with.

the bore spacing measurement of opposing cylinders (on the same crank throw) is the spacing measurement you will need, not bore spacings on the same bank (side) of the engine.

I would still be careful about blaming the rod offset for rod failure. There are too many other issues to take into consideration before putting all you money on the rod issue.






--Russ

slate blue

Russ I did indeed put the rods together and work out the differences.

Greg

largecar379

figured you did--just put my two cents out there for those who are not clear about what is being discussed here.

another thing to consider is core shift relating to actual bore centering and bore machining. if the bore centers are not on-center with the crank throw for that particular cylinder, the rod spacing again will be incorrect.


carry on!






--Russ

GregBBRD

Yes, the spacing of cylinders, next to each other has nothing to do with this.

GregBBRD

Yes, I suspected the same thing, in regard to leverage. Like I said, they didn't do any calculations below .020", so I don't know the answer. The numbers just didn't change significantly between .020" to .050".