offset grinding crank

Slysc

Registered User
I'm rebuilding my shortblock this winter and I'm planning to use Chevy H-beam rods with a 2.0" crank journal. I figure this will allow me to offset grind the crank and get .155" more stroke. My question is, will the 2.0" crank journal weaken my crank? The stock size is 2.31". I'm figureing that 2" of forged steel is gonna be enough but I thought I'd ask before I make a mistake.

Anyone?
 
There was recently a discussion similar to this on the TBU. Basically, Ford recommends only going .010 on the crank main and rod journals. I think much below this and you will be going below the Tuftrided surface. If you really want to stroke the engine, there was a guy on here who either worked for SCAT or dsitributed for them and could get whatever crank we wanted for a reasonable price (read: around $1500). I would go that route rather than risk a big money buildup on a questionable stock crank.
 
you can but...

You can offset grind the crank at your own risk. If you offset grind it to a chevy 2.00 journal you will gain .300 of extra stroke not .155. the strength left would be dependent upon the craftmanship of the crank grinder.

I would like to point out that if you plan to use a small dia (2.00) journal chevy rod it is about .115 of an inch wider than our stock SC rods. So width of the rod would be an issue for you as well. In addition the bearings avalilable for that journal size are SBC sized to and are also TOO wide. I would really think your project out and make sure you can come up with all of your required pieces.

One last thing to remember is you will push your piston up .150 farther than stock. It will require a special custom piston with a very short compression height. It would be a special order from someplace like ROSS Racing pistons, J & E, and others.

Let us know if you do do it and how it turns out.

Chris
 
offset grinding

I hadn't thought about the width difference. That might discourage me from doing the Chevy rod thing. I was already thinking of going with custom pistons. I'm not familiar with the "tuftrided" area of the crank", I wouldn't want to make my shortblock with bulletproof H-rods only to make my crank the weakest link.
Also, taking the crank journal down .300" will only yield .150" in additional stroke.
 
Rods/Pistons

Chris,

I noticed you never replied to the "what rods and pistons are you using" post that was active a little while ago.

So.... What rods and pistons are you using? What sizes, etc?
 
not offset grinding

I talked to Coy Miller about it and he said that he wouldn't recommend doing the offset grind. He said that the design of the crankshaft requires the 2.31" for strength. The Chevy V8 can run the smaller 2.0" journal cause the crank is designed differently.
He said the last guy who tried doing it broke his crank and then ended up buying a custom forged stroker crank. (Fred).
So, since I can't afford the custom crank. I guess I'll just go with a set of 351W H-rods and slightly modified pistons.
 
well....

I am currently using completely custom machined rods and pistons. The rods are an H-beam style 6.000 inches long with a journal of 2.100 and a pin dia of .928. (yes offset stroke to SBC) The rod bearings are completely hand made to fit the crank and the custom rods. The pistons are custom Ross Racing Pistons made for an overbore of .050. Yielding a total of 4.13 liters.

Not to try and disagree with you on the stroke theroy of offset grinding but I have done this several times. So, if you were to offset grind a stock SC crank with rod journal Dia of 2.311 to a small journal SBC journal of 2.000 it would yield .300 of extra stroke. It gives you an additional .150 of stroke at BDC and an additional stroke of .150 at TDC.

I even went to far as to draw a simple picture to full scale so it would make sense to everyone. It took me a long time to understand when I first started stroking car engines so I thought it would help.

The difference in journal sizes is aprox .300. Yet the centerline of each rod journal only moves .150. But you get this additional .150 of stroke at the TDC position and the BDC position yielding and overall gain of .300.

Same would happen if you were to use the stock SBC journal of 2.100. You would gain and additional .100 at BDC and TDC yielding and additional .200 of extra stroke.

If the picture doesn't make sense get a plain piece of paper as I did and draw a straight line on it. Take a compass and draw a 3.39 circle in the middle for the stock SC stroke. (1.695 radius) Then where the circle you have just drawn meets the straight line draw (2) 2.311 stock SC journals. This gives you the TDC positions and the BDC positions. Next draw two 2.000 journals starting with the outer edge of the stock SC journal as I have done. Then you can physically measure them to check. It doens't have to be perfect to get the idea.

offsetgrind2.bmp


I hope this clears up anyones questions to what I have in my engine as far as rods and pistons and how an offset grinding works. There are other issues such as rod clearances with the side of the block and counter weight issues and the piston when it is at BDC but they can be worked out.

Chris
 
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your setup

Chris,

So your set up is very similar to what I was proposing to do.
When I talked with Coy, he thought that it would weaken the crank too much. You're obviously running some big HP numbers and haven't broke your crank yet. What do you think about the crank strength with a 2.0" journal?

Also, if I were to use the Chevy rods, couldn't I make the crank journals wider to fit the rods and the rodbearings instead of thinning the rods and making custom bearings?

And, .. yes, I get it now with the stroke. I was thinking about the radius of the stroke and not the whole diameter. It will add .300" to the diameter by adding .150" to the radius, your right.
 
well

I think widening the journals the .115 it would take to use a SBC Rod would weaken it more than the offset grind itself. Go look at a crank and I think you will see why. 2.000 jornal was more than what I wanted to go to. So if you do go to that size be sure to check the donor crank over really well for micro cracks and all. But sure to look into counter weight to piston clearance as well.

Another thing that people don't realize is rod bearing speed decreases with journal dia. Meaning that when you reduce the diameter of the jornal down the speed on the bearing decreass which will help in reducing the amount of wear the bearing recieves. It does however remove material from the crank which does in fact weaken it to a degree. But if you look at how the jornals are stagared with each other you will see that it is minimal in that area.

Chris
 
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2.0 journal

Why was 2.0" more than you wanted to go?

The reason I ask is that I could buy a slightly used set of Carillo H-beam rods from a guy but I need to know by 6pm tonight if I want them. ($275 for 8 of them). They have the 2.0" journal size and they are 6.2" long. The added length may help my piston to counterweight clearance.
 
compression height

Also, do you happen to know the stock distance from center line of wrist pin to the top of the piston on a SC?
 
Those rods would make your compression height something in the neighbor hood of .800. Not impossible but very small. Also I didn't want to go to 2.000 because it was just too much material to remove.

Chris
 
compression height

So if the 6.2" rod and the extra .300 stroke produces a compression height of .8, then the stock height must be around 1.4", right?

(6.2 - 5.9 stock length = .300 + .300 additional stroke = .6 + the .8 you noted = 1.4")
 
Actuall I think I did my math wrong cause the stock compression height I believe is 1.850 so it would make it

1.850
-.200 for rod length
-.150 for extra stroke (all you get on the top)
_______________
1.500

not as bad as I thought...but the pin dia is .928 which half of that accounts for part of the compression height. So you would have a usable portion of 1.050 for rings and all. It would work. Just try it and hope you don't break the crank.

Chris
 
2" too small?

just wanted to beat this dead horse a little more.

So you said that you thought 2.0" was too much metal to remove.'
Yet yours are 2.10". Do you think another .050 on each side is gonna make a difference? Are you worried about your 2.10" being too much? Is there any way to tell for sure (other than building it up and finding out)?

Just wondering if 2.0 being too small is a SWAG (scientific wild ass guess) or if it's too small because ...

thanks for your advise.
 
its not .050 on each side but more like .100 on one side. Remember you are only removing material from the inside between the rod journal center and the crank center.

Chris
 
Re: well....

wow good thread

Wzenheimer said:
The pistons are custom Ross Racing Pistons made for an overbore of .050.

Does that overbore make you nervous? I heard Morana once
built an engine .060 over but besides that I've never heard of
anyone going past .030. I know for stock block 5.0s going over
.030 is considered taboo and I've read a couple problems with
.050 and .060 overbores on stock 302 blocks. I assume that
the v6 block is just as thin wall as the 302, but I could be wrong.
Do you have info that reassures you enough to go that far out?
I am playing with some big valved heads right now and .050
over would help alot flowwise but I'm scared to try that :)
 
.050 Didn't really bother me with the thickness. The SC block is by far more stout that a 5.0 block. There is more internal webing around the mains, the cylinder walls are thicker, and of course the main caps are just freaking huge.

Also, in the repair manual they show stock replaement pistons in overbores of .020, .030, .040, .060. So going to .050 wasn't a big issue to me.

If I could grow some real big brass ones I would stroke it to .300 then bore it about .090 and use a 6.2 in long rod. It would create a 4.332 liter. However, in the modeling I did for engine power the difference in stroke from what I have now and the extra .100 didn't make that much difference. Also, with the cam that I run the extra .100 of stroke would make my piston speed go up almost to what i would call the safe limit on the H-Beam rods i am using. Which would limit the amount of RPMs I could run. And my cam that I have ground is designed to be a 4000 to 6500 RPM cam and that it is. Check the dyno graphs in the HP section.

Chris
 
SC crank

Chris,

I talked with Tom Morana about doing the offset grind and he said he wouldn't do it either. He says the forged SC crank is brittle. He's got a 4.2 that he recommends for the supercoupe but it has a cast crank. He says it's less brittle than the forged. (?!)
:confused:

How long have you been running your stroker setup? Looks like it's working for you by the dyno numbers your pulling. I really want to try it but I'm afraid of breaking. Did you do anything else to your crank to strengthen it after you did the offset grind?
 
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