.040 vs. .020 over???

[prsrcokr]

I'm looking for opinions of those that have used these bores. Block is a virgin and I bought .040 JE's because I wanted the slight increase in displacement, thinking it might help low end TQ. What are you're opinions, should I go with .020 and have that much of a stronger block? I'm planning for oversized valves which also made the .040 over my choice piston.
Thanks,
Brian

 

[Lurch]

As I recall, .040 over is pushing it. If you have .040 pistons already, I dunno, but right off the bat if I had to overbore I'd do .020, just in case I needed to again down the line. I don't think .040 gives you that much displacement, like .1 liter as I recall.

 

[Race Bred]

As I recall, .040 over is pushing it. If you have .040 pistons already, I dunno, but right off the bat if I had to overbore I'd do .020, just in case I needed to again down the line. I don't think .040 gives you that much displacement, like .1 liter as I recall.

d= r^2*Pi*stroke* # 0f cyls
stock stroke=91mm
stock bore=83mm

mm to in conversion = mm/25.4

stock = 41.5*41.5*3.14*91*6= 2.954191L
.020" over = 41.754*41.754*3.14*91*6= 2.990464L
.040" over = 42.008*42.008*3.14*91*6= 3.026958L
.060" over = 42.262*42.262*3.14*91*6= 3.063674L
.080" over = 42.516*42.516*3.14*91*6= 3.100611L

Displacement gain over stock:
.020"= + .036273L = + 2.2135 in^3
.040"= + .072767L = + 4.4405 in^3
.060"= + .109483L = + 6.6810 in^3
.080"= + .14642L = + 8.9350 in^3

note Pi used in calcs = 3.141592654 (calculator aprox) not 3.14...

Defiant7m has said a 85mm bore (.080) is 'safe' if hard blok is used. Check the 101mm 7m thread for more info.

 

[IJ.]

I'd go for the smallest oversize you can get away with as the displacement increase is minimal and won't make much difference to output but thin walls can tax the cooling system.

 

[flubyux2]

yeah, listen to 1J....

0.040" isnt worth jacking up the block for the miniscule bump in displacment. just go with a 20 over, that way the block is still usable down the road later on.

if you bore it all out this time around, the block will be useless later on, unless you want to ahve someone prototype some cylinder sleeves, but that would be copious amounts of money.

as far as allans setup, id like to run some siamesed cylnder sleeves and use an 86mm bore. if i have cylinder sleeves that are all one piece and fitted to the block, i might be able to run a onrmal cooling system. i dont really want to convert to alcohol, but itd be gangsta.

 

[chevyeater]

Last I checked, Mello was selling his .080" over shortblock. Pick that up and save your block for the future.

 

[bigaaron]

I went .029 over for Ross pistons because that was next size up from the factory .020 (.5mm) overbore size that you could get off the shelf rings for. For turbo cars an extra pound of boost will make as much or more power then the overbore will. .080 over would make the sealing surfaces for the headgasket pretty thin between the cylinders and from the cooling system to the cylinders. Since headgasket problems are built in to the 7m I don't know if I would feel comfortable with any bigger then .040 over.

 

[GrimJack]

I'd keep it to .020 over for the first time - that way you can rebore your block three times if need be.