"Cracked" con rods

[Ross]

Who can explain how these rods are manufactured?
Winfred, aren't the M70 rods that way too? I've not been in the bowels of one but seem to remembr something written about it in the old sales literature.
My understanding is that the process offers perfect alignment of the rod halves. What I don't get is how this can be done without distorting the part.

[Jon K]

Well most rods I know of are forged (yes most stock rods are forged) and they're made from two forgings.

[Jon K]

"The invention concerns an improved manufacturing method of a powder metal connecting rod with a stress riser crease formed in a side face by forging a sintered preform with a generally V-shaped notch in the side face. A one-piece forged rod is produced having a crease therein which is readily separated by a cracking process into a two-piece rod with a separable bearing cap. "

http://www.patentstorm.us/patents/5613182-description.html

[Jon K]

I think I did read that this might be how my rods were made - I am not sure

[Ross]

Very informative. Unless I missed it the article doesn't address final machining but it must be done after the cracking as the force would certainly elongate the hole.
Yours look like sintered, what do the mating surfaces look like? Machined or broken?

[Jon K]

I haven't taken them apart (per engine builder recommendation). I will let you know - I think they would crack them and then machine them, because even on cracked rods you never see the V groove

[winfred]

they are broken, that way it has maximum contact area and locks together, i have not been into the bottom of a v12 yet but the m60 i built a few months ago was powdered/cracked rods, i do know when a m60/62 sucks water #6,7 or 8 usually punches through the valley and out again the side before it stops :D

[Barney Paull-Edwards]

After forging but before machining, they are split in a pre-machined groove by a bloody great hydraulic ram in order to achieve a molecularly perfect fit, Thats my understanding! By co-incidence BMW/Okrasa were the first to do it with the `80`s F1 engine I think,the rods are H beam but 90` from road engines.

[Jon K]

That is my understanding as well Barney

[Bill R.]

newer m3,m5,z8's have them. Its cracked when the forging is still rough and then the sides and bore are machined.. Here's a pretty good description in this article
(http://www.allbusiness.com/professional-scientific/scientific-research-development/155655-1.html)




Who can explain how these rods are manufactured?
Winfred, aren't the M70 rods that way too? I've not been in the bowels of one but seem to remembr something written about it in the old sales literature.
My understanding is that the process offers perfect alignment of the rod halves. What I don't get is how this can be done without distorting the part.

[Ross]

Thanks Bill.
What happens to these sintered rods when they fail? Do they bend or break or just explode into dust?

[Jon K]

they bend - you can break a rod as well, but thats absolutely catastrophic. Usually you will bend them, but thats at such high stress levels its unlikely. Though a bad tune can do anything.

[Robert K]

A typical rod would have machined surfaces where the cap mates to the rod. The only thing that really aligns the two pieces is how tight the fit is between the hole in the cap and the two bolt shoulders protruding through the rod. That's the old way.

With the cracked rods, they actually manufacture the entire rod as one piece. Then, as someone else has said, they take a huge hammer and hit the rod at the point it would normally have been machined in a regular rod. When the cap portion breaks away, the surfaces where the two parts would meet back together now have a rough surface instead of the machined surfaces that a normal rod has. The benefit however is that when you try to put those two rough surfaces back together, they only fit one way-perfectly. Since the two surfaces are irregular shapes that mate perfectly, there is no chance for the cap to slide around on the rod if the bolt shoulder happens to be a few thousandths to small. Funny how low tech really ends up being high tech in this case. Does that make it any clearer?

Robert K
1991 535i

[Jon K]

Ross thought you might enjoy these pics

Shows the quality of machine work between the Eagle and stock 328i connecting rod I have.

http://blowneuroz.com/525/Parts/rodcomparison1.jpg

http://blowneuroz.com/525/Parts/rodcomparison2.jpg

You can't even fill the split with your finger nail if you scrape over it, its nearly impossible to see!

[Ross]

Thanks. The thrust faces appear not to machined.
So when cracked the rod is apparently pulled until it gives at a parting line imparted during forging then the bore is machined. No evidence of the stress riser from forging and no indication of the sides having been machined....hmmm.
Makes the BM rod look stone age.

[Jon K]

Believe it or not there are guys over on bf.c saying that the BMW rod pictured there is "very strong" - I have heard/seen of non vanos rods making more power than on a M50TU/S50/M52/S52 engine, and I think it's because they feel more comfortable with the rod design.

http://blowneuroz.com/rods.jpg

A guy has made 704 rwhp on the stock M52 rods (pictured in previous post) and the rods bent. The swede churn out over 800 rwhp on stock NV rods. I am a firm believer in "girth" and the NV rods have that - the Eagles have it more so, and are better forged and materials.. Hopefully it works out.

[632 Regal]

The thrust faces appear not to machined.

They should be machined to eliminate thrust surface wear but are not, either they may be extra wide for final machining or they just forgot about that part. I haven't seen an Eagle rod in a while but any other performance rod has been machined at exact tolerances, no mix and match with them, you need to get the crank finished if it is off tolerance.

[Jon K]

Carrillo:

http://www.autosportengineering.com/racing/911_vintage/images/3.0%20RSR%20Carillo%20Rod.JPG

Thrust not machined.

http://www.sallee-chevrolet.com/Scat/images/scatrod5.jpg

Scat rod, thrust not machined.

http://www.laraceworks.com/catalog/images/manley%20ibeam%20rod.jpg

Manley rod, not machined.


They all have the fillet area inside the thrust region machined, but not the thrust region.

[Bill R.]

surfaces, thats the only way to ensure that faces are perfectly parallel and the width of each rod is the same, they were probably either shot peened, glass beaded or tumbled in media to stress relieve after the thrusts were machined, any of which would make it look as if it weren't machined

[Ross]

I see now.
Those Carillos must be a joy to install.

[Jon K]

surfaces, thats the only way to ensure that faces are perfectly parallel and the width of each rod is the same, they were probably either shot peened, glass beaded or tumbled in media to stress relieve after the thrusts were machined, any of which would make it look as if it weren't machined


Actually yes i forgot to mention, the Eagles are shot peened as are most.



Ross - why do you say they'd be difficult?