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The BBC looks like the winner for the 235 and GMC because of the big range of lengths, pin sizes, price etc.
The only downside is no offset to speak of, so you're stuck with near-stock stroke (+.1125" max). IMHO small price to pay for such an important step - it means the engine can turn as fast as the valve gear will stand.

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Yeah, thats what I was thinking about more so than the stroker option. The broader range of lengths available for the BBC(up to +.300 longer than stock 235/261 rod) will also allow for a more common SBC forged piston that could be massaged a little to be made to fit the application without too terrible of an extra expense. Unfortunately for the GMC, the BBC rod choice will only be an extra +.100 longer than the stock GMC rod, so it doesn't really give you the same benefits for that engine group as it does the 235/261 engines! Maybe using the shorter deck 248 engine block bored to 4.000", along with the 270/302 crank and BBC rod im using could get you closer to a more common SBC piston. The 270/302 blocks have such a tall deck height, there just isn't much option for a budget rod and piston swap....



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Panic, by 'f/s' I take it you mean Feet per Second. Yes / no?

Yes the 'bearing speed' is the important factor there (as I understand it anyway) and lightweight is always a good thing when you're trying to rev the snot out of something.

As for if they would self cancel, probably not, but it would stand to reason it should be close in your + 10% -10% example; the error would likely be exagerated the larger the numerical change (ie +15% -15%, just for numbers).


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This thread as diverged into two discussions.

Panic, concerning journal bearing oil flow and shaft support:
The shaft is lifted off the bearing surface and support by the oil wedge. This higher pressure film of oil is determined by the physical dimensions of the bearing, the bearing surface speed (affected by engine rpm), and actual oil viscosity (as affected by temperature).

Total journal bearing oil flow, for technical discussion, is divided into hydrodynamic flow (generally 80 to 85%) and pressure flow, sometimes also called side flow(generally 15 to 20%).

For a given journal bearing the hydrodynamic flow is basically caused bt the "pumping" action of the shaft/journal assembly, and is directly proportional to engine rpm.

The pressure flow is basically the additional oil flow to enhance cooling of the bearing, and is directly proportional to supply oil pressure.

A properly design journal bearing will operate with a flooded oil inlet at very low pressure, just several psi. The oil wedge will form a much higher local pressure to support the rotating shaft. Please note that a high pressure may be required at the beginning of an oil header/gallery to provided sufficent flow/pressure at the tail end of the header/gallery.

Unlike most flow/pressure relationships related to the velocity squared, these are directly proportional relationships.

On heavy rotating equiment, in some applications a secondary high pressure lifting oil or jacking oil is supplied to the bottom of the shaft or thrust bearing to phyically lift the shaft at start-up or low rpm's.

One source that discusses and gives calculations for journal bearing oil flow is the "Machinery's Handbook" in the 'journal bearings' section.


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Winter is right we have two discussion here.

Lennart, There are a lot of possible piston that would work with longer rods for the 292. That's the problem longer rods for the 292. There are no longer rods available made of forged steel. The only rods that I'm aware of are custom Aluminum and not recommend for the street, because they stretch. The dragsters that use aluminum rods resize them after every season, maybe more.

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So to paraphrase this entire thread, would you agree that it would be just easier and probably cheaper to buy some forged pistons from JE or Ross or wherever that are made for a 292?


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That's pretty much what it comes down to for a 292, and what most have done. I hope to measure the thickness of the top of the BBC piston this afternoon and post some pictures. Unless someone can look at the numbers and calculate the strength, I don't know that that will change anything.

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Just for everyones reference on cost, the current price for the Ross pistons is $459 for a set of 6. The specs are forged Flat top, 2 valve relief, machined for pin locks, forced pin oiling, machined for 1/16 x 1/16 x 3/16 rings, .927 wrist pin bore(pins not included), any bore size(no extra charge),any compression height(pin placement)(no extra charge)+ shipping! Now we have some dollar #'s to have for our wish list....Who's in!



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There are no longer rods available made of forged steel.

There are many, but not in useful sizes unless you want to spend a lot of money. Carrillo will make you a steel rod to your exact dimensions.

The only rods that I'm aware of are custom Aluminum and not recommend for the street, because they stretch.

If the recip. weight and RPM are kept within reasonable limits, the mileage for an aluminum rod can be quite extensive - over 70,000 reported for some stock engines.

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Re Ross: for an extra charge they may have taper-wall pins and a weight reduction program (milling from inside).
You can also make much higher compression by describing the chamber shape for pop-up (they my even have the chamber profiled on CNC already), or lower CR by giving the ratio and they will make a D cup with enough depth to get there.
Blower also needs the ring stack slightly lower down, and more minimum crown thickness.

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Panic, If you've got the $$$$ there's lots of goodies to had. The motor in the Junior Fuel Dragster that I teamed on had a Titanium crank. The info on the Alum. rods is interesting. Still $1,200 to $1,800 for a set of rods, that's almost half the cost of a crate motor.

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I was able to measure the top of the BBC 366 ci. pistons that I had the tops cut down to work wih the stock rods today. The thinnest spot I could find on the top of one of the spare pistons was .2418. I then measured from the top of the piston to the bottom of the valve relief and that confirmed the .080 the shop told me that they had taken off. So if my math is correct and there is not to much deviation in the piston casting. The pistons should be about .1618 thick at the thinnest point. Anyone with ideas how weak that might be.

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The .241 should be safe for a cast piston. Its the .161 that is going to be kinda' on the fence. How big of an area is it that thin. Now that the tops have been cut, how far down is the top ring. Keep in mind the 366 piston and wrist pin are a whole lot heavier than the stock 292 piston and wrist pin. That will potentially put much more stress on a rod that wasn't engineered for that extra mass hanging on the end of it. So be cautious about RPM, as the piston speed of the 292 will be much higher than the 366 at the same RPM. Just curious, did you check the weight difference between them.



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CNC-Dude, thanks for the consideration. I'll have to go back and map the top of the piston a bit better. The under side of the top is irregular to say the least. Right now I'd say there is an area about the size of a quarter that is .161. I'll map the measurable area on both spare pistons I have. You can still see part of the valve reliefs and notch here's a picture of the piston installed;

http://www.flickr.com/photos/33950672@N08/3222177846/

you can see the notch on the bottom left and part of the valve relief on top left of center. Here is a picture of the piston showing the Teflon skirt coating before the top was cut and two 292 rods. I think the rod in the center has been lightened and the one on the right is stock.

http://www.flickr.com/photos/33950672@N08/3222177844/

For reference here are the three off the shelf pistons for the 292;

http://www.flickr.com/photos/33950672@N08/3222177836/

The one on the left is the low 7.6 compression piston with full dish, the center is the 8.3 compression with the 'D' shaped dish, and the one on the right is LPG piston with the oval dish that is .180 deep. I'll weigh the stock 366 against the two pistons on the left and let you know how they compare.

I did weigh the stock piston and rod against the 366 and lightened rod. The 366 piston and rod were about a 1/2 pound lighter. Here's a picture of the assembled block just because.

http://www.flickr.com/photos/33950672@N08/3222217572/

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JM2¢:
The marks left from the valve notches have to be safe, both because they're original and because they're above the skirt.
The "delicate" areas will be the part of the unsupported crown (away from the skirt and especially away from the pin bosses) closest to the exhaust valve, the plug, and the center of this area.
These will fail first on detonation, but shouldn't give up from inertia at .161".

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I found some Jaguar 3.8 rod data:
Length 7.75"
Big end bore 2.233" (292 is 2.225"), would need an undersize and a smaller undersize on the crank.
Piston pin .875"
Width 1.180"
Based on 292 deck height, the piston CD for zero deck (quench distance is gasket only) is 1.065".

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I looked at Kieth Black web site for Silvo-O-Lite pistons, they have a listing by bore size. The smallest CD I could find for bore of 3.875 was 1.581. Here's the link.

http://kb-silvolite.com/spistons.php?action=diam

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panic,

there are 3.905 dia pistons for the LS1 that come in a 1.115 CD.
Check Wiseco or Diamond to name a few.

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CNC-Dude, I weighed the pistons and the BBC 366 is 28.099, the low compression full dish is 28.423, the 'D' shaped dish is 30.127 ounces. That's piston and pins only.

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larry,
i have been out in my shop checking out weights. a stock 292 piston with no pin weighs about 740 grams. and using 28 grams per oz, yours is 784 grams. so pretty close.
tell me again how you lost 1/2 lb per cyl, that i believe is 6 oz per cyl.

on a side note, i weighed some performance 292 pistons.
a je flat top weight was 640 with rings and pin


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 Originally Posted By: LGriffin_#4385
CNC-Dude, I weighed the pistons and the BBC 366 is 28.099, the low compression full dish is 28.423, the 'D' shaped dish is 30.127 ounces. That's piston and pins only.

Larry
That looks to be a decent piston swap alternative.



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I talked to someone in the business about the flathead Ford rod today, and he said they'd custom machine their H-beam rod to 2.1 Chev rod bearing, but said their forging would be best @ .787 pin. That would give a 7" rod, but would still require custom pistons rather than off the shelf. They would need to have 10 orders to reset the machines and not have to jack the price real bad. They'd still approach $100 each however.

7" H-beam, 2.1" big end, .787 small end.


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CNC dude, thanks for the info on the Ross pistons earlier! Lurker Dave here; This settles it for me--I'm thinking a 292 bored .060 over, stock rods, ross pistons, water heated intake with a 390 holley (I already have that; tho the megasquirt looks nice) to replace the 250 I've got now. (currently set up with headers, clifford intake and 390 holley)...still have the stock head, so that is the first priority.

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292 rods on ebay. The seller might give a few here reason to pause. http://cgi.ebay.com/ebaymotors/Chevy-Inl...A1%7C240%3A1318

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The problem with aluminum rods in a 292,or a 250 for that matter,is you have to use a roller cam. Because you have to grind so much rod clearance in the camshaft to clear the rods,a cast flat tappet camshaft will break in half if you grind them that much....



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