tlebovic

There is a posting on the 944 list about changing the #2 rod bearings in their engines for racing use. Is this applicable to a 928?

https://rennlist.com/forums/924-931-...l#post11169746

No affiliation, just curious...



Tom
89 S4 Auto

Alan

#2 & #6 in a 928 are problematic racing but its an oiling issue rather than just a bearing issue for a 928 (prob in a 944 too...?) - so while bearing attention might be needed for a worn engine - for track longevity you need a way to avoid any oil starvation - be it from oil packing in the heads and uncovering the pickup, excessive aeration, feed issues in the block (specific to 2/6), oil temp issues.

Lots of ideas on how to make it better - no really definitive answers but some good threads in the last few years with some analysis and options.

Alan

Strosek Ultra

Yes, CB-1628H or CB-1628HX having .001 more oil clearance.
Åke

hacker-pschorr

From that thread:

If the idea is to build a race engine that will last, "ballparking" the rod bearing clearance may not be a good place to start.

Hopefully Greg Brown will see this thread and chime in. In another thread he went into great detail about the proper way to clearance rod bearings for a 928. Since the 944 uses the exact same rod bearings and has the same clearance, that procedure applies to those engines as well.

Vilhuer

Changing away from OEM Clyco is just band aid but can be useful to do anyway. Clevite CB-981P works also but will need different cuts to rods than shown in 944 thread.

ptuomov

At least the 928 cranks that I've seen are all very consistent. I've been playing with the digital mikes that I bought last year, and at least with three cranks all rod journals are very close to each other. So that's the good news.

Another piece of good news is that it's not like there's that much variability with the cheap Glyco bearings, so the above solution is at least as good as those in terms of setting the bearing clearance.

Erkka and Ake have Clevite bearings that match the 928 well. One thing I am curious about is whether the 52mm journal Subaru bearing could be used with it's significantly shorter length than the stock 928 bearing or Erkka's and Ake's choices. Short bearing with heavy rods and pistons would be an interesting computation to make.

BC

How wide are these other bearings? The bearing in the 944 thread is too narrow.

ptuomov

In case you are saying that MM's bearing is too short, I have a question to you: Based on what analysis, logic, or math is it too short?

Rob Edwards

Nomenclature check on aisle 9:

By 'short', and 'long', what dimensions are you referring to? Forgive my cloudy brain, I'm just not understanding.

A stock 928 rod bearing is .9449" wide, about 0.0591" thick, and whatever dimension in diameter, +/- whatever eccentricity. They fit on a stock rod journal that is 51.971-51.990 mm.

RE: running those narrow Clevites in a 944 rod, I have no idea how that works better since you're giving up a significant chunk of the surface to distribute an oil film over, but then this whole discussion is way above my paygrade anyway...

Third pic:
https://rennlist.com/forums/11167296-post1.html

ptuomov

It is my understanding that the length of the bearing parallel to the journal centerline is called length and not width. Width is not used, I believe, because that would lead to confusion with journal width. I recall reading this from a tribology book three or so years ago.

The stock 928 bearing length of 0.9449" is longer than any comparable rod bearings made since. It's like a bearing from a diesel truck. Conjecturing from where the later engine models have moved, a modestly shorter bearing will likely perform better not worse than the stock 928 bearing, but of course one should punch the numbers in a bearing calculator if one wants to be sure.

Rob Edwards

Aha, gotcha.

Vilhuer

CB-981P is very close to same as stock. Its for over 40 years old Datsun 2L engine so that might explain its size.

BC

I don't know why short or long means in this case. I am talking about width. Narrow is the word I used. On purpose. Instead of Short or Long. Which usually denotes Length instead of Width.

BC

Its not modest from that picture. And the rod bearings are "narrower" or "Skinnier" because the rods of newer engines are also that as well.

GregBBRD

There's been enough written on this subject that if the OP did a search, he'd spend an entire day weeding through the various threads.

Regarding the Clevite CB981P bearing....it is very narrow. The 944 rod is several millimeters wider than the 928 rod, so it looks even narrower in that application than in the 928 application.

However, the 944 piston pushes squarely down on the 944 piston and rod....there is no rod offset, because it is an inline 4 cylinder and it is a simple thing to make everything push straight down. This is not the case with the 928 engine. There is considerable offset built into the rods, since each rod shares a rod journal on the crankshaft with another rod. This means that the piston never pushes squarely on the crankshaft rod journal.

The result is that the rod has a "twisting moment" and essentially tries to "rock" the rod and the bearing on the rod journal.

The Clevite bearing was narrow enough that the bearing load math was very far off and I completely dismissed this as a possible solution.

That being said, I have never tested them....