Chevrolet Nova Forum banner

Stupid crankshaft question of the day

3337 Views 5 Replies 3 Participants Last post by  69NovaSS
ok now...I'm sure I know the answer to this so please be gentle with me...but just how far can a crankshaft be offset ground before it is nolonger anygood?

Could you offset grind a crankshaft that has rod journals throws of 2.50" enough to accept rods that need a crank with rod throws of 2.20"

So I guess basicily I'm asking if you could grind .300" from a rod journal and still have a crankshaft that is useable?:confused::)
1 - 6 of 6 Posts
I guess you could; some Ford 460 cranks are offset ground quite a bit to build 513's.
The crank will get weak if you turn it a great amount. The best way to describe what happens is if look at a crankshaft from the top and imagine each journal is disk. Now look at the way each of those disks overlap each other. Normally each rod journal overlaps the main by about half. That stiffens the crank. A good example of this is 400SBC mains are .200 bigger than the 350; and the stroke is .270 longer than a 350. That keeps the ratio of journal overlap about the same.
I used to work for a machine shop; the owner would take cranks that had spun bearings that eaten away the upper loaded side of the rod journals. He would offset grind the crank undersize until the journal was usable. It would move the centerline of the throw outward slightly increasing the stroke. Maybe .020 to .030 to slightly increase the stroke of a 400 by a few inches. The reason he did it was 400 cranks were had to come by and expensive.
The size of bearings available limit how much you can take off using stock rods. I think it also decreases the load area of the bearing. The bearing speed decreases; but from what I remeber it weakens the bearing.

350 = 4.000" x 3.48"
400 = 4.125" x 3.75"
See less See more
well this is a super secret squirrel type know hush hush kinda thingy so I have to keep hold my cards close to my chest so I dont let out too much info...wouldnt want to have to kill ya:rolleyes::D

I did a little extra research and found out that yes indeed I can use the rods that I want to use and offset grind the crank to accept the rods and increase the stroke....just not as far as I was hoping it could be offset..BUT hey its all cool...

Now all the following info is based on manufacturer specs and nothing at this time has been measured so as we know these dimensions are subject to change:)

Hopefully after sonic testing the block (to verify wall thickness) a finished bore dia. of 4.250" will be possible. Couple that to a stroker crank with a stroke of 4.530" and the displacement will be in the 514" area.

Other than the crank all the parts are off the shelf parts...the crank itself will require offset grinding which will reduce the rod journal size from 2.5" down to 2.2" (BBC rod journal size) and increase the stroke by aprox .280" total and possibly a touch more depending on how things mock out....(could be a .300"+ stroke increase)

BB Chevy 454 standard bore pistons (1.52" CH)
BB chevy 6.8" rods
The block itself is 10.625" tall +/- .005"

So just based on the basic numbers here when deck height if figured out the pistons is about .040" down the hole so that is why I "think" it might be possible to stretch the stroke out another .040" total or so....anyway...this project will self destruct in the next 30 seriously this on is on the deep back burner waiting for the Nova to hit the streets first....Then it will see the light of day..:)

Nova_Guy thanks for the info...:)

When all is said and done it should make a ton of torque in the mid RPM range (3000rpm or so) and max out at about 5000 rpm or so...just what I wanted...;)

OH ya btw this is not a Ford or Chevy block:)
See less See more
Ok heres a pic of roughly the type of crank I'm talking about. Forget the aluminum rods as I will be using forged steels ones....this is, I believe, a factory stroke crank and I notce there doesnt seem to be any overlap between the rod throws and the mains...(I mention this because on Nova_Guy's comments in his post)..

Anyway...have a good one:)

See less See more
When you get into the 5.5" stroke category (as pictured above) you loose any hope of overlap. You must then rely upon the strength of the material of the crank (in this case 4340 chromemoly alloy) machined out of a solid forged billet.

When Chevy increased the stroke of the 327 to make a 350 they increased the rod journal size on all of their production cranks (cast and forged 1053 steel). They did this to increase the overlap to match the increase in the longer stroke. They did the same thing again when they introduced the 3.75" SBC 400 engine (only offered with a cast nodular iron crank) once again to improve the overlap. Chevy did this because customer's expect to drive a car with poor maintenance at least 100,000 trouble free miles or else they were sold a "Lemon".

You can offset grind cranks and machine custom compression height pistons (usually by machining the tops off off the shelf pistons) but for the modest gains it would have to be for a full endurance race (for that small perceived advantage to build up). I know this is the kind of thing that "Smokey" would do, just keep in mind his cars though fast finished few races.

I personally like to buy a aftermarket block and bore it to the biggest diameter piston made (not custom, sitting on a shelf), I then fit the longest stroke I can find to fit in the block without hitting the cam. Then I decide upon an RPM range I want to build the motor to and buy the parts to get me there for the short block. I then choose heads based upon my RPM range with the highest priority given to valve spring pockets as they will determine how long the engine stays together (I've sucked more than my fair share of valves). Finally I buy the cam. But that just me.

Big Dave
See less See more
Big Dave.....that is really good info thanks....:)

Well I guess I should let the "cat.......meeeeeoooooowwwwwww....outta the bag" (my Seinfeld reference);)

Anyway it really isnt a big secret.:rolleyes: This is an olds 455 motor.:eek: This motor from the factory has 3" mains, 2.5" rod journals and a 4.25" stroke. :)
here are a few of the stroker combos that can be done with the 455 block. said:
473" 455 [4.21"x4.25"] +.085" overbore/455 crank/425 Olds rod/.+060 pontiac 455 piston - pontiac piston fits the olds rod. This would be a good high rpm combo, as the rod/stroke ratio is improved with the 425 rod. A KB # 222 or other pontiac piston may even have valve notches that would work?

482" 455 [4.25"x4.25"] +.125" overbore/455 crank/std bore TRW 427 BB chev piston with dome removed, and olds rod's small end opened to .989"[custom pistons would probably be lighter than trw chev pistons]. YOU MUST USE A SOLID DOME chev piston. Hollow dome pistons cannot have their dome removed.

508" 455 [4.25"x4.48"] +.125" overbore/455 crank offset ground to 4.48" [2.25" pin]. Can use buick 455 rod, but this 6.6" long rod results in a poor rod to stroke ratio. Mondello uses the buick rod in the "motorhome" stroker kits, with a smaller bore. Requires the counter weights to be ground to clear bottom of piston. One could also use a "long" aftermarket pontiac rod[2.25" pin]

514" 455 [4.25"x4.53"] + .125" overbore/455 crank offset ground to 4.53" [2.2" pin]. Must use 6.7" or 6.8" aftermarket bb chev rods [apx c.h needed would be 1.535" and 1.635"]. Note- these combo could use a KB #224 and KB #204 pistons. They are flat top bb chev pistons.
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.