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bore size 302

40K views 16 replies 11 participants last post by  iwantmore 
#1 ·
i measured the bore size of my '84 5.0 and it is 4.039 Inch,

I thought a 5.0 is 4.00 inch.. overmate? mbe other crank 2?
 
#2 ·
Std. bore is 4.00. Yours measures like it has been over bored .030
 
#7 ·
wow... .125 over bore.. geese..

Yes, people would frown when i told them my 302 was bored .060 over..
Never had a problem..

I would like to see a .120 over 302 motor run. Wonder what pistons you could use.
 
#9 ·
WOW!, 60 and 120 over???? Holy crud, I never seen it till now...

Well, I've seen 60 in a few but not many...
I'm apparantly really living on the edge! I have a 4.10 stroke crank to go along with my 4.060 bore to make 424 cubic inches (some round it off to 425). Between the rod ratio and the "thin cylinder walls" some people think it will not last. So far, so good... I guess time will tell! :rolleyes:
 
#10 ·
I would never run a .104 wall. Sure, it would run, but you'd never get the power you would otherwise have due to cylinder distortion. On a high PCP (Peak Cylinder Pressure) engine, like high compression N/A race engines, nitrous or boosted stuff, the thrust side would also crack or split in short order. You might support it with a full block fill if you didn't need a cooling system. Not the thing to do for 18 cubic inches.
 
#14 ·
.040 over is pretty common place. I've done several .060 without any problems. They were street cars, and it's hot here in Florida. Never a problem.

I do prefer to stay at .030 and .040, just incase I run into a "thin wall" block.
 
#16 ·
In extreme cases the cylinder may crack, but there are bigger issues before that. The primary issue is cylinder rigidity. A true and straight cylinder wall is required for best ring sealing. All engines have cylinder wall flex due to the combustion pressures involved and the thrust load of the pistons. If the walls are off-center (casting core-shift) or too thin (overbore) they can flex beyond the point that the rings seal is affected - reducing power potential.

The worst case is an overbore on a shifted core. This causes one side of the cylinder to flex more (imagine egg-shaped) which plays hell with the ring sealing. Best is a centered bore with thick walls. Acceptable may be a shifted core with slightly greater thickness on the piston thrust side (since the thrust load also tries to flex one side of the bore) creating a more even wall loading.

If you think this is getting overly picky, consider one example in side-by-side dyno tests of identical spec ~450hp engines, the power variation was nearly 30hp (Mopar Direct Connection Program, ca. 1980). The more cylinder pressure or RPM you're applying, the greater the effect. This is one reason 1974-'78 351W blocks are often avoided as greater random core shifting was common in that year range and, unless sonic tested, are a greater gamble. Another example is the E8 series of 2.3 Turbo blocks that suffer from common #1 cylinder shift and are carefully scrutinized by performance builders of those as well.

Ford small blocks are lighter than their Chevy counterparts, and thin walls is one reason. This makes it more critical for our use but with greater overall benefit. If you're dumping any serious money into an engine, sonic testing should be first on your list, to avoid power loss from an avoidable cause. Hope that helps.

David
 
#17 ·
Thanks Pounds Per Square Inch Gauge. There were some issues with early Chev's too. I bought a 38 back in the 70's and it bad a seized 283. A lot of guys bored 283's .125 to make a 301 or 302 depending on who you asked. Mine had a 50's block with TRW forged pop up pistons in it. The bottom of quite a few cylinder walls broke off and jammed into the crank. It was some thing to see. The good news was none of the pistons were damaged. I sold them to a friend who used them in a mid 60's 283 and abused that engine for years. So I really don't remember sonic testing at that time but as PSIG said it is the thing to do if you want to know for sure. I have been told the late 289 blocks can have some thick walls.
 
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