Bearing clearances for high rpm sbc?

General engine tech -- Drag Racing to Circle Track

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Fireonthemountain
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Re: Bearing clearances for high rpm sbc?

Post by Fireonthemountain » Wed Oct 25, 2017 1:37 am

MadBill wrote:
Tue Oct 24, 2017 7:00 pm
Fireonthemountain wrote:
Tue Oct 24, 2017 1:32 pm
...And what's the actual harm of bigger clearances?? Oil pressure at idle really matters little, as long as it jumps up with the rpms to good pressure numbers...
The load capacity of the "hydrodynamic wedge' that supports the crank and rods drops rapidly as the clearance increases, so it's important to not run much more than enough to ensure distortion doesn't cause parts to 'ground out'. i.e., the ideal rod/main clearance in a 500 HP @ 7,000 RPM SBC might be 0.002"/0.0025", but the identical parts in a 1500 HP blown 8500 RPM application might need a thous or more greater.

On the other hand, the flow to maintain sufficient pressure will also increase with more clearance, thus increasing the oil's capacity for carrying off heat.. :-k
Keeping the rod side clearance tight, which is the opposite of a lot past thinking on the subject, should slow the flow back down while and forcing more oil to pool in the larger clearances. But if you go bigger clearance and wide side clearances on the rods, then yep you might have an issue of losing the pool and the oil wedge and pressure.

I know of a well known builder that does this narrow side clearances with larger bearing clearances, even on his EMC entry and his race engines. Now this is with Oldsmobiles that are famous for losing bearings, as mainly cast cranks distort at rpms especially with their long strokes on the BBO and larger bearings. But I would think, if it works on them it should work anywhere. Yes, I know that makes me a heretic, but someone has to be.

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Re: Bearing clearances for high rpm sbc?

Post by rebelrouser » Wed Oct 25, 2017 11:59 am

In my experience with a stock block, and stock type crank, that you are most of the time building to about twice its originally engineered horsepower level, loose clearances and thicker oil makes them stay together. The block and crank move around because of the stress .003 on the mains, and .002 on the rods is what I shoot for. with 10w 40 oil or even 20w 50 during hot summer days. If you have a good stiff aftermarket block, good stiff aftermarket crankshaft, then the .001 rods and .002 mains or even tighter with thin oil works good with those builds. I build 4 or 5 engines a year, and most of my customers do not have the money for the really good parts. My personal race car has a 440 Chrysler world products aluminum block with Thompson aluminum rods and both the block and rod manufacturer said to use .003 clearance on rods and mains. In it I run 20w 50 racing oil and it holds 90 lbs cold and about 30 lbs coming up the return road, bearings look like brand new every time its apart to check it out. So I don't think their is one clearance that works with all engines, recommendations from manufactures and experience have to come into play for the final decision.

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Re: Bearing clearances for high rpm sbc?

Post by af2 » Thu Oct 26, 2017 7:48 pm

I am still looking for the high RPM?
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Re: Bearing clearances for high rpm sbc?

Post by houser45 » Sun Oct 29, 2017 8:17 am

.001"per inch, no reason to be any looser than that. The oil pump will work harder and the crank will be whipping all the excess oil around. .002 to .0025 on the mains and .002 to .0022 on the rods. Guys that say it needs to be looser either cant measure clearances accurate enough, Cant machine the clearances accurate enough or both. If you open up the clearances to .003 on both all that will happen is more oil wasted being thrown and you Lose the better oil wedge you had at .002. And the more oil being spun around by the crank= higher oil temps/windage and loss of hp. It would work with .004 on both but will it work? yes

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Re: Bearing clearances for high rpm sbc?

Post by Warp Speed » Sun Oct 29, 2017 9:02 am

houser45 wrote:
Sun Oct 29, 2017 8:17 am
.001"per inch, no reason to be any looser than that. The oil pump will work harder and the crank will be whipping all the excess oil around. .002 to .0025 on the mains and .002 to .0022 on the rods. Guys that say it needs to be looser either cant measure clearances accurate enough, Cant machine the clearances accurate enough or both. If you open up the clearances to .003 on both all that will happen is more oil wasted being thrown and you Lose the better oil wedge you had at .002. And the more oil being spun around by the crank= higher oil temps/windage and loss of hp. It would work with .004 on both but will it work? yes
It's a vicious cycle. That is why it is important to have properly machined parts that are designed to the stress level present.
As mentioned, .001 per inch of shaft diameter will be more than adequate for the OP's rpm goals and parts.

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Re: Bearing clearances for high rpm sbc?

Post by panic » Sun Oct 29, 2017 9:56 am

.001" per inch of shaft diameter: 1914

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Re: Bearing clearances for high rpm sbc?

Post by stealth » Sun Oct 29, 2017 12:22 pm

I kinda like this guys way of thinking:


DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“Almost without exception, engine problems are the result of insufficient clearances.”

We are often told to think big if we want to achieve success, but today I want you to think small. In fact, I want you to think about clearances in a racing engine that are smaller than one thousandth of an inch. These minuscule measurements can mean the difference between long life and sudden death for a racing engine.

Every year we see hundreds of motors come through Reher-Morrison Racing Engines. Some are built by professionals, others are assembled by racers in home workshops. Almost without exception, the problems that we find are the result of insufficient clearances. I’ve come to the conclusion that a little extra clearance is infinitely better than not enough clearance.

There are hundreds of parts in a racing engine. Most of them rub, slide or rotate against another component. In a perfect world, there is no metal-to-metal contact between parts; instead, chains of oil molecules separate the two like atomic roller bearings. (The petroleum experts may disagree with my analogy, but that’s my layman’s explanation.)

A hydrodynamic wedge of oil can withstand thousands of pounds of pressure while producing very little friction. But if this lubricating film breaks down due to inadequate clearance, friction and pressure create heat that microwelds the two surfaces together. As the parts continue to move against each other, pieces of metal are ripped from their surfaces, further accelerating the destructive process. Eventually the two parts seize, producing the blackened and burned appearance that we see in spun bearings, scuffed pistons and galled lifter bodies.

So how can engine builders keep the hundreds of parts in a motor from reaching this dire condition? By providing enough clearance for the lubricating molecules to do their job.

The cardinal rule of engine building is to measure every clearance. The one dimension that you assume is correct is the one that will destroy your engine. At our shop we use build sheets to record every vital dimension – main bearing bore diameters, bearing thicknesses, crankshaft journal diameters, connecting rod bores, and on and on. Whether you build one engine or 50 engines in a season, filling out a build sheet reminds you to check every piece that should be checked. If you don’t want to design your own assembly records, the Chevrolet Power manual has sample sheets that you can photocopy and use for your engine building projects.

I encourage you to purchase a good set of micrometers, a dial indicator and calipers. The price of precision tools has dropped dramatically, and good tools are an investment that lasts a lifetime. If you prevent just one catastrophic failure by finding an out-of-tolerance clearance, you’ve paid for the cost of your tools.

All manufactured components are built to tolerances. By checking parts before assembly, we learn exactly what those tolerances are. For example, your machinist may finish your block’s main bearing housings on the small side of the specified plus-or-minus dimension. The crankshaft you bought from a friend may be on the high side of the range of outside diameters. Both parts are within their respective tolerances, but in combination they can produce bearing clearances that are dangerously tight. It’s better to find a problem like this on the workbench than to find it when pieces of bearing are lying in the bottom of the oil pan.

One of the persistent problems we encounter is piston ring end gaps that are too small. Many racers are squeezing the end gaps in order to improve leakdown test results. Well, if you want to see zero leakage on your meter, just put O-rings on your pistons. But if you want to make power where it counts – on the race track – you must have enough clearance to prevent the ends of the piston rings from butting under actual operating conditions. Even if you don’t reach the point of damaging the cylinder walls, you risk impairing the ring seal if the end gaps are too small. In our Pro Stock engines, we run generous .028-inch gaps on the top rings. Believe me, if I thought there was power in tighter end gaps, I’d reduce the clearance in a heartbeat.

Wrist pin galling is becoming an epidemic in racing engines, and I believe the causes are insufficient pin clearance and inadequate lubrication. Today’s dry-sump oil pans are very efficient at scavenging oil, and vacuum pumps further reduce the amount of oil circulating in the crankcase. The unintended consequence of these improvements is that the wrist pins, which depend entirely on splash lubrication, are now starved for oil. We mill slots in the cheeks of the connecting rods in our Pro Stock engines to direct a spray of oil to the underside of the pistons; this both cools the pistons and lubricates the pins.

Valve guide clearances that are too tight are another common problem. I think it’s a mistake to reduce the valve guide clearance in the hope that you can control where the valve seals on its seat. In fact, the valves are bending, flexing and bouncing like crazy at high rpm. I’ve never seen a valve stick in its guide with .002-inch clearance, but I’ve seen lots of them stick with tight clearances. My advice is to play it safe and open up the clearance; you might even find a little power.

It’s become fashionable to brag about tight piston-to-wall clearance because this supposedly stabilizes the pistons and improves power. But where are you measuring the clearance? One piston manufacturer may specify the clearance under the oil ring, while another might specify it at the skirt tang. With tapered and barrel-ground skirts, where you measure the piston diameter makes a huge difference.

If you see heavy outlines of the piston skirts on the cylinder walls, that is a clear warning sign that the piston-to-wall clearance is too tight. From the first day that I installed a set of custom pistons, I’ve found that a little extra clearance never hurts. More clearance definitely adds a safety margin in case you misinterpret the manufacturer’s measurement instructions – and in most instances, it increases power, too.

You do check your lifter clearances in the block, don’t you? Aggressive cam profiles and steeply angled pushrods impose high side loads on the lifters. You can help keep them alive by making sure that they have adequate clearance in their bores. Repairing a galled lifter bore isn’t cheap, and the damage done by a broken lifter is never pretty. Please do me a favor and check your lifter clearances.

Building an engine is like painting a car: the real work is in the preparation. A drag racing engine only runs in competition for a few seconds at a time, but it can take days to check and assemble it properly. Whether you are a professional or a novice, it’s time well spent.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man.

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Re: Bearing clearances for high rpm sbc?

Post by MadBill » Sun Oct 29, 2017 5:14 pm

Reher's mention of rod cheek reliefs to improve pin lube rings a bell, in that I have seen same in 60+ year-old designs but not in modern production/race engines. Anybody got anything to share on the subject?
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Re: Bearing clearances for high rpm sbc?

Post by Walter R. Malik » Mon Oct 30, 2017 1:26 am

MadBill wrote:
Sun Oct 29, 2017 5:14 pm
Reher's mention of rod cheek reliefs to improve pin lube rings a bell, in that I have seen same in 60+ year-old designs but not in modern production/race engines. Anybody got anything to share on the subject?
Older O.E.M. V8 connecting rods had some kind of "pee" slot or hole in them to lubricate the pin and cylinder on the opposite bank.
Over the years they went away and newer rod bearings which fit those older rods don't even have a relief for those slots or holes, anymore.
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Re: Bearing clearances for high rpm sbc?

Post by cjperformance » Mon Oct 30, 2017 2:16 am

Walter R. Malik wrote:
Mon Oct 30, 2017 1:26 am
MadBill wrote:
Sun Oct 29, 2017 5:14 pm
Reher's mention of rod cheek reliefs to improve pin lube rings a bell, in that I have seen same in 60+ year-old designs but not in modern production/race engines. Anybody got anything to share on the subject?
Older O.E.M. V8 connecting rods had some kind of "pee" slot or hole in them to lubricate the pin and cylinder on the opposite bank.
Over the years they went away and newer rod bearings which fit those older rods don't even have a relief for those slots or holes, anymore.
On a couple of occasions i have run an engine with no oil pan fitted and the pump picking up from a seperate container. It is a bit of an eye opener (and an oil shower :lol: ) to see just how much oil flys out off of the crankcase even at idle speed, let alone with a few more rpms.!
Piston squirters, edm'd rods etc to get the oil exactly where you want it fine and of course are quite different to just oil throw off. But there is also an amazing ammount of oil just getting tossed around in there.
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Re: Bearing clearances for high rpm sbc?

Post by Amilcar » Mon Oct 30, 2017 8:21 am

cjperformance wrote:
Mon Oct 30, 2017 2:16 am
Walter R. Malik wrote:
Mon Oct 30, 2017 1:26 am
MadBill wrote:
Sun Oct 29, 2017 5:14 pm
Reher's mention of rod cheek reliefs to improve pin lube rings a bell, in that I have seen same in 60+ year-old designs but not in modern production/race engines. Anybody got anything to share on the subject?
Older O.E.M. V8 connecting rods had some kind of "pee" slot or hole in them to lubricate the pin and cylinder on the opposite bank.
Over the years they went away and newer rod bearings which fit those older rods don't even have a relief for those slots or holes, anymore.
On a couple of occasions i have run an engine with no oil pan fitted and the pump picking up from a seperate container. It is a bit of an eye opener (and an oil shower :lol: ) to see just how much oil flys out off of the crankcase even at idle speed, let alone with a few more rpms.!
Piston squirters, edm'd rods etc to get the oil exactly where you want it fine and of course are quite different to just oil throw off. But there is also an amazing ammount of oil just getting tossed around in there.

Have you or anyone here, plugged a clear hose on the drain plug, leave the other side open and high to vent? On an intent to see how much oil level drops with a running engine ?
Does it works?

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Re: Bearing clearances for high rpm sbc?

Post by nickpohlaandp » Mon Oct 30, 2017 8:36 am

cjperformance wrote:
Mon Oct 30, 2017 2:16 am
Walter R. Malik wrote:
Mon Oct 30, 2017 1:26 am
MadBill wrote:
Sun Oct 29, 2017 5:14 pm
Reher's mention of rod cheek reliefs to improve pin lube rings a bell, in that I have seen same in 60+ year-old designs but not in modern production/race engines. Anybody got anything to share on the subject?
Older O.E.M. V8 connecting rods had some kind of "pee" slot or hole in them to lubricate the pin and cylinder on the opposite bank.
Over the years they went away and newer rod bearings which fit those older rods don't even have a relief for those slots or holes, anymore.
On a couple of occasions i have run an engine with no oil pan fitted and the pump picking up from a seperate container. It is a bit of an eye opener (and an oil shower :lol: ) to see just how much oil flys out off of the crankcase even at idle speed, let alone with a few more rpms.!
Piston squirters, edm'd rods etc to get the oil exactly where you want it fine and of course are quite different to just oil throw off. But there is also an amazing ammount of oil just getting tossed around in there.
I would LOVE to see that. Surely you took video!
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