I've heard so many times that when LT1's are upgraded by better heads and cams that there is a tendency to spin a bearing or two. Why is that? Is it because the rpm is increased? Is it poor bearing design? Or is it the balance problem with the LT1 bottom end?

 

You know, I've heard that but I've not personally seen that. I've taken apart engines that were heads/cam with 150,000 miles on them and they looked fine. I'm certain that some have upgraded and had spun bearings, and I'd guess that some have put on a heads and cam and then got hit by a truck a week later also. It's just a small block Chevy. Folks have been putting heads and cams on these things since the 50's.

Since a lot of these engines are almost 20 years old maybe their bearing clearances are opening up some and the added power is just the straw that broke the camels back. I know a lot of people have had success but I would call that luck. If you're going to put a H/C combination on an old engine you're asking for trouble.

 

usually people dont touch the bottom end when upgrading heads/cam. (for the most part)

WHen you get the added power, people drive it harder. If the bottom end wasnt touched and is a bit tired, itll show faster with the added hp and more spirited driving, but may not have shown if the heads/cam were not upgraded.

 

I know many LT1's that go many miles with no problems.My take on the spun bearings is that Chevy changed there rod design to powdered metal and these rods do not hold up to the rpm's that the LT series motors are capable of.

 

So the LS1, LS2 and LS3's aren't capable of revving? They use PM rods up to what.......430 Horsepower?

 

I don't think it has anything to do with the stock balancing, it's decent from the factory. The clearances are failry tight on the LT1, this may be the main cause. But, probably many fail from a bad install. You get many doing cam changes that have never touch an engine.

 

I have always been suspicious this has been related to people going to higher oil viscosity on the stock bottom designed for lower oil viscosity....

...plus then exceeding the rpm capabilities of the stock LT1 bottom..

???

 

I know one of the differences between the LT1 and the LT4 engine is the crank and piston design. But the change in crank design is mostly for durabilty and strength. The piston design is just to increase compression ratio and allow for the different ring design. I don't see any differences in the bearing design. So, maybe it is the old or worn out bottom ends that are the cause of bearings spinning.

 

Well I just spun/ locked up a STOCK LT1 engine yesterday at the track.
It will be a few months before I pull it apart, but I'll bet it had a moment of low oil flow and zip bang boom there goes the engine.

I'll come back to this post in a few months and let ya'll know what happened.

 

Those sweeper turns are not friendly to the normal oil pick up system of the LT1/4. At one of the driving schools they told us to add an extra quart of oil to help, but a dry sump is better. I've backed off of the road course and autocross weekends as it got too expensive. I'll be watching to see your results.

 

I bet the LS series rods are a different design than the LT1's

 

I bet it is more road course than auto-x causing oil starvation. You don't stay in one direction long in auto-x. I was told to run .5 qt. extra to help address this issue.

 

seen it on an oval track

seen it when one was overreved

so it's true

 

Yep, I keep an extra qt in it, must have burned part of it off and it was a little under 1 qt over when it happened.
Oh well, done for the year. We'll see what the fundage looks like for next year.

Dry sump for the LT1 is an expensive and rare beast. North of $4k easy

 

I've been reading this forum for several years and the LT "spun rod bearing" stories are numerous, so numerous, that there must be some common thread; and that is:

because LT oiling systems suck.

everything was sacrificed on the almight altar of fuel economy and the oilins system was its first victim.

(1) one thing I noticed with my LT is that the oil pressure is extremely sensitve to temperature (regardless of what weight fully synthetic oil I run) and of course on the LT engines the oil cooler was eliminated;

(2) the factory recommends oil that has the viscosity of tap water...this is supposed to "free up horsepower" but again I believe it done in the interests of (again) fuel economy;

(3) everyone believes 10 psi pressure for every 1000 rpm is adeqate. Oh come on now. For the sake of argument, when my engine is pulling a vehicle on the freeway at 80 miles an hour for hundreds of miles at a time, I'm supposed to feel confident with 18 psi oil pressure?

(4) don't even get me started on the 1960 era flat oil pan baffle the factory installs; much more modern curved and louvered windage trays are available ; they control the oil better and they DO free up horsepiower;

(5) so many people shy away from using a high volume, high pressure pump in these engines; citing that they rob power (what three to five horsepower at the most ?) and that they put too much wear on the cam gear and oil pump drive stub gear. Just for the record, small block chevys and big block chevy's both use the same oil pump drive gear, so either every big block chevy has a worn out cam gear and worn out distributor gear or this argument is fallicious. Granted, given the shallowness of a corvette oil pan (6&7/8ths inches, plus pan gasket) fitting an high volume pump is not always easy, but there are pump and pick up combo's specifically designed for these applications.

(6) most aftermarket cranks are crossdrilled, the stock crank is not. this means that with the stock crank a rod journal only receives oil through 180 degrees of its rotation; with a cross drilled crank the rod journal is oiled through the entire 360 degrees of rotation.

(7) the main purpose of any oil system is to get oil to the bearing; you can have all the oil pressure in the world, if the oil is backed up by a series of resrtictions and not getting to the bearings it aint doin its job. For better understanding of how to maximize the chevy oiling system, go back to the days when NASCAR stock cars ran STOCK engines.

Suggested reading: RACING The Small Block CHEVY, 4th ed
by John Thawley
A STEVE SMITH AUTOSPORTS PUBLICATION

which devotes extennsive discussion to eliminating various restrictions within the block iself; and,

CHEVY PERFORMANCE, volume 1
by JOHN MICELSEN
S-A DESIGN BOOKS

"HOW TO BUILD A 383 CI SMALL BLOCK FOR STREET AND/OR STRIP SUPREMENCY" at page 122:
" To insure an adequate oil supply to the main bearings (and thence to the rods) Joe drills the oiling passage in the (main) bearing to match the diameter of the oiling holes in the cylinder block"

'nuff said.

 

After the last post by mtwoolford, I need a high volume oil pump, a new after market crank with cross drilling and then drill the oil passages to match each other. Maybe all this is why GM dumped the LTx motors. I'll have to think about this new wrinkle in my build plan, but it looks like I'll just balance the bottom end and install new bearings after the crank is polished. It seems like I take one step forward and get a two step back penalty.

 

Housing bores do go out of round a a very basic thing that needs checking whenever doing a rebuild as well as actual diameter to insure proper bearing crush. The rod and main bolts can and do fatigue and can easily cause a bearing to spin from lack of bearing crush.

 

Ddahlgren has hit on the right reason for the LT motors bearing failure.Compare an old style rod to the new style powdered metal rod.The bottom cap on the powdered metal rod is very thin and I feel is more susceptible to rod stretch thus losing the bearing crush which leads to spun rod bearings.