Hi all.

I have a question about determining when an aluminum rod may be at the end of it's life. I tore down my motor due to a main bearing failure. As I've progressed to the rods, I noticed they have opened up from their original state. The ID bore of the big end of the rod is slightly bigger, most are about .0004-.0006 larger. That measurement was taken from the vertical axis, from around the bearing pin. The horizontal axis is much tighter. Is this normal?

What has the experienced builder determined as to how much these thing will open up before failure? Also, one rod is nearly .002 bigger on the vertical axis and +/-.004 on the horizontal axis. This one I think is toast.

They are MGP billet rods. Cheers.

 

how much service have they seen?

 

About what would be equivalent to 60 passes, unfortunetley, all but about 5 were on the dyno. Motor is tuned to 700 give or take a few at 26psi.

 

ouch. they really shouldn't be toast though, unless you are making gods chariot power on the dyno constantly.

 

It wasn't by choice to make that many, tunning issues racked the count up.

 

Wow, you may want to give the guy's at BME a call. I know they can make a rod that will stand up to the abuse.

http://www.bmeltd.com/

 

There's absolutely nothing wrong with MGP rods. In my opinion there one of the best rods out there right now..

 

Are those the rods you are running Enzo??

 

maybe...

 

This

 

Btw to the original poster I would replace those as there getting beat up..
Blaming the rod is like blaming the piston for melting..
There's other reasons why this is happening..

 

Could a machine shop cut the caps down and re hone the big end opening?

We've never had it done to an alum rod but we have it done all the time to forged rods that are egged or have had bearing issues.

 

Clearances?

 

Why did the main bearing fail? By chance due to lack of oil?
What did the rod bearings look like?
What fasteners do these rods use?

The most common reason I can think of for failure would be heat or bad tune, but you should see that in the bearings....

 

they are aluminum ya kno they do wear out they dont last for ever

 

Let me try to get to some of these responses.

Yes, I'm working on that one, but no definitive reason only much speculation.


I'm not sure if these can be reworked as the mating surface on the cap is serrated. I think that's what it's called.


.0028 across all rods. perhaps on the snug side as MGP recommends .003 - .0035, when it goes back together it will have .003.


I lost the no. 3 bearing which was caused by a thrust bearing failure. Part of the thrust made it's way into the main. Lucky for me I had just finished making the pass and upon return to the pits notice my oil pressure had dropped to 60 and wouldn't go any higher. Racepak data showed oil pressure drop just before I let off, from 90 to 60. Lucky me, literally. Come to find out my tranny pump put out too much pressure and pushed the converter forward and waxed the thrust.

The rod bearings were on their way out. There isn't an aftermarket bearing for the J32 Motor that I have been able to find and so I'm stuck with factory bearings on this motor until the new motor is done. That one will be special. One or two of the rod bearings looked OK, one took some debris somehow when the thrust took a dump and was pretty scored, the others looked flakey or somewhat pitted if I had to describe it. If I can get 50 or so runs out of factory bearings, I'll take it.

I don't have the experience to read bearings, but will chalk this one up as a learner. The fasteners used were provided by MGP, that was not the problem. I can say the bearings did not see extreme heat as all the rod bearings had the appropriate crush as did all the mains except for the one that bit the dust. There was no discoloration.

..

 

Are you using a more aggressive R/S ratio?
What did you rev to?
Were you able to see which side of the rod elongated? I know it's the big end, but cap side or rod side?
Were the rod bolts torqued or stretched?

Sorry for asking so many questions. Just trying the good old process of elimination.

It's pretty interesting that they have all been stretched or elongated evenly....

 

Well without looking at any of the parts it's hard to say without guessing.
But If I was to guess lol. I would say the fact that you say your using factory bearings I would guess they are very soft. So now with a tune that u say was having some issues and required alot of Dyno time I'm going to guess you put a good beating on those bearings and pounded them. With pounded bearings now the rod housings was getting pounded.

I would replace all rods and now start monitoring bearings after every event or after every couple of runs until you get your tuneup perfect.

You might wanna get a billet crank with a more common rod journal so bearing choice isn't a problem either. You should definitely be using a race bearing of some sort.

And lastly listen to what Mgp recommends for clearances. If he stated .003-.0035 then run that. Too much is better then not enough.

 

Are u running a dry sump?

 

Not only are they very soft, they are very thin as well. It's remarkable that they held up like they did. I'm with you on following MGP's instruction but .0002 diff couldn't make that much difference and the bearings didn't show a tight situation. But when it goes together it will have the .003. next motor will have the billet crank setup to use off the shelf bearings. yes, it is a dry sump.

Didn't change the ratio at all. After reading up on it, rule of thumb is your rods should be 1.75 times the length of your stroke. I wish my rod was 1.75 times the length of my stroke...hehe.

Bolts were stretched, not a clamping issue. Most measurements were consistant, just the one that came in .002 over what the others measured at.

 

Give the manufacture a call and ask them directlly at what temperature you should run them at. That will affect the rod in more ways than you can think.

Get another set, replace them and send them to the manufacure for inspection.

Stefan