Odd main bearings in LT1 engines

In disassembling my 96 RMW wagon 140K engine for rebuild and upgrade I pretty much measured everything to to determine what I wanted to replace, change, etc.

Quite a surprise on the main bearings. I used plastigauge on them after just removing the caps and no more disassembly. They checked, roughly, front to rear, .0013,.0015,.0014,.0015,.0015. Plastigauge actual numbers are always iffy, but comparison between bearings backed it up as checking. All bearings looked to be in very good condition. After disassembling it all, I cleaned up the bearing backs and got "interesting" information. All the bearings carried 96 date on them so original.

Number 1 main std/std

Number 2,3,4 mains .0006 upper/std lower

Number 5 main .001/.001

So 2,3,4 are corrected .0003

And 5 is corrected .0010

I have never heard of a factory engine correcting anything less than .001 in the past, and .0003 is nearly nothing.

I went digging for information and found this post on another forum about the issue and it is really quite interesting and backed up by TSB.

Quote :

All 1996-99 5.0L (L30) and 5.7L (L31) engines were built with 0.001 in. undersized number 5 crankshaft bearing inserts.


Engine Bearing Knock Noise (Re-suppport Crankshaft, Select-fit Rod Bearings) #99-06-01-003 - (03/29/1999)
Engine Bearing Knock Noise, (Re-support Crankshaft, Select-fit Undersize Connecting Rod Bearings)
1999 Cadillac Escalade

1996-99 Chevrolet and GMC C/K, G, P Models

with 5.0L or 5.7L Engine (VINs M, R - RPOs L30, L31)

Important
GM Canada Dealers require prior DSM authorization to apply this bulletin.

Condition
Some customers may comment about an engine "knocking" noise.

A condition may exist in some engines where, the crankshaft is NOT being evenly supported by all five crankshaft bearing inserts. In these engines, the number 1, 2, 3, and 4 crankshaft bearing inserts are supporting the crankshaft, and the number 5 crankshaft journal (rear) has excessive clearance relative to the number 5 crankshaft (main) bearing cap insert. In this condition, the crankshaft flexes under load, and pounds on the lower number 5 crankshaft (main) bearing insert creating the knocking sound. The engines were originally built with 0.0006 in. undersize crankshaft (main) bearing inserts in the number 2, 3, and 4 crankshaft (main) bearing locations and 0.001 in. undersize insert in the number 5 crankshaft (main) bearing location. The service procedure listed below addresses the above condition by lowering the crankshaft at the number 2, 3, and 4 crankshaft (main) bearing positions (increased crankshaft (main) bearing size of the lower crankshaft (main) bearing inserts) and raising the number 5 lower crankshaft (main) bearing insert (undersized insert) in order to properly contact the number 5 crankshaft journal surface.

Not exactly how mine is but very close and they did not say really if it was half shell change or both on 2,3,4 or not.

Perhaps this has to do with some of the mystery "knocking" we seen to hear about pretty regularly. My guess is that plenty of the same thing has happened to previous years, but they didn't want to get into fixing them. Didn't even list the application, but it got the fix.

Anybody heard of this before?

Here is the GM TSB that this is referring to,

[You must be registered and logged in to see this link.]

Kind of interesting information.  Definitely says you want to stay on the low end of clearances if you are trying to reduce noise.  It also pretty much gets rid of the larger clearance on the #5 main, which I never really understood on the auto trans cars.  Often needed for the manuals when the thrust bearing is in #5, though, or on #3 if it is there.

The obvious solution for those of us rebuilding or building and engine is to get the main bores straight and all the same size, and the crank similarly all the same journals. All the same clearance, in the lower clearance range for street cars, bigger for more output stuff.

If you've ever tried to find exact replacements for the original springs in a vehicle... you quickly find that the factory had a much larger selection than the aftermarket. It is very likely that crankshaft bearings had a similar situation, where the factory had lots of custom size bearings to allow fine adjustment, while the aftermarket only has std, +0.001, +0.010, +0.020, and +0.030. This is also why people suggest keeping an untouched factory shortblock if you're not increasing displacement, vs. an aftermarket rebuild. If I ever got another low mileage short block, I would blueprint and port the stock oil pump and rear main cap, and leave the rest alone.

In the '96 LT1 with 170k I recently put new main and rod bearings in, the crank measured fine but the bearings definitely had absorbed a lot of trash and had some weird markings, possibly from corrosion from sitting in old oil for a very long period of time (the car was dormant for as many as 7 years).

This is common practice on the newer generation of GM engines. All of my 6.2 and 6.5 diesels had "selective fit" bearings. After the final line boring, the bore is scanned and the appropriate upper and lower bearings are selected. Typically they stamped the specific upper and lower bearings codes on the adjacent bearing bosses.

Bill

Is that TSB for LT1s? It refers to trucks

Swaggerwagon wrote:

Is that TSB for LT1s? It refers to trucks

That is part of the reason this seemed so odd to me. I have not been around the GM stuff all that much, but have many, many, times about the mystery knocking sounds that are attributed to piston slap and other things and it is seemed to be broadly across the entire product line. Manufacturers have been doing some select fit bearings for decades, but it was usually to correct errors and in the more normal undersize type fixes and stamped on the pan rail or elsewhere. No stamps on this engine.

My guess is that they probably used this same fix in many of their engines but never issued a TSB for them so they didn't give customers something to point to when they tried to get the noises addressed.

I did go back and looked again at the wear patterns on the mains I removed and the rear main looked like it had hardly been touched. That would probably mean it was not down on the bottom bearing as they were trying to do, but also not up high either. The other mains all showed light, normal pattern, wear that was quite light for the miles and as is normal a bit more on the lower shell.