Conrod rocking. Is this normal?

MBradbury · 11-05-2003, 06:20 PM

Help everyone,

This engine rebuild is racking my nerves

I just closed the crankcase a few days ago, and now I am really concerned about the amount of rocking movement on all the conrods. The crankshaft was not reground, but I took extensive measurements with an engineering micrometer and it was absolutely within standard specifications with no measurable out of round. The old bearings were in near perfect condition when removed. I fitted new standard Glyco bearings, but DID NOT perform the plastigauge clearance check. I did perform the visual drop swing test, and they all seemed ok. Now the crankcase is closed, I can deffinitely rock the conrods. At first I thought it was lateral movement, but now I am convinced there is a distinct sideways rocking when I apply a finger-thumb force.

The only variable I can attribute is a temperature difference of about 20F between when I assembled the crankshaft and today's test. I don't know the exact coefficent of expanison of the crankshaft journal, but working on an average of 0.000008" per degree F, I can't believe this would account for the rocking motion.

Is it normal for the conrod to rock to the extent that you can feel a distinct click? Has anyone observed similar? If the experience of this board is there should be no rocking, I am prepared to spilt the case again and perform the plastigauge test. I wish I done it the first time

Mark Bradbury
1983 911 SC

Rot 911 · 11-05-2003, 06:39 PM

Mark I will be interested in the replies. When I was tearing down my 3.2 I noticed that I could rock the rods as you describe. The bearing looked just fine when I took the rods off the crank. I just assumed there has to be some clearance. How else could oil get between the crank and the rod bearings?

Jay Auskin · 11-05-2003, 08:54 PM

I was finishing the longblock on my 2.4L today, and was messing around with the con rod movement too. I concluded that it is indeed lateral movement. With that assembly lube providing a nice slick surface, you're getting the "illusion" of rod rocking. I truly think you're OK. (at least it helps me sleep at night)

HawgRyder · HawgRyder's Garage

When rebuilding Harley engines...the amount of "rock" in the top of the front rod is the key to judging clearances(normal is about 1/32").
I would say that because of the width of porsche rods...the amount of true "rock" would be much less.
You might be seeing a "slide" effect of the rod moving from side to side on the journal.
Bob

911pcars · 11-06-2003, 12:11 AM

I have no standard for what constitutes a certain oil clearance using the tilt motion method. Maybe others can provide more info and assurances.

Lacking any insight, what are you going to do, button it up and hope/wish it's okay? Better safe than sorry. It's not too late to check oil clearance w/Plastigage. You can do it with the crankcase together. However, you'll probably have to replace the rod bolts. Use the "old" ones to check the clearance. BTW, did you check the main bearing oil clearance?

There doesn't have to be a lot of "space" for oil. The lubrication system is under pressure and the bearings receive oil through drilled passages in the crank.

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

Porsche_monkey · 11-06-2003, 05:39 AM

I just plastigaged one of my con-rods. While it feels like there is a suprising amount of slop, the plastigage showed there was not. I think the issue is the side to side slop, not clearance in line with the con-rod.

If you carefully pull and push on the rod you will probably feel almost no play. Don't let it move side-to-side, just pull and push in-line with the rod.

I would plastiage one con rod (and replace the bolts after) just for peace of mind.

john walker's workshop · 11-06-2003, 07:47 AM

they all have side movement. if the crank was fine, and you used new bearings, don't worry about it.

Porsche_monkey · 11-06-2003, 07:49 AM

I just checked mine, yes they rock, yes they click. They are also right on spec. Don't take your case apart, move on, drive your car.

Jay Auskin · 11-06-2003, 03:21 PM

I plastigage'd mine too. I forgot to include that. Like PBH said, if you isolate the side to side movement, there is no radial play. A slight wiggle of the small end does give an illusion of "rocking".

MBradbury · 11-06-2003, 07:16 PM

HI,

Many thanks to everyone who responded.

The concensus appears to be that everything should be ok. I ommitted from my original post that if you push and pull on the conrod there is zero movement. I recognise that is a good sign. Maybe the lateral movement amplifies the illusion of rocking, but if I am honest with myself there is still a radial feel to the play.

One useful piece of information I gathered is that I don't have to split the case to perform the plastigauge test. However, if the test were to fail I would have to bite the bullet and undo the work so far. My decision is to go ahead and continue i.e. finish the short case assembly and perform the piston deck clearance checks. I will order a couple of rod bolts and perform the plastigauge check on one of the conrods for peace of mind before going to the next stage..

Regards
Mark Bradbury
1983 911 SC (3.0L to 3.2L shortstroke in progress)
1988 Carrera 3.2 (insurance writeoff, will be a 2.7 RS lookalike one day)

snowman · 11-06-2003, 09:52 PM

Rest assured that you are probably ok. The plastiguage measurement is second to the actual measurement of the crank journals and rod bearing diameters. That is it is less accurate. But not much less.

I use plastiguage as a sanity check, ie not as accurate as actual measurements of the crank and rod, but a very very good second check. If the actual measurements are good it is GOOD, period. Have some faith in your measurements.

911pcars · 11-07-2003, 01:41 AM

Mark,
While chances are everything is probably okay, I think you're doing the prudent thing by actually checking the clearance and not assuming it's correct.

If you had measured all the rod journals and all the installed rod bearings, I wouldn't have taken pause. However, you stated you only measured the journals and that was it. You'll have much better piece of mind knowing the oil clearance is to spec at this point. JMHO. Best wishes on your project.

Better safe than sorry or...... a roll of the dice
Sherwood

john walker's workshop · 11-07-2003, 07:56 AM

you have to hold the rod perfectly still to use plastigauge. do you think you can do that with it hanging out of the case? it tends to smear a bit if the rod or journal is moved. then you will be freaking over that.

snowman · 11-07-2003, 09:34 PM

You can remove a rod and its bearings without breaking the case. Since you have already measured the crank journals, all you have to do is measure the id of the rod bearings, assembled into the connecting rod and torqued down to about 3/4 of the recommended torque. Then measure the id, subtract the od of the crank and you have the oil clearence.

Another suggestion, being a little less anal about the whole thing is to do this. You know the crank journals od. Check the box the rod bearings came in and see if its the correct size for that journal. If it is you are ok.

911pcars · 11-07-2003, 10:09 PM

Snowman,
Odds are everything is okay and Mark has little to worry about in this regard.

However, I don't want to leave the impression that it's okay to assume everyone did their job from manufacturing to packaging to machining/rebuilding engine parts. Things happen.

For example, it'd be hard for me to believe that Glyco (or any other manufacturer) never had an incorrect bearing inserted into a box. Mark seems to have standard bearings, standard rod journals and seems to have decent "play" in the rod "drop" test, so the chances of having an oversize set of rod bearings is minimal. But what if the actual big end of a rod happens to be out of spec (too large) and the machine shop didn't check it? There wouldn't be enough bearing crush and Mark could soon have a spun bearing (Mark, you should check this dimension too).

I'm not usually this anal, but I think you have to force yourself to become "one with the sphincter" (that's engine zen-talk) or at least very thorough considering the cost of engine parts and potentially wasted time.

Sherwood

Wayne 962 · Wayne 962's Garage

Do you have the engine book? There is a spec for side clearances of the rod in the back. I think that this is what you guys are talking about when you say "rod knock." In reality, I would call it axial play.

On page 205 of the Engine Rebuild Book, I list the connecting rod widths and their respective clearances to the crankshaft sidewall.

-Wayne

snowman · 11-09-2003, 05:52 PM

Quote:

Originally posted by 911pcars
Snowman,
Odds are everything is okay and Mark has little to worry about in this regard.

....

I'm not usually this anal, but I think you have to force yourself to become "one with the sphincter" (that's engine zen-talk) or at least very thorough considering the cost of engine parts and potentially wasted time.

Sherwood

I think thats why I first suggested measuring the big end ID with the bearing installed, it includes everything, including proper crush, and leaves nothing to chance.

On the other hand, knowing the crank journals were measured and that they are not to tight, and looking at the box and its dimension, leaves only a very minute chance anything would be wrong. Personally I would have measured EVERYTHING twice, and checked it with plastiguage too. I would not have assembled the case without knowing the answers. On the other hand if I were to come across a pre assembled case, knowing the same set of facts, I might well have done what I said in the second suggesiton and let it go as chances of a problem would be almost zero.

MBradbury · 11-09-2003, 05:53 PM

HI everyone,

Once again, thanks everyone for the helpful suggestions and support. Just to finish this one off.

I read the responses with interest. Just a couple of remarks from my side to help clarify why I didn't think it necessary to perform the plastigauge first time round. Firstly, this engine rebuild is not being performed because of any component failures, although seat of the pants feel suggests power is down a bit. The rebuild is an attempt to increase power substantially by means of a 3.0L to 3.2L short stroke conversion with 964 cam regrind. The car already has all the bolt on performance enhancements, e,g, SSIs. Obviously, while everything is apart, it's a good excuse to replace, refurbish many parts e.g. valve train, etc. It is my firm belief that the case has never been split during the car's previous 85k miles. The previous owner's looked after the car and it shows no sign of any butchering (until now). Knowing this, together with the almost perfect state of the bearings when I broke the engine down led me to perhaps a false sense of security. In retrospect, I should have measured the ID of the assembled conrod bearing and I wouldn't be worrying now. I measured just about everything else when the case was split and all is within specification.

To Wayne's comments, I did check the lateral movement between the condrod and the crankshaft and it is within the specification as listed in his engine rebuild book.

This rebuild is not over by a long chalk. I have no doubt more questions and progress reports will be posted soon.

Thanks
Mark Bradbury
1983 911 SC 3.0L, soon to be a 3.2L short stroke.
1988 Carrera not so soon to be a '73 Carrera 2.7 lookalike.