[Rick Jones]

Hello all:

I'm getting a snug .012" rod side clearance using a feeler gauge on all journals. This is a stock N crank and Eagle H beam 6.625 rods, and the chamfers are oriented correctly.

Is this too tight of clearance?

Thanks, Rick

[hal101]

i would say yes

[Sirrotica]

If it were my engine I'd be looking for .022-.024 side clearance because it decreases as the rods heat up. You surely don't want the side clearance too tight, and have the sides of the rods seize against the fillets of the crank, or each other.

[gtofreek]

What bearings are you using? If they are performance rod bearings, they will be specific for either the upper, or lower position. If you have a lower shell in the rod, or vice-versa, then you will have tight rod side clearance. I would like to see a bare minimum of .016"-.018" but would much prefer over .020". More doesn't hurt anything either. I once built a BB Chrysler engine that used BB Chevy rods[as per customers recipe]. It had .080" side clearance. I was worried about it, customer wasn't. It's been running for over 15 years like that now. As long as the oil holes aren't exposed, more is no problem.

[Sirrotica]

Originally Posted by gtofreek

What bearings are you using? If they are performance rod bearings, they will be specific for either the upper, or lower position. If you have a lower shell in the rod, or vice-versa, then you will have tight rod side clearance. I would like to see a bare minimum of .016"-.018" but would much prefer over .020". More doesn't hurt anything either. I once built a BB Chrysler engine that used BB Chevy rods[as per customers recipe]. It had .080" side clearance. I was worried about it, customer wasn't. It's been running for over 15 years like that now. As long as the oil holes aren't exposed, more is no problem.

The NASCAR builders run huge side clearances, granted they only run the engine one race, but .045-.090 is the normal range. That surprised me until I read their reasons for using that much side clearance. The builders say wide side clearances have little effect on oil pressure or durability. They are more concerned with low drag and plenty of oil able to flow through the bearing area for cooling. I had it in my mind that side clearances that wide would have a negative effect on oil pressure, builders say that the bearing clearance has much more effect on the oil pressure than side clearance does.

Their contention is that the piston is all that is needed to keep the rod aligned. 20 years ago that kind of side clearance would be met with gasps....LOL

[hurryinhoosier62]

I agree with Paul. Make sure the bearings are in the correct positions and remeasure .012 is simply too tight.

[dataway]

I'm curious .... as a novice engine guy ... I don't see much of a downside to large rod side clearance, but do see a down side to a too small clearance. The rods, if given room (within reason) pick their happy place and stay there correct?

[Cliff R]

I wouldn't worry about .012", a little on the "tight" side for aftermarket rods but you'll be fine there.......Cliff

[Tom Vaught]

Originally Posted by dataway

I'm curious .... as a novice engine guy ... I don't see much of a downside to large rod side clearance, but do see a down side to a too small clearance. The rods, if given room (within reason) pick their happy place and stay there correct?

If you push in the clutch and have proper crank Thrust Clearance they might move from the "Happy Place" a few thousands of an inch and then return to the "Happy Place" after the shifting is done.

Not much of an issue on a Drag Car but a NASCAR engine or a short track engine would see more thrust wear over the race.
Their Duty Cycle is high rpm for many laps vs a quarter mile race and then back to the pits.
You need more side clearance on the rods to allow for that additional oil temp and wear.

We instrumented rod bearings in a running engine for temps and I was amazed how quickly the rod bearing oil temps went up with a increase in engine rpm. Very hot temps. Need to get that HOT OIL out of there asap.

Tom V.

[Rick Jones]

Originally Posted by gtofreek

What bearings are you using? If they are performance rod bearings, they will be specific for either the upper, or lower position. If you have a lower shell in the rod, or vice-versa, then you will have tight rod side clearance. I would like to see a bare minimum of .016"-.018" but would much prefer over .020". More doesn't hurt anything either. I once built a BB Chrysler engine that used BB Chevy rods[as per customers recipe]. It had .080" side clearance. I was worried about it, customer wasn't. It's been running for over 15 years like that now. As long as the oil holes aren't exposed, more is no problem.

Paul, they are the Sealed Power 81555CPA bearings. Not sure if these are side specific so I will take a look. Thanks for the info.

Rick

[mgarblik]

Originally Posted by Cliff R

I wouldn't worry about .012", a little on the "tight" side for aftermarket rods but you'll be fine there.......Cliff

I know you like tight thrust clearance also. With a tight thrust clearance, say .003-.005", the .012" assuming the bearings are installed correctly would live. However, if the thrust clearance was at the outside of the factory tolerance, say .007", that would only leave .005" per rod pair at room temperature, which I think is a little tight. Paul stated .016-.018" as a minimum range, which is what I have always used also. I have never run anything over .030", but that doesn't mean it would be a problem. Sounds like NASCAR runs them much wider than .030". For reference in our race cars with aluminum rods. .020" produces some light scuffing between rods. .022-.023, perfect and look like new on inspection. We run .004-.005" thrust.

[gtofreek]

Originally Posted by Rick Jones

Paul, they are the Sealed Power 81555CPA bearings. Not sure if these are side specific so I will take a look. Thanks for the info.

Rick

1555CPA's are NOT side specific so you're fine in that aspect.

[Cliff R]

I wouldn't assemble one of these engines with main bearing thrust clearances up near the upper end of the range. I consider .008" the absolutely maximum acceptable end play for the crank. Most of the Pontiac builds we've done here have been around .003-.005".

Many years ago I put one in service that was a little looser and guess what, it knocked the thrust out of it in about 6 months of street driving. I still remember not having a "warm and fuzzy" with that engine, and wasn't comfortable putting it together with that much end play. One of the few times in my life I didn't stick with my instincts on something got bit in the back side pretty hard for it.

Good thing it was my own engine, a 428. Since then I've stayed down on the lower end of the range and zero issues.....IMHO......Cliff

As for .012" side clearance on the rods, with .003-.005" main bearing clearance it will run forever like that, and have really good oil pressure as well.......Cliff

[unruhjonny]

incase anyone cares, this is from the 1970 Factory Service Manual:

[gtofreek]

Having thrust clearance over .008" isn't a problem as long as it isn't much over. I've built at least 1-2 dozen Pontiacs with thrust play in the .008"-.012" range with never a single problem with any of them to date. The real problem is with the thrust bearing itself. Both Cleavite and FM thrust bearings have an issue. They just don't know how to make a thrust bearing. I've complained about this many times over the years. ACL has got it figured out, but unfortunately, they don't make bearings for a Pontiac. Never tried a King bearing yet so I can't comment on them. The problem with these thrust bearings is the flanges measure .003"-.005" different in their thickness from one side to the other. Both bearings will measure the same, and they will vary .001"-.002" in thickness on the same side if you measure the different segments around the flange. The .003"-.005" difference from side to side wouldn't be a problem if they were both either thick or thin on the backside, but they are not. Each bearing is made the same, so when you flip the one over to mate it to the other, one is thick on the backside while the other is thin. If this is left alone and just installed in the engine, the thicker side will be absorbing ALL the thrust pressure and will burn up. As it burns up, it takes out the other half with it, so it ends up just looking like a thrust failure. The thrust faces need to be sanded down to match each other on the back side. Front side doesn't matter as it sees no real pressure. So this means I first sand the thinnest flange down, using my fingers to exert more pressure in the thicker parts to even the flange out along its length. I take off only enough to get the thick parts the same as the thin parts. Then I sand the thicker thrust bearing rear flange to match the one I just did. Usually there is a .003"-.005" difference that needs to come off. The front side of each bearing, I just lightly sand them just to clean up any burrs or small high spots that might be present.

Then, the most important part of this all is once installed in the engine, I do not use a dial indicator on the front of the block to measure crankshaft endplay. Instead, I use a feeler gauge on the rear side of the thrust bearing to measure the clearance that way. By doing this, I can check the clearance on each half of the rear side of the bearing to see if they are equal. If I just use a dial indicator on the front, it will only tell me how much clearance I have on the thickest side of the flange bearing. It will NOT tell me if both halves are equal, and if you don't sand your bearings to match each other, I guarantee you, they will not have the same clearance between halves.

[SRR]

I have to ask, which rod grows more as it heats up, a forging or a casting?

[gtofreek]

I would venture to say a forging would expand slightly more since there is more material compressed into the same area. More mass, more expansion.

[mgarblik]

Originally Posted by gtofreek

Having thrust clearance over .008" isn't a problem as long as it isn't much over. I've built at least 1-2 dozen Pontiacs with thrust play in the .008"-.012" range with never a single problem with any of them to date. The real problem is with the thrust bearing itself. Both Cleavite and FM thrust bearings have an issue. They just don't know how to make a thrust bearing. I've complained about this many times over the years. ACL has got it figured out, but unfortunately, they don't make bearings for a Pontiac. Never tried a King bearing yet so I can't comment on them. The problem with these thrust bearings is the flanges measure .003"-.005" different in their thickness from one side to the other. Both bearings will measure the same, and they will vary .001"-.002" in thickness on the same side if you measure the different segments around the flange. The .003"-.005" difference from side to side wouldn't be a problem if they were both either thick or thin on the backside, but they are not. Each bearing is made the same, so when you flip the one over to mate it to the other, one is thick on the backside while the other is thin. If this is left alone and just installed in the engine, the thicker side will be absorbing ALL the thrust pressure and will burn up. As it burns up, it takes out the other half with it, so it ends up just looking like a thrust failure. The thrust faces need to be sanded down to match each other on the back side. Front side doesn't matter as it sees no real pressure. So this means I first sand the thinnest flange down, using my fingers to exert more pressure in the thicker parts to even the flange out along its length. I take off only enough to get the thick parts the same as the thin parts. Then I sand the thicker thrust bearing rear flange to match the one I just did. Usually there is a .003"-.005" difference that needs to come off. The front side of each bearing, I just lightly sand them just to clean up any burrs or small high spots that might be present.

Then, the most important part of this all is once installed in the engine, I do not use a dial indicator on the front of the block to measure crankshaft endplay. Instead, I use a feeler gauge on the rear side of the thrust bearing to measure the clearance that way. By doing this, I can check the clearance on each half of the rear side of the bearing to see if they are equal. If I just use a dial indicator on the front, it will only tell me how much clearance I have on the thickest side of the flange bearing. It will NOT tell me if both halves are equal, and if you don't sand your bearings to match each other, I guarantee you, they will not have the same clearance between halves.

This is exactly how you deal with thrust bearings. I use a surface plate and oil soaked wet paper to equalize the clearance. Follow the tips above and no thrust issues. Began using the King Race bearings last year. Very impressed and they are closer.

[Rick Jones]

Thanks to all for the information. This forum is outstanding because of all of the experience and knowledge shared!

I remeasured and have two journals less then .012 so time to do some more work to make it right.

Thanks, Rick