The shop rebuilding my 383 stroker (420-440rwhp) say they won't build an LT1 stroker with .0020 to .0025 clearance on the mains. They want to go .0030 on all mains because they say the bearings will live longer and there won't be a hp penalty.

Any truth to this?

 

First the rear main needs more clearance than the front 4.

They must have never used any light weight synthetic oil in motors before.

Bearing clearance goes hand in hand with oil temp and weight.

I don't know why the bearings will live longer with more clearance, if you run to thin of a oil the bearings are not going to live longer and you'll make less HP.

Bret

 

From the How to Rebuild LT1 Engines book:

Crankshaft Main Bearing Clearance #1 = 0.0007-0.0021"
Crankshaft Main Bearing Clearance #2,3,4 = 0.0009-0.0024"
Crankshaft Main Bearing Clearance #5 = 0.0010-0.0027"

I do not know if these are GM specs or what.

 

My mains on my 385 #1- .0025
#2- .0024
#3- .0026
#4-. 0027
#5- .0029

After break in I have used Mobil 1 10-30 in winter and 15-50 in summer.

 

That's not bad for a blown stroker. That's about where I would put them.

Bret

 

The rear mains on the sbc cranks usually are a slightly smaller journal OD on that rear main anyway so they automatically have slightly more clearance back there at the thrust bearing.

Yeah you don't need to run so much clearance nowadays but it probably wouldn't hurt although you may have low oil pressure when the engine is hot with lightweight oil if you have way more like the .003-.004 I see many people run! This in turn might hurt the rods since the oil pressure at the mains is what feeds the rods if you really have low oil pressure. We run around .0025-.0030 on them with no trouble but I'd rather be around .0025 than .0030 usually. The weaker the caps are the more they move around and some people DO run more if they have a lot of power and they see touching between freshens. It all depends on the rpm and the crank and the block and caps.

With "normal" stuff you should be perfectly fine with .0025 or so. I think the general rule of thumb is .001 per inch of shaft diameter and that's what I see most in the industry running from PS to NASCAR to nice street stuff.

Something where the cranks is crap or the crank is distorting under massive load like Fuel cars they really do run crazy .005-.007 clearance with 50 weight + oil otherwise the cranks contact the bearings a LOT plus they run very soft babbit style bearings that can take some contact too and live at least a few hundred revolutions.

 

Straight from Clevitte
Mains--.0027-.003
Rods--.0025-.0027
Cam--.004
They build them they ought to know.
Thats in line with what your builder wants.
This is for their "H" series brgs which it should be the same for most all.
Tight clearances don't give you the oil cushion you need to make the brgs live at a high HP level.
To run these clearances you have to have an oil pump to keep up with them and a pan.

 

The oil "cushion" is really a load transfering device....

A large bearing clearance has a narrow "cushion" to spread the load from the crank on to the bearings with. A tighter clearance allows a wider "cushion" for the bearins to spread the load too. That's the reason you run heavy oil with a larger clearance so you can spread the load out onto the bearing, since the oil develops a wider wedge. The problem as Eric said is that you can starve the rods easily.

For me you get less protection with more clearance than needed and you get much less oil pressure at idle which ends up killing lifters and the like. So it all depends on what you want.

Bret

 

I just talked to him last night and here's what he said: First 4 main journals: around 30 thousandths. Last main journal: around 35 thousandths.

He runs a circle track car and has built a few 10 sec all motor SBFs.

 

He must mean 3 thousandths and 3 and a 1/2 thousandths.

 

I'm sure he does.

Sometimes folks talk in "tenths" rather than thousandths, where .0001 (one ten-thousandth of an inch) is considered "one" so 35 would be 35 * .0001 = .0035. That's convenient when the difference between .0029 and .0034 is being discussed, for example.

IOW, mentally he is shifting the decimal point four places to the right.

We often do a similar thing with a shift of 3 places to the right when we talk about "30 over" (.030 actually) bore or "10 under" (-.010) bearings.

Then there are those of us who suffer from that dreaded malady: decilexiphobia or the improper placement of decimal points. It's pandemic in industry.

 

Annnnnnnnnnnnnnnnnnyways.

Skipping over the decimal lingo, he still maintains that my bearings will live longer this way.

 

hmmmm........wonder if i can get my company's payroll department to develop that problem for my paychecks.

 

I can tell you from experience that if they aren't linehoning for instance a LOT of guys run that much since the bores aren't all in line basically too but .003 isn't gonna hurt anything probably but low rpm idle oil pressure when the engine is hot.

 

Have run 0-20 Royal Purple with these clearances and no ill effects after tear down at the end of the year,Just got to have an oil pump to keep up with it.Engine made 668FWHP n/a then we put a 200 shot on it.
I would run 30 weight with these clearances for everyday use.