Thread:

BBC Cranks 4.840 vs 4.900

What is the difference ?

Does the 4.900 fit in a "regular" block ?

Originally Posted by glastornjet73

Idk what you consider a "regular" block 9.8 deck gm block i know will not fit a 4.500 stroke but a gm 10.2 will fit a 4.500 and no more as far as aftermarket 9.8 blocks i believe will fit a 4.500 and am not sure about aftermarket 10.2 ones

SAY WHAT?

Originally Posted by wizbang

What is the difference ?

Does the 4.900 fit in a "regular" block ?

Whats the difference? 160 thousandths
Wiz, the standard GM block is 4.84.

4.90 is aftermarket.

I guess I should be more specific.

I have a brand new Bryant crank. It is for a 4.900 bore space application.

Can I use it in a factory Chevrolet "4.840" bore space block ?

Can I just bore the block using a 4.900 plate to center the bores over the 4.900 bs crank ?

Will the heads , like a Dart, be useable ?

Or was the 4.900 thing an engine of it's own ( big block Chevy ) ?

Originally Posted by wizbang

I guess I should be more specific.

I have a brand new Bryant crank. It is for a 4.900 bore space application.

Can I use it in a factory Chevrolet "4.840" bore space block ?

Can I just bore the block using a 4.900 plate to center the bores over the 4.900 bs crank ?

Will the heads , like a Dart, be useable ?

Or was the 4.900 thing an engine of it's own ( big block Chevy ) ?

Just messing with you Wiz, because you need it. I believe you'll find the main centers on the crank will not line up with the mains in a standard block.

Do this. I think if you lay a tape from the outside of the front counterweight, to the outside of the back counter weight, the crank will be longer.

You're looking at a Dart, Brodix, KB, World Merlin(not a II or III) or a DRCE 2

Hey Wizbang, a really, really, really nice guy e-mailed these to me a while back at my request. Could be very helpfull.

I'll stand both cranks up on the rock and measure them, then I'll report back !!!!

Wiz,

GN7 is correct on what he is telling you. A standard GM Big Block has the bore spacing of 4.84 and had since the inception. But the desire to build larger cubic inch engines and utilize larger bores, blocks had to be reconfigured to move the bores further apart to allow for the larger bore. Generally the 4.84 blocks with thick walls (bowtie, Dart, Merlin) will go to a maximum of 4.625 bore. When blocks of wider bore spacing such as 4.90 were developed the bore could now go to something in the range of 4.680 or so. Then 5.0 bore spacing and now we see 5.30 bore spacing, etc in special aftermarket blocks. Each time there is a desire for a bigger bore a "longer" block has to be developed. And this bigger bore along with long stroke cranks has allowed the advent of the 700, 800, 900 and now 1000 cubic inch BBC style engies.

So to answer your original question, If you have a standard BBC block (the most common) it is of 4.84 bore spacing. And if you have a 4.90 bore spacing crankshaft, it will not fit in the block as it will be too long and hang on the sides of the main webs.

Kind of a long explanation, but I hope this helps.

Gear

Crank comparison 4.840 vs 4.900 ( bore centers )
Cranks were stood up on flywheel end and measured in this manner. Journals are measured on center.

Item measured 4.840 4.900 4.900 difference

Thrust 1.9450 1.9450 0

# 5 Main 3.7646 3.7639 -.0007

# 7/8 Rod 6.0699 5.9832 -.0867

# 4 Main 8.4814 8.4314 -.0500

# 5/6 Rod 10.9172 10.8843 -.0329

# 3 Main 13.3372 13.3319 -.0053

# 3/4 Rod 15.7589 15.7850 +.0261

# 2 Main 18.1598 18.2330 +.0732

# 1/2 Rod 20.5947 20.6858 +.0911

# 1 Main 22.8793 22.8787 +.0006

# 1 Main end 24.1120 24.0602 -.0518

Crank Gear Surface 24.5358 24.5427 +.0069

Total OAL 27.1901 27.1941 +.004

Now , will it fit in a 4.840 bs block ?

Originally Posted by wizbang

Now , will it fit in a 4.840 bs block ?

I know now, as I have tried to lay it in a block ?

Are you SURE that crank is for a 4.900 BC?

Originally Posted by Oldsquirt

Pretty sure that 4.900 bore space crank is specifically for the DRCE blocks.
Your numbers make it look like the front cylinders have been moved forward from block center and rears moved rearward. #1 & 2 approx .090" forward, #3 & 4 approx .030" forward, #5 & 6 approx .030" rearward and #7 & 8 approx .090" rearward. That would accomplish the additional .060" bore spacing inside the original BBC architecture. Just a mildly educated guess, though.............

wizbang, look at your numbers. OldSquirt is right. You would need to measure every main and rod journal from the center out. Measuring from the back makes the back four cylinders show a negative number, the front four a postive. When you start adding the - and the + together, they cancel each other out and you end up with a .004 difference which is most likely a measuring error on your behalf.