Purpose building Derby engine.

[bluredvision]

Piston squirters aside....


I'm going to put together a purpose built big block 400 Mopar. I've got an old set of 915 closed chamber heads (should bring compression up to around 9.5" ish" stock crank and pistons. bigger cam with around 510 lift, hydraulic lifters, stock rockers double honeycomb springs yadda yadda. my big question is about the main bearings and cam bearings. is there any brand in particular that are derby approved? 
knowing nothing about building a derby motor....
I'm thinking of buying a half dozen different bearing brands and tossing them in the oven. measure with calipers before and after to see what brand expands more, do you guys think one of each is enough get proper test results with or should i get a half dozen of each brand for a broader average range ? has anyone done this type of thing or am i just overthinking?

[mopar17]

Not saying they're the best but I've had good luck with Clevite.

[chevykillerx13]

I think what your shooting at is right, but IMO, if possible, I'd find another engine that has at least the crank and rods, then install the bearings as you would normally, then heat the bearings up to your desired temperature. This would simulate the situation a little better, as I think in the oven the bearings would become distorted and alter your result, since they would be more or less out in the open in the oven, with nothing preventing them from distorting in a manner that would be impossible in an engine without total catastrophic failure, such as flattening out or twisting sideways. You could go one step further, and take an electric motor, run a belt around the crank pulley, and spin the engine consistently at whatever you think the average RPM for the engine would be, I'd say somewhere in the ballpark of 2-3,000 RPM. Doing this would better simulate the stress on the bearings, both the rotating and heat, giving you a more accurate result. I understand that this seems excessive, but to get the best results, I think the more factors you address at the same time, the better your results will be. Hell, you could even rig it up to an excersize bike or something if you didn't have a motor that would be able to turn that amount of RPMs. Maybe I'm going a little too far into this, but I'm thinking in order to come to a more accurate answer to your question, you need the rotational stress. This would also potentially simulate a lock up, although you wouldn't have piston caps or cylinder heads on the engine. And no oil kinda throws the answer off also. Maybe even go as far as having a variable speed motor, and watch a derby video, mimicking the throttle of the car you're watching, to really create the derby scenario. Again, maybe I'm crazy and delving too far into this, but I don't think simply putting bearings in the oven is going to give you much of an accurate answer, since certain bearings will be made of other alloys and act different than the others when subjected to an ominous heat with nothing forcing them to maintain their shape. You'll probably notice cracks that are caused only because of the stress from the abnormal distortion.

[dm440c]

Interesting discussion.

What you are talking about is the rate of thermal expansion, which can be expressed as a linear distance if you throw in a few other factors. The take-away though is that it is material dependent and specific to the chemical composition of the materials in question. Strictly speaking, it is an index number- a coefficient- for example, in general aluminum has a coefficient of thermal expansion 2x that of steel. To put that in simpler terms, an aluminum object will grow in size by a factor twice as big as a steel object when subjected to the same temperature elevation. To clarify that- it isn't saying the aluminum object will be twice as big as its original size, it is saying that the amount of linear growth of the aluminum object will be twice as much as that of the similar object made from steel.

Circling back, it is mainly about the material composition of the object in question. Engine bearings are multi-layered with a lot of technology involved for the materials of the different layers (aluminum, copper, and lead are common materials found in the layers) plus there are treatments that can affect thermal stability of metal objects, such as cryogenics. That's a whole 'nother discussion. I doubt any bearing manufacturers will be very forthcoming about their specific materials or processes used to the point that you could work it out on paper.

If the experiment is to compare different brands of bearings to see who's product expands the least, I would concentrate on the effects of heat as it is both the biggest contributor and the easiest to simulate and measure. Heating them in the free state should suffice for your purposes. It is an interesting discussion above about simulating engine conditions but I would argue that, relatively speaking, main bearings don't see much stress in this example since the crank spins on a common axis. Stress on the mains is more of an issue in terms of balancing against the pulses of the rod/piston assembly bob weights and from combustion events so the only way you would truly simulate that is with a running engine. The rod bearings are the ones that take a beating but again it is from the constant sudden changes in direction experienced by the pistons combined with the violence of the combustion event.

In any of the engine bearings operation it is the oil film that is critical. The crank and rods (and camshaft) ride on a cushion of oil that separates them from the mating bearings. Most bearing wear happens during start-up while there is no oil pressure, and this wear happens over time not all at once... until or unless something disturbs the oil film, typically surface imperfections that degrade over time/ oil mechanical property breakdown/ loss of oil pressure due to blockage or pump failure. The stock design has a wide margin that makes it very difficult for operating conditions to overpower the oil film short term so if the engine and its parts are made correctly early bearing failure should not happen.

What about excessive heat, such as a derby application? Definitely a concern but unless your engine has unusually tight clearances the real story here is with the oil. So, considering all of this, my theory is to build with loose clearances and use heat tolerant oils that will still do their job when the conditions suck.

[STROMI 121]

Apr 8, 2017 11:22:00 GMT -5 dm440c said:

Interesting discussion.

What you are talking about is the rate of thermal expansion, which can be expressed as a linear distance if you throw in a few other factors. The take-away though is that it is material dependent and specific to the chemical composition of the materials in question. Strictly speaking, it is an index number- a coefficient- for example, in general aluminum has a coefficient of thermal expansion 2x that of steel. To put that in simpler terms, an aluminum object will grow in size by a factor twice as big as a steel object when subjected to the same temperature elevation. To clarify that- it isn't saying the aluminum object will be twice as big as its original size, it is saying that the amount of linear growth of the aluminum object will be twice as much as that of the similar object made from steel.

Circling back, it is mainly about the material composition of the object in question. Engine bearings are multi-layered with a lot of technology involved for the materials of the different layers (aluminum, copper, and lead are common materials found in the layers) plus there are treatments that can affect thermal stability of metal objects, such as cryogenics. That's a whole 'nother discussion. I doubt any bearing manufacturers will be very forthcoming about their specific materials or processes used to the point that you could work it out on paper.

If the experiment is to compare different brands of bearings to see who's product expands the least, I would concentrate on the effects of heat as it is both the biggest contributor and the easiest to simulate and measure. Heating them in the free state should suffice for your purposes. It is an interesting discussion above about simulating engine conditions but I would argue that, relatively speaking, main bearings don't see much stress in this example since the crank spins on a common axis. Stress on the mains is more of an issue in terms of balancing against the pulses of the rod/piston assembly bob weights and from combustion events so the only way you would truly simulate that is with a running engine. The rod bearings are the ones that take a beating but again it is from the constant sudden changes in direction experienced by the pistons combined with the violence of the combustion event.

In any of the engine bearings operation it is the oil film that is critical. The crank and rods (and camshaft) ride on a cushion of oil that separates them from the mating bearings. Most bearing wear happens during start-up while there is no oil pressure, and this wear happens over time not all at once... until or unless something disturbs the oil film, typically surface imperfections that degrade over time/ oil mechanical property breakdown/ loss of oil pressure due to blockage or pump failure. The stock design has a wide margin that makes it very difficult for operating conditions to overpower the oil film short term so if the engine and its parts are made correctly early bearing failure should not happen.

What about excessive heat, such as a derby application? Definitely a concern but unless your engine has unusually tight clearances the real story here is with the oil. So, considering all of this, my theory is to build with loose clearances and use heat tolerant oils that will still do their job when the conditions suck.

I was thinking the exact same thing, but it was no where near that scientific in my head.

[bluredvision]

DM440, I always thought that engine bearings were to blame for heat ceasing. bearings expanding= no room for oil= chirp-chirp-klunk...or so i had always thought anyways. how loose are you thinking? say for example machine down the crank .002? or use slightly undersized bearings throughout ? maybe pistons with 3 or 4 rings with a say.010 taken off the tip of each end of the ring to allow for more expansion? or are you thinking something along the lines of a .030 over engine with .025 pistons?

[noser23x]

Apr 8, 2017 11:22:00 GMT -5 dm440c said:

Strictly speaking, it is an index number- a coefficient- for example, in general aluminum has a coefficient of thermal expansion 2x that of steel. To put that in simpler terms, an aluminum object will grow in size by a factor twice as big as a steel object when subjected to the same temperature elevation. To clarify that- it isn't saying the aluminum object will be twice as big as its original size, it is saying that the amount of linear growth of the aluminum object will be twice as much as that of the similar object

Just checking my knowledge on the subject. So say at (T) temperature, the rate of growth of say steel is (y-x) [Y being the measurement of said thing after expanding, and X being the original measurement before expanding], so aluminum would be (2(y-x)) correct? Or in simpler terms, say that a steel object expands .003, the aluminum object would expand .006?

[dm440c]

Apr 9, 2017 18:41:07 GMT -5 bluredvision said:

DM440, I always thought that engine bearings were to blame for heat ceasing. bearings expanding= no room for oil= chirp-chirp-klunk...or so i had always thought anyways. how loose are you thinking? say for example machine down the crank .002? or use slightly undersized bearings throughout ? maybe pistons with 3 or 4 rings with a say.010 taken off the tip of each end of the ring to allow for more expansion? or are you thinking something along the lines of a .030 over engine with .025 pistons?

It's easier to say that everything expands with heat, but with all other factors held equal objects made from two different materials will reach two different final shapes.

In this example the bearings and the crank are not only two different materials but two very different shapes and volume/mass. To address your post and the one below it in the vaguest possible terms it's pretty darn complicated to calculate down to a real number how much "bigger" each engine part will be for a given thermal change, so what they actually teach you in Engineering school is to decide first whether you really need that exact number or if you just need to understand the relationship to the system the number represents. Reminds me of a joke I heard once...


An engineer, a mathematician, and a physicist are each presented with a beautiful woman 8 feet away and the stipulation that at each time interval, they may move half of the remaining distance towards her.

The mathematician concludes that after N iterations there will be 8 divided by 2N feet remaining which will never equal zero so he gives up on the spot.

The physicist opines that if each iteration requires a finite amount of energy then the energy expended in the approach will be inversely proportional to the distance remaining and gives up on the spot.

The engineer says "8 feet, 4 feet, 2 feet, 1 foot, 6 inches, close enough for practical purposes".


Point is, there is already an established specification for bearing clearances that is used to determine the need for a rebuild or whether the parts were machined correctly. When confronted with the problem of the complex computational analysis needed to get it down to a number that represents maximum room for thermal growth without detrimental effect on lubrication I stick to the upper end of the tolerances and the rest works itself out.

[dm440c]

Apr 13, 2017 21:29:37 GMT -5 noser23x said:

Apr 8, 2017 11:22:00 GMT -5 dm440c said:

Strictly speaking, it is an index number- a coefficient- for example, in general aluminum has a coefficient of thermal expansion 2x that of steel. To put that in simpler terms, an aluminum object will grow in size by a factor twice as big as a steel object when subjected to the same temperature elevation. To clarify that- it isn't saying the aluminum object will be twice as big as its original size, it is saying that the amount of linear growth of the aluminum object will be twice as much as that of the similar object

Just checking my knowledge on the subject. So say at (T) temperature, the rate of growth of say steel is (y-x) [Y being the measurement of said thing after expanding, and X being the original measurement before expanding], so aluminum would be (2(y-x)) correct? Or in simpler terms, say that a steel object expands .003, the aluminum object would expand .006?

keep in mind we are using a super simplified version of how linear thermal expansion works here but yes, the way you presented it works for the purposes of this discussion. Looks like you have some experience with math concepts so if you have a curiosity for a more accurate computational approach try searching some form of "rate of thermal expansion of metals" and you can go deeper into it. I took those classes 20 years ago and really dug it but once I entered the working world all I really needed to retain was the concepts, so if you really want to get into the numbers you'll get more from a book than you will from me lol

[crusher71]

DRYBLOCK= NO WORRIES......LOL I LOVE MY "THROW AWAY" ENGINES. THEY OWE ME NOTHING, THEY WRECK STUFF AS WELL AS A 10 GRAND DERBY ENGINE AND THEY ARE A TON OF FUN WITHOUT THE COST.. BUT I WILL STICK WITH MY PURPOSE BUILT 355 FOR A SERIOUS SHOW...... 30 YEARS AGO WE RAN 80 - 90 GEAR LUBE IN PLACE OF 3 QUARTS OF OIL, IT SURE STUNK WHEN IT WAS DONE BUT THEY LASTED QUITE A WHILE AT LOCAL SHOWS... NOW ITS EXPENSIVE, AND 90% OR THE DRIVERS HAVE A BAZILLION DOLLARS INVESTED...

[noser23x]

Apr 16, 2017 8:24:15 GMT -5 dm440c said:

Apr 13, 2017 21:29:37 GMT -5 noser23x said:

Just checking my knowledge on the subject. So say at (T) temperature, the rate of growth of say steel is (y-x) [Y being the measurement of said thing after expanding, and X being the original measurement before expanding], so aluminum would be (2(y-x)) correct? Or in simpler terms, say that a steel object expands .003, the aluminum object would expand .006?

keep in mind we are using a super simplified version of how linear thermal expansion works here but yes, the way you presented it works for the purposes of this discussion. Looks like you have some experience with math concepts so if you have a curiosity for a more accurate computational approach try searching some form of "rate of thermal expansion of metals" and you can go deeper into it. I took those classes 20 years ago and really dug it but once I entered the working world all I really needed to retain was the concepts, so if you really want to get into the numbers you'll get more from a book than you will from me lol

I kinda figured simpled down, I try to tuck as many concepts in the back as much as I can. Seem to come in handy one day.

[CHUNKY MONKEY 13]

I'd just buy a decent brand bearing, run them a little loose, file your ring end gaps loose and run the hell out of it lol,

[bluredvision]

Im getting close to final assembly. my Crank is just at the machine shop getting pollished. I'm wondering if i should have the crank ground just a little extra on the mains. maybe half a thou or so or maybe a guy should keep the main and rod bearings mid range spec and be more concerned about a little extra on the bore and ring gaps as ive already had done. ?

[owen11x]

And wrist pins