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Author Topic:   3.875 question
Dirt Full Roller

Total posts: 53
posted October 18, 2002 09:08 PM
How much notching do i have to do and how much trouble is it to put a 3.875 stroke crank in a small block 400. wanting to build a 6 inch rod 3.875 stroke motor for next year. also what kinda rpms would this motor like to turn. i know i turned my 355 8000 to 8300 all year last year in my late model. would this motor take that or would it be more of a low end motor?? thanks


Dirt Full Roller

Total posts: 62
posted October 18, 2002 11:06 PM
Not to discourage you or anything, we have a motor in the works with the same stroke you mentioned, and we had to cut, grind, and clearance like crazy. Where we had clearance problems was on the oil pan rail, we cut enough off and got the rods to clear the pan rail, and then we had to grind on the rod bolts so that it would clear the cam. Now we are just waiting on the heads to get back from the machine shop. This is the first real exotic engine we have built, and we are expecting at least 8000rpm out of it, everything is super nice in it. just my 2 cents!!

Dirt Freak

Total posts: 353
posted October 19, 2002 01:44 AM
Your talkin 421 cubes of pure power. You can crank 'em!

Dirt Roller

Total posts: 13
posted October 19, 2002 06:22 PM
think about what you are building, small block bore with big block stroke, 6 inch rods are going to help with turning more rpm. personally i think a 6.125 rod with 3.750 stroke is more what your looking for still big cu. in but you can twist it harder than 421. there is a formula you can use to determine safe max. rpm for different stroke rod length combinations( with good quality parts).

Dirt Administrator

Total posts: 5556
posted October 19, 2002 08:21 PM UIN: 16262997
It is called piston speed....check out the toolbox, if you will play with this a little, you will see where the longevity breaks over on RPM on what we have all be told should be the max rpm of specific engines....pretty well holds true with this calculator as well.


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