Ok. Now that I know more about the 400 BB motors here's the deal. 16 years ago I rebuilt my 400 olds that was an B block and A heads (65 motor). Nice right? However, the motor was rebuilt because it had spun rod journals and the crank was out of round at these journals as well as cracked. I believe if I remember right I ordered a crank from a shop in Southern Wisconsin, if my memory serves me right, but I never knew that there was two types of cranks for this motor back then. Forged and Cast. How do I know this guy didn't give me the less desireable cast crank and are there any differences in terms of stroke that would affect hp or bore/stroke ration being under/oversquare. I can't remember that far back, but there may be markings on the crank, no? The motor is currently sitting in my garage, so access to it is very simple being out of the car and on a stand. Thanks.

 

All of the early 400 (1965-67) and 425 engines used exclusively forged cranks. So unless you got a 1968-69 400 crankshaft by mistake your good. The back of the crank where the flywheel/flexplate attaches the forged cranks have sort of a check mark notch in the round mounting flange. The cast ones have a letter U cut into it,

 

There are early cranks and later cranks (68 up). The early cranks were all forged and had a flywheel mounting bolt pattern different than the later cranks. I believe all early cranks were forged so if your original flywheel/flexplate bolted up you have the good crank. It doesnt really matter though for a street car which one you have. The later 400 crank if bolted in the early 400 engine would no longer give 400 cubes as stroke is different.

 

How is it different? Longer stroke you think?

 

The 425 and early 400s used a forged crank with a 3.980" stroke. The 455 and late 400s used a cast crank with a 4.250" stroke. You can't interchange the two without changing rods and pistons as well.

 

That's what I kinda figured. Why would you have to buy new pistons though. I could see rods, but pistons if you keep the same bore?

 

Pistons are different heights too.

 

gotcha. Makes sense now.

 

Early pistons have 1.61" compression height and a 7" rod. When they increased the stroke a bit over .25", that would mean a "stack" of piston and rod would be 1/8" less overall. The factory went to 6.735" rods and a compression height of 1.74" to do this. By the way, they also "profiled" the rods at the top of the rod bolts and used shorter bolts to gain more clearance to the camshaft. You can't install factory 7" rods on the 4.25" stroke crank and clear the camshaft lobes, even if you find a special piston with the short required compression height (centerline of pin to top) of about 1.49".