Noticed in Leo's book that slipper pistons cause cracking issues in 292's due to the local loads of the piston on the cylinder walls. I think I've figured out a way to drop a 292 crank into a 250 using 5.7 rods, but the off the shelf pistons are slipper type. Can I expect the same issue, or even worse due to the rod angle?
Just for amusement these pistons would also allow 250 crank with 6" rods, and stil be able to deck the block....
The 250 with the 6" rod swap would be a better combo to build. You will be introducing a much more severe C/L ratio into the engine by putting a 292 crank into a 250 block, and you will be making the problem much worse.
In a perfect world, you would need a piston with a compression height between 1.380"-1.385" to use a 6" rod in a 250 block with a 250 crank to allow a light cut off of the deck to bring you to near zero deck height. That's also assuming your using a block that hasn't ever been decked before.
Don't forget the 292 crank counterweights will have to be trimmed. Also watch connecting rod to camshaft clearances. Use a 292 cam and electric fuel pump will work. Buying pistons is the easy part......
An old post. I know there are others here too but this took an hour to find.
292 crank ib 250
1.313 CH 3.898 bore pistons
250 crank, 1.756 stroke, 6" rods, 9.078"
292 crank, 2.06 stroke, 5.7" rods, 9.073"
IIRC factory deck is 9.15 +/-.007" CH. So at min spec, with a .038 gasket, you'd be right in there.
problem is these are slipper skirt design. So my question was if the cylinder failure noted in the 292's was seen in 250's, particularly if running a 292 crank (I would think yes, but haven't built one to try....).
I've built a couple strokers (4.062 stroke, 5.7 rods) at 4" bore in a 250 block. No breakage but there was evidence of bore distortion.
Latest build is at 3.935 bore (4.062 stroke, 6.0 rods) and seems much more stable in terms of bore distortion. Pistons are very short custom JE. I've run it hard without problems.
Search for "Nova project progress" for more details. I don't know how to attach a link, perhaps someone can help.
I think the original intention of guys swapping the 292 crank into the 250 blocks was to cheat in racing by trying to fool the tech guys with the short deck blocks, and they weren't concerned to much with the consequences of what they were bringing into this type of swap. As stroker pointed out, it has been done by a lot of guys, and even though it doesn't provide the best of design criteria, there are ways you can help make it better, but it can get costly.
This question came about because I found some reasonably priced pistons, with short CH, but without a full skirt. Also some good steel rods, reasonably priced, in 5.7 or 6.0. Could put together complete piston/rod package for about $700.
I have toyed with the idea of a 292 crank, have half a dozen 250 blocks, no 292 blocks. With the slipper pistons the crank to piston skirt issue is much less. electric fuel pump and a grinder fixes the fuel pump lobe problem.
I also have toyed with restroking the crank by getting it turned to the smaller journal size. Could be used to fine tune CH rather than decking the block as much. Would let me use the cheap (relatively) pistons and tailor my CH to exactly what I want.
just thinking of my future. Right now I have to get the lathe a Bridgeport put back together so I can make my own rocker shafts for my Allis Chalmers. Then I can make roo in the shop and get back to my current L6, then think about LS hybrid head or the new head.... etc.
The only 5.7" or 6" rod available for these cranks is for the 250 rod journal, so at the minimum you would have to regrind the 292 rod journals to use them, so offsetting the journal can help adjust the deck height as well.
I'm not a real fan of slipper skirt pistons for high hour uses such as street engines. My experience is that they are hard on cylinder walls, causing relatively thin walls to distort.
Sometimes a good deal on "cheap" alternatives is not such a good solution.