I've seen the topic of bearing clearances discussed here at LS1Tech.com in other threads. I've also done several searches (when the search engine works) and tried to see if the subject has been difinitively discussed before and I think that the anser is no. So here is my thread starter.

What considerations should be made in engine build-up regarding main and rod bearing clearances, piston pins, even cam bearings for normally aspirated versus power adders or for vehicles that are primarily street driven on a daily basis versus track-only vehicles. Another variation might be drag racing versus road racing (closed circuit).

Thanks for your input in advance.

Steve

 

TTT,
I've noticed that a number of folks have looked at this post. Does anyone have any ideas about it yet?

Thanks,

Steve

 

This would be an interesting topic, but I don't know that most builders are
going to give out certain specs.

I would love to know how tight quench would be on a pure strip motor, or
how far PTV can be pushed.

One of the tips I've read about is to use oversized main bearings to reduce
bearing speed. Apparently this reduces heat and helps create a "hydrodynamic wedge" (oil barrier) between the crank and bearings.

As for the clearances, I've read typical values of 0.0015-0.002" and this is
matched to the oiling system for proper oil volume and pressure.

 

Depends on your motor your working with.... LS1 aluminum motors are different from iron SBC's.

Same goes with bearing sizes... the larger they are the more clearance you need.

Bret

 

I can imagine a lighter rotating assembly would allow for tighter clearances;
at least that's what I think you're referring to?

How does the bearing clearance change however? Do you mean the
crank to bearing clearance for 0.0015-0.002"?

I would have thought the clearance would need to be in that range to maintain pressure.

 

Here's what got me to thinking about this. I've bought a good used (very low mileage) 6.0 L block that has been cleaned, inspected and measured by my machine shop. Everything is well within tolerance except for one main bearing bore which is .0005 oversize. I'm going to build a 408 out of this block and reuse my heads, etc on my daily driver, NA, truck. My goals are improved torque with OEM durability.

My factory maintenance manual shows the bearing clearances for a new production engine to be:
Main - .0006" to .00248" (approximately .00015" nominal)
Rod - .0007" to .00212" (approximately .00014" nominal)

My Haynes manual says about the same (which I would expect).

Now, I've been reading articles and have queried a few builders and they say that I should should shoot for .0025" to .003" for the mains and .0018" to .0022" for the rods. However, the builders say that more clearance should be allowed for these clearances as well as the wrist pins if a power adder us used.

Note, these clearances are much smaller than what I'm used to on a Gen I or II small block.

What do you think?

Thanks,

Steve

 

Quick calc on LS1 bearing size = 65mm x .0011" = .0028" clearance, but you could go tighter than that. (This is for a Fe block)

Bret

 

Bret,
So is there a rule of thumb that says eleven ten thousands per millimeter?
Where could I find some guidance information on this?
2.8 thousandths sounds more reasonable to me than 1.4 to 1.5 thousandths recommended by the factory. I would think that I would have good oil pressure at 2.8, but I hate to set up a new engine that is in conflict with the GM recommendations . . . assuming that they had a good reason for it in the first place which I'm not sure about.

Any more thoughts?

Thanks again,

Steve

BTW, I'm not trying to be argumentative, but I get 71.5 thousandths on the calculation that you are showing.

 

It's per inch.... I just used the mm measurement because that's what the LS is.

2.559 x .0011" = .0028" clearance.

That's just a general rule to apply to all iron block motors.

I think the GM recomendations are for a Al block not a Fe(iron) one.

Bret

 

I've built several sportbike engines using around .002 clearance rod and mains on 1.400 journals(rod and main). I'm really surprised GM would allow clearances so tight. I have a hard time understanding how oil could properly form it's protective wedge at so tight of clearance.

 

My factory service manual and the Haynes manuals are actually for the cast iron (Fe) blocks 4.8/5.3/6.0 blocks for the 2000 year model. I had initially only checked the 6.0 L but I've gone back and checked the 4.8 and 5.3. It just seems like to me that such tight clearances are a recipie for oil starvation.

I'm still curious to know if any professional engine builders recommend different clearances for power adder engines or road racers. Also what are the rules of thumb for aluminum (Al) blocks?

Thanks,

Steve

 

I've only heard of power adders affecting piston to bore and ring end-gaps since that's where all the extra heat is.
You increase cooling capacity with a forced induction setup (if needed) to keep the coolant temps in the same operating range. Wouldn't this mean the bottom end stays the same temp?

I have heard that you increase the bottom end tolerances for higher rpm operation such as a race motor vs. a street motor but that's related to rpms and not power output specifically.

 

I've been reading some of the guidance on the Clevite 77 (Dana) website http://www.engineparts.com/techbulle...L77-1-205R.pdf. Pretty interesting stuff on daily drivers versus performance applications (although power adder issues are not directly addressed.

I definately believe that adequate cooling capacity for both oil and the engine block and heads is always a necessity.

Steve

 

I'd have to go back to the textbooks to verify, but I believe the 'hydrodynamic wedge' is much thinner than the bearing clearance. As I recall, the clearance mainly determines oil flow rate, and thus cooling effect.

 

I thought the OEM main spec was no more than .0028, and some guy was complaining in a thread I read recently, that that clearance causes low oil pressure problems. He said to shoot for .0015 to keep the oil pressure up. I was talking to a guy in a machine shop that is balancing a clutch for me. and was installing a Callies crank in a LSX block and he told me he was mixing and matching main bearing halves to tighten things up. Go figure.

 

I have put together quite a few LSX engines and have had the best results with .0015" mains and .0018"-.0020" rods. The aluminum blocks grow a ton after heating up and if you don't keep the clearances tight they are too loose after warm up. This will lead to low oil pressure's when warm. A lot of "old school" engine builders loose site of these newer aluminum blocks and there associated parts. This is the problem with most machine shops as there mentality is the older specs.

I also mix a match as needed to get exact clearances. This is why building these motors is not a "evening" build if you take all the right steps. Another tip to consider is letting the block sit at room temperature before checking clearances. They will be different if the block is cold! I noticed this with our Bridgeport Milling machine before we put radiant heat in the shop! I have a digital readout that measures to .0001" and and used to be able to watch the deck grow over a 25 degree temperature differential.

 

Very impessive input, as dubious as some of the information may be on this site, this is refreshing.

 

bigdsz - I agree! I appreciate everyone who is commenting on this thread.

9D9LS - Would you recommend the same clearances on a street driven cast iron block? The block material should make no difference on the rod clearances (assuming that forged steel rods are used in both applications). However, maybe with cast iron, maybe the main bearing clearances should be about .0022", since the coefficient of expansion is much less with iron compared to aluminum. What do you think?

All my best,
Steve

 

Steve,

I run SBC (2.45") Iron blocks at .00225-.0025" so maybe a little higher than that on the LS1 (2.559")

 

Steve,

The interesting thing with these clearance topics is what GM considers to be the "normal" oil clearance range. I believe the mains are range is .0007"-.00212" and the rods are .0006"-.00248". As far as I'm concerned this is quite a spread. I have noticed on the larger displacement (heat building) motors that they are more sensitive to the correct clearance to keep oil pressure from being low when the motor heats up. It is quite amazing how fast the oil temperature rises with a 422 motor vs. a 346 on a light trot around the block. This has to have some impact on the main bore diameter with the aluminum blocks....

I figure that the .0015" mains is a good mid point to keep the clearance at based upon GM's view. I'm sure they more about this stuff then we do! The few Iron blocks I have built where setup with a clearance on the mains from .0015-.0018". There is still growth with an Iron block but not nearly as much with aluminum. The rods are more forgiving as it is subject to the same growth rates regardless.

Back to the oil temperature topic; I believe that the oil temp has a huge impact on how your clearances are setup. I don't even think twice on larger high strung motors to install a good oil cooler system as part of the "package". The hotter the oil both metal growth and oil thinning are working against you.

This is just my opinion from my own experiences. Great topic and I hope we can continue to get feedback!