Wobbly Rods

[1972_911T]

A few weeks ago I finnished assembly of my bottom end and crank, and have been bugged since because of the side to side movement of some of my rods which I fear may be excessive. I first noticed this on strip down the rods for cylinder 5 and 6 seemed to have alot more side to side movement than the rest with cylinder 1 virtually having none this is part of what prompted me to do a bottom end rebuild as my intensions were just to do the top end/piston rings.

The engine has had all the bearings changed and tolerences checked and everything is within spec so after assembly I was hoping this movement would be gone/even across all rods, but its not and again its most noticable on the rods for cylinders 5 - 6 with cylinder 1 hardly having any. The movement is not massive but the thing thats making me worry more than anything is that it seems to be more noticeable on two or three of my rods if they were all the same I proberbly wouldnt have been concerned.

The other thing I noticed is rather than it been the whole rod that is moving from side to side its more like the rod is turning slightly I,e the front half or the rod moves to the left and the back half moves right pivoting on the crank journal I suppose this exagerates any fine tolerance as the rod length acts a lever moving only say 0.05mm at the pivot but maybee 1mm at the end of the rod length

Is this normal or is it something I need to be concerned about?

Steve

[Jim Sims]

Were the connecting rods rebuilt (big end resized and small end rebushed)?

[1972_911T]

The small ends were re bushed the big ends were measured and were in spec and didnt need resizing.

[Wayne 962]

Doesn't quite sound right, but the answer is in the measurements - all the specs are in my Engine Rebuild Book in the back, including side-to-side clearance...

-Wayne

[1972_911T]

What doesn't sound right Wayne the fact that they move at all? or that there is a difference between the rods? or both? I did chk all the dI'mensions against the ones in your book I'm now questioning if my measurements are accurate enough.

Steve

[Henry Schmidt]

I can't say whether or not you have a problem but before you assemble the engine, do what it takes to get comfortable with the project. Take your time and even if you have to ask someone more experienced check it for you, it's worth it.
The fact that you noticed some extra play and it doesn't feel just right puts you miles ahead of even the professional that has no feel.
Experience is great but you've got to have the touch and it sounds to me like you do.
That said, some engines feel looser than others and the loose ones make the horse power. If you are within specs (even the loose end) run it.

Good luck

ONE MORE THING: connecting rods should be rebuilt after every separation. When they run, the stress and the heat causes the rod pieces to take a set and when separated that set can never again be achieved. by rebuilding the big end you start over and the rod can take a new set with the new bearing and new rod bolts.
Most failures with titanium rods happened because the big end wasn't round when retorqued.

[1972_911T]

OK I'm now considering tearing down my bottom end again to double chk my measurements, I am quite reluctant to do this though as this rebuild was never because of engine failure this engine ran well with good power and throttle response however it had the usual 3.2 valve guide wear problems and the subsequent smoking been the cause for the rebuild mainly because the engine was already out of the car at the tI'me I decided A top end rebuild wouldn't go a miss this ending up been a case split job.

Before I decide to tear into the case again I would like to clarify a few points. From my research on pelican others had reported wobbly rods or has they put it conrod rock, the advice they got was mixed and ranged from the advice to strip it down and re check the big end clearances and axial play to others saying it is normal and to proceed with putting it back together.

If this is abnormal what could be the cause the two things I can think of are either excessive side clearance/axial play and or elongation of the big ends making the bearing fit more loosely at the top and bottom of the rod resulting in not as snug fit on the journal however I thin this is less likely as this would probable still require excess axial play to be noticed in the way I'm noticing it

If this is abnormal and is due to excessive axial play what problems will this cause me if I decide its not worth re checking it and go with my original measurement and continue with my rebuild?

If I do find it to be abnormal what is likely to be at fault worn crank worn conrod sides or both and what will the repair entail?

Any Advice appreciated as I really can't make up my mind and if at all possible want to avoid re splitting the case that said I also don't want to be pulling a noisy and or damaged engine in a few thousand miles.

Steve

[davidppp]

Hello Steve.

I'd listen to Henry..

your instinct sounds good to me..

and if there is any difference between the rods, I'd want to know why before going any further..

One tip I have found useful is to simply allow each rod to drop under its own weight from the horizontal..the speed and sound they make should be the same..

I'm sure you already checked the side clearance with feeler guages..

I persoanlly do not find Platigage to have much application..but this might be a case where it could help..

I'd be suspecting the rods were not accurately gauged..

Kind regards
David

[Wayne 962]

Did you have the rods resized, like Henry mentioned?

-Wayne

[1972_911T]

No the rods were measured and checked against the dimentions in your book and all were in spec by my measurements.

I am now considering tearing into the engine again to have these re checked by a machinist as they proberbly have more acurate measuring devices than my digiatal calipers.

If an excessive side clearance issue exists would rod resizing deal with this or do they only resize the big end bore and not the rod thickness? Would it also mean that the crank side wall could be excessively worn or is it nomally the rod side that wears?

[Porsche_monkey]

Just going by feel I was sure my con rods were too loose when tore my engine apart. After measuring them they were fine.

I don't want to stop you from being 100% sure of your ow engine, but a tiny bit of clearance can feel like a lot sometimes.

[davidppp]

Sounds like you do need to take the rods off again IMHO.

I do a quick check for ovality and size with a snap guage but the Sunnen rod hone is more accurate..

No way would calipers suffice..

The side clearance is easy to check with a feeler guage, and is not at all critical..( IMHO)
( Some say it has a role in maintaining the oil film..I can't asee this and have built engines with very wide clearances when using odd rods/cranks..with piston centred rods..)

Neither the rod nor the crank side face wear at all unless something is misaligned.

Or the crank has been ground with too wide a wheel..

The need for accurate measuring tools, and great care, for the rod and crank bigends should be emphasised.

Where in UK are you?

Kind regards
David

[1972_911T]

Hi David

Im in South Yorkshire

Steve

[Henry Schmidt]

Originally posted by davidppp
............................The side clearance is easy to check with a feeler guage, and is not at all critical..( IMHO).......................
David

I agree to a point. As with everything, extremes are dangerous but in general side clearance will offer little or no real service issues.

Neither Al Franken or Michael Savage bring anything to the discussion but entertainment. Be careful of extremes.

[1972_911T]

Just out of curiosity is there a remedy for excess side clearence if it is present? of is it new crank and rods?

Steve

[Henry Schmidt]

I would say new rods if the problem is that the rods are too narrow.
Custom rods if the crank journal is too wide.

That said: if this is a low to medium rpm street motor and the clearence is less than .030" run it.

[davidppp]

Henry..

Can I tap your thoughts on this?

Obviously, a clearance of 30 thou is massive by any standard...and I guess you would agree there is no real issue re oiling here?

What in your opinion is the limiting factor..is it the offset load on the piston pin causing the piston to cock?

Thats what I've always assumed?

BTW, it is possible to build up the thrust faces with moly etc..the stuff used on piston skirts can be applied quite thick..the guys in UK who sell the stuff tell me they use it on Titanium rods for this purpose..when putting F1 rods into odd engines..then just machine them back..

But Ti rods have galling problems unless treated on the ide face anyway..

Thanks
David

[1972_911T]

Ok I think im been a bit paranoid here I just rechecked my side clearences and there all within the tight 0.1 - 0.2 mm range, This been an 89 Engine. There is a bit of a difference between some of the rods the two ones that I was concerned were noticably looser by hand were about 0.17mm while the other 4 were tight with the 0.15mm gauge. If this was an 85 engine it would be out of speck for been too tight (0.2 - 0.35mm)

Any one know Y porsche tightend up this range on the 86 - 89 engines?

Wish I could recheck the big ends as easy, its a tough call wether or not to split the case to recheck these.

[SchnellSchweitz]

I am finding a similar issue in my rebuild at the 45lbs torque. All of my rods (big and small ends) were rebuilt to spec and the crank checks out (std/std), but I still get some side-to-side play on all of my rods. Three have a tiny bit more than the others. Infact, they do make contact with the associated crankshaft knife.

I just want to hear if this is normal? It sounds like a tiny bit of side-to-side play is acceptable so long as the measurements are good.

Do new engines come from the factory with a little side-to-side play as well? Just curious.

[BURN-BROS]

Keep in mind, there is a big difference in the "feel" of a rod when it is dry as opposed to having a generous amount of oil between the cheek of the crank and the side of the rod.