LS race engine bearing clearance suggestions

[Racer Buzz]

Corvette Team Members, Can I get some feedback from those in the know on the suggested clearances for both the main bearings and the rod bearings in an LS series race engine running an Aviaid dry sump oiling system? Anybody have a connection with a LS series real race engine shop where they might share their suggestions that you could then pass along? The GM factory specs says: Crankshaft Main Bearing Clearance - Production 0.0008-0.0021 in Crankshaft Main Bearing Clearance - Service 0.0008-0.0025 in but that seams like a pretty big spread. My engine guy says my motor is running .0022 to .0025 and I have now lost the #4 main bearing twice. Once in a wet sump 383 CID motor and now again in a dry sump 405 CID setup. Strange that the main would go before a rod bearing as the mains are fed first! And if the oil feeds from the rear of the engine also strange that the second to the rear main brg would go before the front main or front rod brgs. This is actually a continuation of my earlier thread on my race setup oiling system problems. I've received a lot of helpful input and suggestions from the members on several new ideas to incorporate but I'm now struggling with what clearance to run in the rebuilding of my 405 CID motor. I certainly appreciate any input. __________________

[Deakins]

Has this block ever honed on the bottom end? It is very strange that you keep loosing the same main bearing.... Can you see any problems with the other bearings that are coming out of this one? If not I would look at some of the components for the cause, like the block or the crank.

[Racer Buzz]

Yes, both of the blocks (the old 383 and the new 405) were line honed to fit the crank as a matter of procedure. I think it probably has something to do with the current .0022 main bearing clearance that the shop set. Perhaps it should be even tighter like .0015 to .002 as the GM spec is .0008 to .0025.

I'm looking for what some other LS race engine builders are running in a high HP road race motor. Mine is 700 HP with a dry sump.

Thanks for your suggestions.

[ralph]

Tighter would be worse IMO. At that power level you should have at least 2.5, but i don't think 2.2 would kill it either. And what do you mean you lost it? Did it spin or burn or fatigue, all of the above? Did the bearing give any clues as to what happened. Usual failures are due to oil stavation, misaligment or clearance (lack there of).

[Racer Buzz]

Ralph,

By lost it I mean the oil pressure finally went to zero during the race after slowly fading down from 65# and the bearing turned in its journal. I did not see the problem #4 main bearing just the signs of it turning in the block. Pictures available. But I cannot post pics. The motor was align honed and the bearings measured and fit. As a side note my engine builder says he has only recently discovered that Chevy CAM bearings have considerably tighter clearance than the aftermarket CAM bearings and he feels that was a contributing part of the pressure loss problem.

What are your thoughts on the main bearing clearances growing more in an aluminum block over an iron block and if so by how much? If the aluminum block expands more / faster than the steel crank does what happens to the clearances if they start at .0022? Does it grow by a thousanth to .0032 or more? If Chevrolet says .0008 is OK on the tight side what does that tell us we should be doing to keep the oil pressure high as the bearing clearances grow while the oil temp hits 240 to 260 degrees and up. 240 is probably more than what a typical street engine sees and consiquently there is more clearance in a hot race engine and then more oil loss occurs and then I end up with lower oil pressure.

Am I going anywhere with that thought process? I'm thinking the mains should be on the tight side say .0015 to .002 in an aluminum block race motor that also sees higher oil temps like you find in road racing.

[ralph]

Buzz, i didn't read your title carefully enough and didn't realize this was a LS application. I have no experience with those engines, but generally in performance applications, you what a little more clearance than a stocker. And if you're going to Err, giving it more is much better than not enough.

[CFI-EFI]

I too, am not familiar with the LS series engines, nor working with aluminum blocks. However, reading the posts makes me wonder about the bearing fit into the align honed aluminum block. You might want to take a look at the bearing to bearing saddle fit and the bearing "crush", as well as the bearing to crank journal oil clearance. I may be wrong, but with the oil clearances you've posted (they sound good), I think you might be looking at the wrong side of the bearing shell, especially in an aluminum block. For what it's worth.

RACE ON!!!

[TimAT]

The new Chevy Power book has a sidebar written by Katech Performance- details the info on the LS engines and how they make them live. The article could be titled "All you ever wanted to know about LS engines- but were afraid to ask". I don't even have one and it was a real eye-opener.