Rod install: One is "looser"

dtw · 09-13-2003, 09:18 PM

Got the crank assembled up today and everything went smoothly. Except, the very last rod closest to the flywheel is just a bit "looser" than the others. If you point it straight up and let it fall, it falls noticeably faster than the other 5 rods, which are all fairly similar in the fall - which is slow and deliberate. The oddball rod still feels really good, and if all 6 felt like it I would be fine with it. I guess what bugs me is that there isn't consistency. Should I just forget about it and move on, or tear down that rod again? If I do, what is my recourse?

911pcars · 09-13-2003, 09:56 PM

You might have a rod with excessive oil clearance or a worn crankshaft rod journal. I think you should measure the oil clearance with something like Plastigage. The gravity test isn't too accurate. The objective is for all rods to have the same length, thus rebuilding (or at least checking) the rods is something that should be done.

If it's the connecting rod, the machine shop will resize the big ends so the ID for standard size bearing halves (big end) are identical. The small ends are rebushed and reamed so the piston pin-to-rod oil clearance is to spec - all while maintaining the same rod length. You might get away with an "undersized" bearing (actually thicker bearing shell) to maintain the correct oil clearance. Not sure of their availability and not the ideal method for a fix. Typically, if a journal is worn, all six are remachined to the next "undersize", then combined with compatible undersize bearings.

Hope this wasn't too confusing,
Sherwood Lee
http://members.rennlist.org/911pcars

dtw · 09-13-2003, 11:07 PM

Sorry, guess I should have clarified- I this is on a std/std crank with reconditioned rods.

ChrisBennet · ChrisBennet's Garage

"Trust no one, measure everything yourself." is my motto.
-Chris

davidl · 09-14-2003, 11:11 AM

Sorry, this will not do.

thee is definitely something wrong, amaybe serious.

I'd strip the loose rod and check for something stopping it bolting up fully.

If Not, take it back to the machinist and get it checked for size.

But unless you are lucky, you will end up stripping all 6 to measure evrything.

Kind regards
david

dtw · 09-14-2003, 03:54 PM

OK, last night was really late and I was tired when I posted and followed up. Here's some more info:

-This rod (#3) was the only one that gave us static. When tightening the nuts to 20nm but before the 2nd torquing, the rod froze on the crank. It was disassembled and inspected without incident and reinstalled. No trouble at that time, final torque was applied, and everything seemed fine. It wasn't until installation in the case that I noticed the rod swung a lot more freely than the others. Today I removed the crank from the case and stripped down rod #3. I doublechecked clearance with plastigauge and noticed that the clearance was ~.06mm, whereas the other journals measured .04mm. All are within spec, but #3 seems to be a bit sloppier. I am getting a new set of rod bearings to replace the one on #3 - I don't trust the bearing after that first strange seizure on install. Of course I'm also getting a new pair of rod bolts and nuts.

As a "seat of the pants" comparison- all the other rods have a slow, deliberate fall when dropped from vertical. The #3 had a slow initial fall, then a very fast fall toward the end. I would subjectively rate the feeling of all the rods as "excellent" but #3 was just "different".

I'm interested on what those with more experience than me have to say about this...

911pcars · 09-14-2003, 09:02 PM

DtW,
The oil clearance for #3 (the loose one) is .06 mm (0.0023")
The oil clearance for 5 of the con rods is .04 mm (0.0016"). That's a difference of about .018 mm (0.0007").

How about measuring and comparing the connecting rod journals? You might find the #3 journal is 0.0007" smaller in ID (the difference in oil clearance from the rest). If so, then you make the decision as to whether the "within spec" numbers are acceptable or whether you want to regrind all the con rod journals to the next undersize ("1 under") in order to have equal oil clearances. However, this option requires new "1 under" (thicker) rod bearings, not as desireable a setup as standard.

If it's within factory specs and everything else is copasetic, I say use it. Others, of course, may have different opinions which I'm perfectly open to.

You can also ask yourself this question. If Hans were assembling this new engine at the factory and if this con rod was within specs, what would he do?

Sherwood Lee
http://members.rennlist.org/911pcars

Wayne 962 · Wayne 962's Garage

It's late and I'm tired, but one thing I wanted to add is that you can get too carried away with tests like these. The garvity test is best able to tell if a rod is binding - not typically too loose, although you can certainly tell as well. If everything is in spec, bolt it up and go. Use new rod bolts and nuts if you've already torqued them down...

-Wayne