Max boring an aftermarket BBC cast-iron block?

pdq67 · Post by **pdq67** » Mon Dec 30, 2019 5:07 pm

Has anybody ever offset bored either a Merlin III or a Dart BBC cast-iron block out to like 4.75" and then rework the crank counterweights to line up the rods??

I guess that I am poorly asking what's the biggest bore these blocks can be bored out to?

And how much trouble is it to install a 4.50" crank in a 9.8" deck BBC block?

Just thinking about max cubes in a standard deck height aftermarket BBC block is all...

pdq67

econo racer · Post by **econo racer** » Mon Dec 30, 2019 10:19 pm

There is a guy on here that has 620 inches in his shop truck.

Post by **Orr89rocz** » Mon Dec 30, 2019 11:48 pm

Dart big m says 4.625 still has .300 wall thickness. On a na deal you could go thinner on walls but not sure what that gives you on a standard bore space motor as a max.
Guys have done 4.65 bore on dart big m’s

4.5” crank fits

englertracing · Post by **englertracing** » Tue Dec 31, 2019 5:58 am

Hmm
Long stroke...

Pdq was gone for a while.
Maybe abducted by aliens?

Just kidding.

It would probably be most effective to bore it max without having to offset it, and use the time/expense saved on the cylinder heads to net more than that the extra 0.080 or 0.100 would "get" you.

mag2555 · Post by **mag2555** » Tue Dec 31, 2019 6:08 am

More important then boring for max cid is that during the process you offset the Bore to both unshroud the Intake valve and ease the thrust loading on cylinder wall, or atleast unshroud the valve to the max!

Alaskaracer · Post by **Alaskaracer** » Tue Dec 31, 2019 8:31 am

Honestly, I don't know why you'd want to go through the trouble of trying to offset bore the block, unless you've encountered bore shift and are trying to correct that. GM has their GMPP racing block with 4.9" bore centers. It was developed for Pro Stock racing, and one of the issues it addressed was bore sizing to unshroud the valves. You can offset bore a standard block, but that's not necessarily going to address that issue, and will likely cause other issues that need to be addressed as well. More hassle than it's worth.....

bobmc · Post by **bobmc** » Tue Dec 31, 2019 9:06 am

I think you could use a larger bore with iron heads than alum. 4.70" bore would leave .140" between cylinders, warm up the engine with external heat source before starting and give it a try, if I wanted to disguise cubic inches from onlookers I would take a tall deck block, cut it down to maybe 10" deck
and relocate the water pump holes, use a billet water pump housing cut to spec

mt-engines · Post by **mt-engines** » Tue Dec 31, 2019 10:18 am

pdq67 wrote: ↑Mon Dec 30, 2019 5:07 pm Has anybody ever offset bored either a Merlin III or a Dart BBC cast-iron block out to like 4.75" and then rework the crank counterweights to line up the rods??

I guess that I am poorly asking what's the biggest bore these blocks can be bored out to?

And how much trouble is it to install a 4.50" crank in a 9.8" deck BBC block?

Just thinking about max cubes in a standard deck height aftermarket BBC block is all...

pdq67

I've moved the outers .100 and inner. 050 on a dart pro. That bore was 4.670

I have gone 4.640 std borespace without offsetting the bores for an NA engine call a ring manufacturer first to see what rings are available for your desired bore size. Not a lot of options. And a lot of work for nothing. Might as well for 5.0 bs if you want that large of a bore

Abbottracingheads · Post by **Abbottracingheads** » Tue Dec 31, 2019 10:27 am

On a Dart Big M block you can go 4.650. There is still plenty of cylinder wall thickness, but there is no more room for the head gasket. I have run customer engines at 4.650 no problem with custom Cometic head gaskets.

pdq67 · Post by **pdq67** » Sat Jan 04, 2020 3:53 pm

The 4.90" bore Chevy BB block has to have a 4.90" spaced crank, doesn't it?

A 5" B/S block, better look up PackardV8 and see if he will sell me a Packard block. Then also stroke a Packard crank...

I would have to sleeve it to get any sort of big bore out of it though.

pdq67

mt-engines · Post by **mt-engines** » Sat Jan 04, 2020 10:10 pm

pdq67 wrote: ↑Sat Jan 04, 2020 3:53 pm The 4.90" bore Chevy BB block has to have a 4.90" spaced crank, doesn't it?

A 5" B/S block, better look up PackardV8 and see if he will sell me a Packard block. Then also stroke a Packard crank...

I would have to sleeve it to get any sort of big bore out of it though.

pdq67

No. You can have a 4.84 crank with a 4.9bs

pdq67 · Post by **pdq67** » Sun Jan 05, 2020 3:08 pm

"No. You can have a 4.84 crank with a 4.9bs"

mt,

I have to ask if the 4.84" bore spaced BBC heads will also fit IF CENTERED RIGHT like the crank and also if you know how thick the cylinder walls are before any boring is done so I can see how big a bore to bore to?

Thanks,

pdq67

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