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Old 05-08-2007, 03:09 PM
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Thermal expansion of pistons

I know Aluminum expands at a higher rate than cast Iron, but what are those rates in the realm of pistons and engine blocks? I've found several different coefficients of thermal expansion online, but do those values transfer directly to the application of pistons and bores?

I run my SBC in a demolition derby car. Even after the radiator is destroyed, the engine gets run until the derby is over - no matter what. Normally the engine doesn't quit, but I have had occasions when it was so hot that the pistons stuck in the bores. I'm looking for how much maximum piston clearance I can go. The machine shop says anything over .005" will break piston skirts. The engine is low compression, is never started at temperatures below 50 degrees, and is always allowed proper warm-up before it is driven hard.

I realize for most of you real racers out there this sounds like a total waste of time, (like the fella at the machine shop), but it's gearhead competition all the same - and anyone out there who can help me out will be appreciated.

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Old 05-08-2007, 04:13 PM
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it is a total waste of time really, why rebuild an engine for a derby car when it's only gonna get ran and overheated badly ( and Almost always equates to a cracked head/ block when there run that hot, warpage at the very least ). my local U pull it gets $155 per engine, for any engine, and there's always 350's in that yard, lot less than an engine rebuild, just buy another 1 when your current 1 quits.

but, your machinist is right, too much pistion clearence and you will get pistion slap, which can and will break a skirt eventully

forged pistions don't expand as much as cast, these may work better for you, if your willing to pay $50-$100 per pistion for a motor, your just gonna kill in a few 'runs'

and just an FYI, if your engine got so hot, the pistions seized in the bore, the block is likely shot as it has gotten so hot
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Old 05-08-2007, 04:44 PM
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I don't think so, forged expand more than cast. thats why my Vette sounds like a diesel on a cold morning.

But Who cares if he brakes a skirt, bore it big and let er go. Just bore out a worn block and use stock pistons.

I assume having a broken skirt is less of a deal than a siezed engine in a demo derby. A broken skirt will still go.
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Old 05-08-2007, 06:44 PM
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Quote:
Originally Posted by 427v8
I don't think so, forged expand more than cast. thats why my Vette sounds like a diesel on a cold morning.

But Who cares if he brakes a skirt, bore it big and let er go. Just bore out a worn block and use stock pistons.

I assume having a broken skirt is less of a deal than a siezed engine in a demo derby. A broken skirt will still go.
yeah, I thought I had it backwards ( cast vs forged ). ither way, it dosn't make sense to build a engine for a derby car. just run a junk yard engine. even if 1 junkyard engine is bad, parts from a couple bad 1's could make an engine good enough to run in a derby.
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Old 05-08-2007, 07:01 PM
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matt167- I disagree. You may think it doesnt make sense to build an engine for a derby car. But obviously this person doesnt. I personally think it makes absolutly no sense to put a great big turbo on a civic with a folgers coffee can tip. But guess what? The person that drives that civic loves their car and loves working on their car and loves doing whatever they do in their car. So if someone wants to build a sbc for a derby car why not let him go for it? Obviously he knows whats gonna happen but that is his way of enjoying that vehicle/engine/sport whatever. Ya know? If hes willing to build a sbc for his derby car let him, more sbc's out there for the rest of us that arent destroyed . I dunno thats just my point of view, sorry for the rant.

Steve
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Old 05-08-2007, 07:19 PM
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now you all have that figured out (more expansion rate in forgings)

there are different alloys of forgings
vms-75
2618
4032

4032 is best for the street(more silicon)
vms-75 (1-2%less than 4032)
2618 race mostly or only

4032 has the least expansion rate
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Old 05-08-2007, 09:54 PM
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I have heard of people running up to .009" piston-to-wall clearance. I wonder if loose bearing clearances and a high volume oil pump would cool the bearings a bit.
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Old 05-09-2007, 12:30 PM
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Just a thought.... but it seems like you should run larger ring end gaps, they do on nitrous and supercharger setups because of the heat. I read an article somewhere on big ring end gaps and it did'nt make much difference on oil consumption.

Can you run an oil cooler with an electric fan? Maybe set it up in the cabin?
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Old 05-09-2007, 12:35 PM
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Hey! That woulda made a good project car .... dang it now all there is to choose from for a cheap project car is, um, that astro van over there.

where's all the fwd demolition derbies?

*p.s.* sorry to rant, but felt the need to.
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Old 05-10-2007, 04:11 PM
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I went to watch the top-fuel dragsters last weekend - most of their engine parts are only good for (1) 4 second pass... but man is that 4 seconds impressive to watch!

Not to try to put demo in the same line as top-fuel, but the object is to beat the other competitors. Often, good running junkyard motors are too tight, and running them a few minutes with no water will lock them up. Sitting in the middle of the track in a car built hell for stout - and a motor that won't run... is no place to be. Therefore, there is a need to put a nice cheap rebuild kit, in a well worn block with plenty of clearance.

Squeezer, thanks for the alloy numbers. I'll check some vendors and see if anyone posts the expansion rates.
Fiscus, I was looking to go to 0.010" this time, but was wanting someone else to tell me I wasn't insane for doing so. If you're running 0.009", surely I can go 0.010"... what's a few thousandths amongst friends? Also, I keep all the bearings at the loose end, but still in factory spec. The bearings have never come out scorched or damaged - it's always the pistons.
xxllmm4, someone else recently mentioned the end gap - it makes sense, I believe I will give it a try. Engine oil coolers are outlawed in our demo circut.

This website is awesome! Thanks everyone for your comments.
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Old 05-10-2007, 05:12 PM
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Back in the "good ol' days" there was a process called knurling. The tool pressed a cross hatch pattern into the piston skirt. It was used to inexpensively restore clearance when reusing an engine's old pistons on a restored bore. Restoration of the bore was to break the ridge and rehone the walls. Much less expensive than rebore and new pistons back when people didn't have as much money as today.

The racing people picked up this process as it allowed a tighter clearance to be run in an "overheated" engine without damage to the pistons or bores. The reason is the knurl traps a lot of oil on the skirt which cools it and offers some cushion against the forces reacting against the bore walls. It gradually disappeared over the years as Teflon thrust buttons and later expensive and quirky rocket science coatings be came the rage. But the old fashion knurling still works, all you've got to do is find somebody with the old tools.

Bogie
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Old 05-10-2007, 05:51 PM
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That sounds like a very interesting process. Hmmm I think I may research that some more, see what I can come up with.

Steve
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Old 05-11-2007, 12:18 AM
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I did exactly that a long time ago to a set of forged pistons that were standard bore. Instead of the automotive knurling tool I set these up in the lathe and used an industrial knurling tool and knurled the daylights out of them up to a .030 over bore. The pattern was about like that of a good pistol grip!! These went into a dirt racer and ran for 2 seasons before the car was totally wrecked. the motor still ran fine.

Along the same line since this motor were are discussing can be run without water Why not use a high volume oil pump and tap and oil galley for piston oilers like the diesels have. They have been using these since time began. You could probably just use a copper tube with some holes drilled in it to squirt right up into the head of the piston. The diesel ones are little J hook shaped things that peek around the bottom of the cylinder bore. Also the NASCAR guys use these. Some carefull drilling and tapping for attaching might be required but nothing that can't be done at home. If one gets plugged up in a diesel it will really toast a piston.!!! So they really do work.
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Old 05-12-2007, 03:20 PM
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Well said.....

Quote:
Originally Posted by steveyb4342
matt167- I disagree. You may think it doesnt make sense to build an engine for a derby car. But obviously this person doesnt. I personally think it makes absolutly no sense to put a great big turbo on a civic with a folgers coffee can tip. But guess what? The person that drives that civic loves their car and loves working on their car and loves doing whatever they do in their car. So if someone wants to build a sbc for a derby car why not let him go for it? Obviously he knows whats gonna happen but that is his way of enjoying that vehicle/engine/sport whatever. Ya know? If hes willing to build a sbc for his derby car let him, more sbc's out there for the rest of us that arent destroyed . I dunno thats just my point of view, sorry for the rant.

Steve
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Old 05-12-2007, 03:26 PM
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I have a buddy that built a junker 400sbc for a drag car on a budget. It went together with .008 piston clearance and ran to 7200 RPM without failure.

Why dont you just get an oil cooler and a 2-3 gallon tank for water storage and ditch the radiator. It should give you enough cooling and thermal capacity to run a long time without totally killing things, and you can hide it in a safe place so it stay in tact.
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