I was wondering if going with a smaller rod journal, such as 2.0" or 1.889" rod journal would be worth it for ease of clearancing a 3.75" stroke crank? I would assume the smaller journal would allow more clearance with the block, and that combined with a 'stroker clearanced' I-beam rod would make the task easier? Or is there more of a downside to a smaller rod journal than would be worth it?

 

In the early days, GM built SBCs with rods having 2.0" journals. Later, GM moved to the larger journal size and I suspect the reason was a durability issue, although I've never read about the 'why' that move was made.

Many years ago I owned a 86 Vette and built a 415 cid L98 for it and, for clearance issues in the 400 block, I used the GM rods which had 2.0" journals. I never had a rod bearing problem, but had I to do it over again, I wouldn't have used those 2.0" journal rods.

Bottom line for me: Since there's now such a wide selection of rods available for stroker engines, I wouldn't go with rods that have a 2.0" journal. For a 3.750 stroke crank, clearancing the block isn't that difficult to do.

Just my view.

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

Metallurgy has improved dramatically in the 40ish years since they changed journal size, so what once was a durability thing is no longer.

Hell look at the used high end race stuff on ebay, it is all smaller than 2.00" Today's metallurgy and machining make things stronger.

I don't think it really matters either way for your build.
Just shooting down a bad argument.

 

I would go small rod, just because it has less surface to create friction on. Just my point of view. I will always run small journal cranks/rods in my 327 because I love it to rev HIGH and want the least amount of friction (contact) as possible ---

 

Less surface means higher unit area loadings..... the total drag on the bearing is the coefficient of friction X the load. Spreading it over a smaller area does not reduce the total friction drag. It just increases the stress on the bearing. The smaller journal offers the advantage of less rotating and reciprocating mass.

 

So does that mean that the larger journal would mean better reliability due to less stress on the bearings?

 

Theoretically, yes. The question is "is the bearing metallurgy adequate to support the loads with a smaller journal?" If the bearing isn't overloaded, it become a non-issue.

 

So if I'm going with a forged crank and coated bearings, small vs large journal is a non-issue?

 

I'd also consider the piston and rod weights, the stroke length and the anticipated redline.

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

My personal choice would be the larger journal. Clearancing for a 3.75" stroke shouldn't be that big an issue.

It takes an experienced engine builder to answer your question, taking into consideration the specific bearings that are being used, and items mentioned by Jake. There is no "one size fits all" answer.

 

I totally agree with Injuneer. I'm not qualified to intelligently discuss the difference in bearing loads or materials. So, basically, what I do is try to apply logical thinking.

First, I realize that when GM designed the BB Chevy engine the engineers used a lot of information that derived from the small block build, since the small block preceded it. They discovered areas that needed improvement and many of them were incorporated in the BB design.

Over the years, GM continued to improve both engines, both for power improvement, fuel economy and longevity. Less warranty claims puts more money in GM's bank account. So when GM moved to larger rod journals, I ask myself "Why?"

There must have been a compelling reason since GM had to off-set the increased re-tooling costs involved.

So, then, to me, the question becomes "why would I consider using a crank with smaller rod journals?". If it's just for clearance issues, then is that concern something that can't be easily addressed without moving to a small journal crank?

From all the stroked engines now in play, apparently clearancing isn't a major concern, having already been addressed by the parts manufacturers. The clearancing for a large journal crank is easily dealt with, so what's actually gained by running a small journal crank? Especially if there's even the most remote possibility that durability could be sacrificed.

Also, I think you'll find that most, if not all, engines that have small journal cranks also have relatively short strokes and probably much lighter rods and pistons.

Another thing to consider is the journal diameters that are currently being used in GM's Show Case engines, the LS series of thumpers. Isn't GM now using exotic metals probably to lessen bearing loads. I believe I read where one engine is using titanium rods. Think about the cost of those suckas and then ask yourself "why'd they go with those?"

So, to me, absent any convincing evidence that a small journal crank is marketly better than a large journal version, I'd go with the engineers' latest offerings. Of course, with some one-off engine build, a small journal crank may be the only thing that'll work. For me, it would be a last resort measure.

Just throwing this out as the way I think about the issue. Great question though.

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

Again look at high end racing they use small rod journals. TODAY'S materials and machining mean it can work just fine. Under 1.900" is common and reliable.

Again for a 383 I don't think it matters one way or the other but I also think some people are pushing bad conclusions.

 

My only take on this is the surface travel speed on the bearing face. A bigger journal should mean faster spin, while a smaller would mean slower, do you think this could translate to different wear/tear?

On another note,do you think a smaller journal would mean a more concentrated force on a smaller area on the up/down stroke, while a bigger diameter could help spread the load and lengthen the time it takes warp or wear?

I'm siding with the opinion that it doesn't matter considering the life of an engine isn't usually more than 200k and any failure that could ever be attributed to journal size would probably take 1 million+ miles (if your internals are correctly balanced)

 

This quote is taken from "How to Rebuild Small-Block Chevy LT-1/LT-4 Engines" page 89.

"Undersizing Crankshafts

Chances are, some folks get a bit squeamish if their existing crank needs to be ground more than -.010” on the rods and mains. By the same token, they may not want to buy a remanufactured crank if it’s been ground -.010” or further. Their reasons typically include concern about the durability of the crank (“the smaller journals make the crank too weak”) or surface harness (“gee, if it’s ground that far, I’ll loose the hardness and my crank will break”).
In a nutshell, that’s sheer baloney. For one thing, both cast and forged crankshafts are extremely over-engineered from a strength standpoint. The OEMs design all crankshafts with a wide margin of error, simply to ensure against any potential for crankshaft breakage. Yes, cranks do break from time to time, but not because they were undersized. If arger removing as little as say .040” of material from the diameter of a main or rod journal, the crank breaks during engine operation, then the crank had some serious flaws to begin with, and the undersizing was not a contributing factor.
Back in the 1930s and early 1940s or so, if a crank was ground to an undersize, bearing makers compensated for the additional clearance by making the bearings with the same thickness of steel backing as their standard bearings, but simply added to the thickness of the liner (babbit) to take up the additional clearance and to maintain the specified oil clearance. If the crank was a bit on the unbalanced or distorted side, the eccentric movement of the crank tended to beat out the thicker soft Babbitt lining, which created excessive clearance, which dropped oil film pressure, which took out the bearings and maybe the crank itself. Unfortunately, this old wives’ tale has perpetuated over the years, and for absolutely no good reason. Today an undersized bearing employs a “standard thickness” liner, with a thicker steel backing, so there’s no excessively thick, soft, mushy liner to “beat out”.
Besides, with today’s bi-metal and tri-metal bearing construction (a tri-metal bearing features a steel shell for support, a middle copper/lead layer, and journal-facing babbit for embedability and conformability), lining thickness becomes much less of a significant issue in terms of fatigue resistance issues.

Hardening Loss?
As far as hardening is concerned, production cast cranks are usually not surface-hardened in the first place (something most folks don’t realize). In practical terms, the only real advantage of a concentrated hardening layer is for bearing failure service. If you wipe out a bearing (due to a non-crank-related cause), it’s easier to scrape the bearing material off of the journal during the repair and cleanup. That’s it. In addition, if the “hardening” of the crank is so much of a concern, consider this: if the crank is placed in a situation where the bare journal contacts the rod or main bore, you have a drastic problem to begin with that is in no way a dimensional issue. Remember that the crank journals ride on an oil film that is pumped in between the bearings and the journals. Granted, the bottom of the crank journals tend to rest (and makes slight contact) with the lower bearings when the engine is at rest, but as soon as the engine begins to crank, pressurized oil shoves the needed oil film in between the bearings and journals. In short, surface hardening is simply not an issue in terms of grinding a crank to a moderate-to-severe underside.

How far is too far?
Let’s put it this way: if an established bearing supplier makes a bearing in a specific undersize, it’s safe to grind the crank accordingly. They simply would not offer undersizes if they thought a problem existed in their use.
Many of today’s leading, cutting-edge competition engine builders routinely grind cranks as much as .300” underside in order to reduce mass and the resulting frictional energy. A specific example involves some NASCAR Winston Cup engine builders who reduce connecting rod journal size by .210” to achieve a 1.88” rod journal, and using Honda dimensioned connecting rods. If these guys are whacking off that much meat, and still running 8,000 or so rpm, there shouldn’t be any concern about taking a mere .010” or .040” from the journals for any street or high performance crankshaft application. Again, if you break the crank, it wasn’t because the journals were undersized. Instead, the culprit could be over-revving, gross imbalance, extraordinarily high cylinder pressures, oil starvation, improper oil clearances, a failed connecting rod (which subsequently beats on and breaks the crank) or improper assembly, or a crank that had too much runout or that featured pre-existing cracks. "

 

Go Honda journals or go home!