Connecting Rods - Big end Dia

[mjobrien]

I'm in the process of diagnosing a hard knock that developed and found a few issues I wanted to bounce one off the group...

Man - could Use Dave's Help for sure, does anyone have an update on his condition?

-Connecting rods all mic out at anywhere from .002 to .007 larger on the big end bore than spec. Seems like this could aid in possible slap on the Crank.

Question: Any thoughts on acceptable tolerances
on the big ends? Going by the book they are out for sure.


Michael
55 BN1

 

[roscoe]

If you go by the measurements in the 100 shop manual the size of the big end HOUSING (i.e. without the bearing in it) should be 2.11" to 2.1105". That is converting the metric numbers which are 53.595 to 53.608mm. Are you saying that you are getting up to 2.118", or are you saying .007mm. If your measurement is in inches, yes, that is too large. Do remember that the bearing half shells should be flush with the half of the rod that they fit into. They should not extend beyond the housing where they clamp up, nor should there be a gap. The really important number is the bearing clearance, which the book says should be .0005 to .002 ". This is measured with plasti-gage when the rod is clamped onto the crankshaft. I do not like to measure the bearings in the rod when snugged together (as with a diameter snap gage) for risk of scoring the bearing. If you are .007 over the max housing diameter it is probably out of round. Put the rod together without the bearings and check the diameter at 4 or five places. They may be below minimum size in one direction, and over in another. Those rods would be bad. Hope I'm not stating the obvious to you. Let us know what you find.
I should also add that diameters of this sort should be measured with a snap type gage and then a micrometer, or a real bore gage which most folks don't have. The crank journal should be measured with a micrometer. Calipers are frequently used for these measurements and I find that the more times I measure with calipers, the more variance I get. Snap gages and micrometers are the way to go.

 

[Randy Forbes]

Jon,

I agree with everything you said, except for this:

... Do remember that the bearing half shells should be flush with the half of the rod that they fit into. They should not extend beyond the housing where they clamp up, nor should there be a gap...

All the engines I've built (numbering well over 100) have the (main and) connecting rod bearing inserts standing slightly <span style="font-style: italic">proud</span> of the rod and cap. That's to apply a bit of <span style="font-style: italic">crush</span> to the bearing inserts when torqued to specification.

Yes, where's Dave; he'd back me up :wink:

I've gotten pretty intimate with connecting rod bearings lately; in the past year, I've changed out the rod bearings (only) on about five (5) BMW M Roadsters with the 315 bhp (S-54) engine. BMW was kind enough to provide sub-standard bearings (and some say rods too) on the 2001-2003 M3s (which get the same engine as the Z3 based 2001-2002 M Rdstrs).

First time in my life I have taken bearings out of an engine that have rotated in their rods by up to 3/8" as seen below.

 

[Randy Forbes]

Michael,

I'm betting that you found your source of knock, with the clearance you stated.

 

[roscoe]

Randy, I have no problem with being corrected. "Slightly" is the operative word here. I've never seen a number for how much is too much.

 

[mjobrien]

Jon, Randy,

Thanks, I was using a caliper and took several measurements at different places. As Jon stated hard to get good readings, but seem to get a set of consistent numbers. So what I'm getting is

(Stated Dia. 2.114in)

Rod Dia in.
(4) 2.116
(3) 2.120
(2) 2.121
(1) 2.118

The largest is #2 and it has some clear signs of indents on the journal as well.


I'll look to get some plastic gauge and locate a snap gauge as well...

Michael.

 

[roscoe]

Michael,
The caliper is probably fine if you are getting good consistent measurements. Sometimes folks don't wiggle them enough to get them settled in because you should only be using the knife edge tips. Calipers have flat machined surfaces and are better for shafts than for holes, so if anything you tend to underestimate bore sizes when using them.
You certainly seem to be out of limits on the rods, but what I'm not sure of is if you put the bearings in and they fit as they should (standing proud a tiny bit; thanks Randy) and then you do the plasti-gage measurements, clamped on the crank and get the proper clearance, where is the rod nock coming from. You didn't mention the small end. How is the fit of the wrist pin in the small end? I also have no clue how much slop you can have before you're going to hear it. Randy?

 

[mjobrien]

Jon,

Need to check the small ends next....

On the bearings, I'll check again but I don’t think any of them were actually proud of the flange. They seemed to be either flush or slightly under, I'll check again. Also an observation is that the wear on the bearings is almost 100% top and bottom, almost none on the sides, which leads me to believe that these were not tight all around.....


Michael

 

[Randy Forbes]

Reconditioning the connecting rods is a common machine shop practice. They grind a small amount of material off the mating surface of the rod/cap and then torque them back together. At this point, the bore is oval (shallower at the top/bottom) and then they're recut to a perfect circle. In your case, 2.114"

Not sure how the gudgeon (wrist) pin is held on the 2.6 Four, but all the later sixes use a floating pin with a bushing in the small end of the rod. Replace the bushings and you're as good as new.

If you have pins that are clamped in the rod (Longbridge__2-port__100/6s are this way) then you likely have too much clearance in the piston pin bores and they'd need to be replaced.

 

[Randy Forbes]

The initial "bearing crush" may now be lost, as they've already been torqued to spec, but they shouldn't be below the mating surface of the cap/rod. That would indicate (as do your measurments) that the rod bore is too big.

Looks like reconditioning of the rods is in order.

Now what about piston slap? what is the clearance of the piston in the cylinder bore? Did the knock go away once the engine was hot?

 

[mjobrien]

Small ends do indeed clamp with a bolt, I need to take a look at those next, could call for new pistons. Knock was present at high end of range, near 60 mph in high gear. The engine was at operating temp when this knock happened....

Based on the scratches in the bearings, material in the oil, oversized rods, and overall sloppiness of the rebuild I think It's logical that these rods were the issue.....

I'll get a few snap gauges and take some measurements on the bores as well.

Thanks for all the help on this!!!!

PS: Basil - People will pay for this type of help and collaboration.....


Michael.