Guys I need some advise on bearings..

[cspeier]

First off, I'm a huge fan of .0030- .0035 rod and main clearance. But, I'm in sort of a bind. I'm putting together my 4 cylinder. It's a KRP block, Moldex crank, GRP rods, 15.2 compression, external oil pump, lots of vacuum. SB2.2 head, Hogan manifold w/ Book dominator. Comp deal, 10k plus rpm. Ultra LW oil.

This is my problem. I can only get one type main bearing. FM6689M or a ACL 5M1733P. I have a set of Calico coated 5M1733P. The problem, to me, is I only have .0025 clearance. It goes against what I feel is not enough. With a standard "P", I have .0030. I can live with this, but it's a "P" bearing with this application. THERE ARE NO X's for this, I"M STUCK!

What would you do? Run it with the coated bearings and forget it? Run the non coated bearing. Send the crank back to Moldex and have him build my .001 into it. What makes me sick is I miked the block and bearing and that was suppose to have happened. It didn't..

Need some advise. THANKS A MILLION!!

Chad

[rmcomprandy]

.0030" to .0035" bearing clearances with "ultra Light weight oil", aluminum rods, a strong "MOLDEX" crank and "LOT's of crankcase vaccuum"...???

I also want to read what responses this brings ...

[cspeier]

So you think that's too much?

[rskrause]

Those seem like large clearances. Why do you prefer that big a clearance?

Richard

[cspeier]

Because I've never found anything in running them any tighter. I've also NEVER hurt anything running them at .003-.0035. I know RM lives by this rule. I also know GRP recommends this. However, I'm in a pinch and I really don't want to make a $30k mistake!

I guess I'm looking for some wisdom from experience. My past tells me NOT to run them that tight. Maybe I can read something that might change my thinking.

That's the truth!

[jdeleon]

Chad,
Will there be an interferance problem with the P bearing and and the crank radius?

[cspeier]

Not at all, even the coated bearings are P's. As far as I know, noone makes a H for a Pontiac SD block.

[needforspeed66gt]

Can you get King bearings for that possibly? I would like to see the clearance a little tighter than you run it at. .003" main/.0025" rod would be my choice.

[Ed-vancedEngines]

Chad,
A couple of thoughts.

Unless you have a really large main journal, I think that Brian at GRP would be recommending right in the area of where you are wondering if will work. on normal aspirated engine. If I remember correctly he has given me normal aspirated recommended clearance of .0025 to .003. Call him and ask him.

For many years I have sucessfully been using both rod and main bearing oil clearances of close to .0025 with steel rods. I have even on ocassion gone down to .0022 and up to .0028 with normal applications.

With dead serious high horsepower N2O engines or Supercharged engines using the GRP Rods I am usually wanting close to .0035 because of the extra abuse they have to endure.

You did mention that you are using a light weight oil, so that lube should help quite a lot if you do need to use a little tighter clearance than you are comfortable with.

I do not think the sizes of the rod big end is changing as much as some do think when using the Highest quality aluminum rods, in any application.

You can also polish .0001 or more off the crank journals too.

I am also kinda wondering if it is any problems if you were to mix and match bearings of the different brands, but I have never tried. Whatever you do, be sure that the Radi is clearing the rod bearing.

When using the high priced professional oil pumps like the Titan, Barnes, Peterson, etc and with external and dry sumps with external pumps, we all have a luxury of increasing the oil pressure to make up for the internal oil leak we make when increasing bearing clearances. Those with stock pumps get limited in just how large of clearances they can get by with, and still have enough restriction to allow oil pressures.

You are using external pump and light oil. My suggestion is to go for it or try polishing the crank and then going for it. I have sucessfully in times past spun some pretty tight rpm using the P bearings with sides chanfered.

Ed

[cspeier]

Thanks Ed

This is a Moroso single stage with a Star Pump. The mains are 2.300. The rods are standard 2.0 pin. The rod bearings won't be a problem. However being a queer main, I just might need to run it.

[MadBill]

I'm sure there's a ton of guys here who have found out the hard way what if any changes to make to bearing clearances for aluminum rods, but the physics of it is that the difference in the coefficient of thermal expansion between steel and most aluminum alloys is on the order of 6.5 parts per million.

So if the room temperature bearing clearance with an aluminum rod is 0.003", at 200° oil/rod/crank temp it should increase by 6.5 x 2.0 x 130/1,000,000 = 0.0017", giving a hot clearance of almost 0.005"... (and for any circle track guys, at 270° F., it will be nearer to 0.006"...)

[needforspeed66gt]

The crank grows too...

[Ed-vancedEngines]

BUT'
Bill,
Does the circle of an expanded piece of aluminum grow to be larger in diameter, of does it grow to be smaller? I will agree that the overall mass must surely increase in size.

WE all know the length increases. Those of us who have learned by trial and error have learned we need or like close to .060 for expansion clearances between block, cam and anything the rod gets near. We also have learned that the steel rod will also grow, but not as much. So .040 clearances are pretty much accepted by many of us between steel rods and blocks, cams etc.


Let us let our minds go on a little trip and look at things from different perspectives than usually accepted.

So if you were to cut a hole in an aluminum billet sheet and then heat it, will the hole get larger or will it get smaller? Or can the hole actually stay close to the same size, because of the mass surrounding the material on the perimeter of the hole expanding outward and at the same time pushing around the material of the hole perimeter?

We all know and we all do recogize that all materials do have a different rate of expansion when heated. The question I am posing is in which direction does it grow or expand? Or does it expand in all areas?

So Let us discuss;
Does the hole get larger, or smaller? Or should it stay close to the same, because of the opposing directions of expansion as the material is heated?

Hmm"

Did I open another can of worms

Ed

[needforspeed66gt]

The hole gets bigger. I assume you are asking just to get people to think?

Doesn't matter what material you are talking about, all that changes is the coefficient of expansion.

Installing a ring-gear on a flywheel is an exercise in thermal expansion, you heat the ring until it smokes, then it drops in place...it cools and is seized to the flywheel(assuming the press was correct)

Now in reverse you can install valve seats by supercooling them with liquid nitrogen and they will drop right into place - no pounding needed.

[cspeier]

For what it's worth, the crank is on the way back to Moldex. Just can't run it that tight. Thanks for all the replies..

Chad