Ok

I have bought a set of PM rods or so he says there PM rods off a guy in the for sale section. These rods have a hump in the bottom of the rod cap and he telling me that these are them, now I know what LT1 rods look like but I just what to use this two show him that these are not LT1 rods. So if you guys can reply to this and let him know that Pm rods have a smooth rod cap with no hump at all.

Thanks
Phil

 

http://www.automotiverebuilder.com/ar/ar99928.htm

A picture is worth a thousand words.

-Mindgame

 

I'm the guy who sold him the LT1 rods. The rods I sold him are in fact from an LT1, but are not PM rods. To save face, he is getting refunded. It should also be noted that he is apparently under the impression that all LT1s have PM rods.

I was under the impression all LT1 motors had PM rods in them. Therefore, when I pulled these rods from the stock '94Z28, I didn't realize they weren't PM rods (even though they didn't have the balance pads. I don't pull motors apart everyday). Talking to some other people, they mentioned in other LT1 motors they've pulled apart (mainly '93 and '94 vintage) they haven't been PM rods. So, I have two questions:

1) When did GM make the switch from the rods w/ the balance pads to the PM rods?
2) What are the limitations of the rods in the LT1 motors that are not PM rods.

Thanks,


Steve.

 

Steve,
In your defense bro, I think you're right! I seem to remember reading something about the change to PM rods, but there were definitely LT1's built (early) with the conventional rod.
Although I'm sure someone here knows for sure, I'll try to remember where I got that info from.

Regards

 

'95

 

I do believe that's right, the '93-4 used the old style rods. Of course, they are better than the PM rods, so I think the buyer outsmarted himself (unless he wanted stock replacements to avoid rebalancing).

Rich Krause

 

Link to info on variations in LT1 engines, with pics:

Rebuilding the LT1

A quote from the article:

 

What makes you say that? Everything that I’ve seen seems to suggest that the PM rods are probably the best factory SBC rods out there.

Injuneer… I’ve pulled apart a ’94 ‘vette engine that I know for a fact was original (and actually built in '94, not late 93 like most '94's), was otherwise correct and most definitely did not have PM rods. I’m guessing that it might have been phased in like the non hardened pushrods in mid ’95 and you’ll find some with and without them, but I wouldn’t count on finding them in anything before ’95.

I always found it interesting that with the LT1’s, it seemed that vettes got stuff first, then b-bodies, and then finally f-bodies…

 

The opinion re PM rods is not based on much fact, so it is just an opinion. But consider this: GM first replaced forged cranks on production small blocks with cast (in the 70's, I think), saying they were perfectly adequate. Then they started using hypereutectic pistons instead of forged. This may have been an "improvement" in some respects, but clearly not in others. Then we get the PM rods, which GM says are lighter and stronger than forged steel rods. Perhaps I'm suspicious, but I think it's notable that Oliver, Callies, Carrilo, Lunati, Scat, etc. aren't selling any PM rods. Also, I have never seen them on any of the other contemporary high end motors that I have been inside of. The lower end production stuff has them though.

Here's an article I found on the web: http://www.autofieldguide.com/articles/099903.html Note that they refer to lower costs, and PM being "almost as strong" as a forging. Not what I want in a high performance part. Here's another more technical discussion: http://www.mpif.org/design/conventional.pdf The authors are obviously PM enthusiasts but sum it up by saying "The real advantage of P/M is to cost effectively produce large volume, highly toleranced, metal components. The ongoing debate has always centered on properties, such as strength and size. As you can see from the above comments, P/M is rapidly overcoming these modest objections as material selection and material characteristics are being engineered to specific applications and requirements. Flexibility and knowledge of process variables is the key for both the end-user and the component manufacturer. Non-conventional P/M applications are effectively dealing with size constraints and metal injection molding can deal with any part geometry you can imagine."

Still doesn't sound like these rods would be better than a forging, at best they are nearly as good. In the absence of any hard data I have come to the conclusion that the PM rods were a cost saving measure, not a performance improvement. At best, they may be like the hyper pistons. Truly superior for a stock setup, but less suitable then what they replaced for modified motors. If someone has data to the contrary, I will be happy to change my opinion.

Rich Krause

 

And some more reading: http://www.metal-powder.net/mayfeat1.html

... which quotes PM as being as strong as a forging but with better "Y" axis stiffness. Now, that's compared to an "AISI 1141" forged connecting rod..... not a much stronger 4340 rod.

I think for the most part, they're just as strong as factory forgings. Of course Oliver, Carillo, or any other 4340 forged rod manufacturer, etc., have nothing to worry about there as their product is still superior.

As for the Corvette, it's always gotten "stuff" first.... latest engines, latest transmissions, you name it...... it's GM's premier sports car. Been that way long before the LT came around.

-Mindgame

 

My point wasn’t that I was surprised that the ‘vettes got the good stuff first, but that the b-bodies seemed to get it second.

Like mindgame basically said, in most cases PM rods are similar or slightly better then forgings in compression/stretch, most of what most of our engines see, but I was under the impression (possibly incorrectly) that prior to getting the PM rods in ’95 most LT1’s used cast rods, for that matter, I was pretty certain that most of the rotating assembly was very similar from ’87-94 (yea, I realize that till ’92 that rotating assembly was in an SBC as a tpi engine, but most of it appears to be identical), based on the 5 ’87-95 engines that I currently have sitting in my garage, not counting what I’ve got in my cars (I’ve got ’87, ’92 and ’97 350 powered vehicles).

Maybe I should pop the pan off of the ’87 TPI and ‘94LT1’s in the garage and actually look… I know that the ’95 B-body engine that I have actually has the PM rods, later, vented opti…

 

Mark,
I seem to remember reading that Chevrolet has never used a "cast" rod. They've all been forgings of various quality and strengths. I'll try to dig up the source of that info but I believe it was either Smokey or Jenkins who said that.

-Mindgame

 

I very much doubt there ever was a "cast" rod in a SBC. The rods in the LT1 were forged, until the PM rod came along. Most of them are from 1040 steel, if memory serves.

Rich Krause

 

That makes sense for production reasons. Chevy had a couple of forge plants for cranks, rods, axle shafts, R & P gears, etc. For this reason they had no real reason to use cast steel ("Armasteel") or ductile iron rods like Pontiac, and maybe some other divisions.

Pontiac, which did not have forging capability, used cast cranks and rods (from the late 50's on) with very few exceptions mainly for this reason. At that time, Armasteel was produced by Central Foundry Division (of GM) so Chevrolet would have had to purchase rod castings rather than forge them in house.

I've heard it said that Chevy used forged cranks and rods for so long because they wanted to have the strongest parts for their engines. IMO, it was as much production economy as anything else. Today cast ductile cranks and pm rods are better, more consistent and less expensive than even 'in-house' forgings, and I believe there are fewer forge plants.

 

Hum… interesting… I did some digging through my library early this morning.

I couldn’t find any mention of castings and all the traditional SBC rod pics that I could find were clearly forgings (wide parting lines…), but I did find out that they were forged from 1038 (not really what I would call the strongest material), and that in ’89 GM went from their traditional .570-.575” beam width to roughly a .505” beam width in all their small blocks, including the LT1’s till ’94. Funny that it weighted the same or slightly more then the old wider rods since they moved the weight over to the balance pads. That made it one of the weakest factory rods and apparently the ’92-95 LT1’s were the only modern SBC that had any significant occurrence of rod failure in stock engines.

You learn something new every day, in this case:
- PM rods are probably stronger in most applications then the old “pink” rods, which were just cleaned up and magnafluxed production forgings
- As far as I can tell, all factory SBC rods are 1038 forgings, the goodwrench race ones are 4140 forgings but too heavy to consider for anything
- Stay away from the ’89-94 rods at all costs, they’re just crap.