I am getting ready to take my pistons to the machine shop and was wondering what is the rod length when offset grinding the crank shaft and puttin a big block chevy rod in it?
Not enough info. What pistons and what pin height? If the pistons are replacement type with stock 1.98 pin height (like most standard replacement piston) then you can use a 6.635 BBC rod with no offset, but widened to accept the wider BBC rod. The piston will be .035 higher in the cylinder than the standard 6.60 BBB rod, which reduces the amount of decking required to get the deck height nearer to zero for optimum quench. There are other pistons out there with higher pin height to accept a longer rod like a BBC 6.70 or more. All depends on what piston you have.
The Wiseco's have a 1.980 pin height(identical to stock). If you order BBC 6.635 rods, you cut the crank .050 to 2.200 BBC size. No offset needed, but you do have to widen the rod journals to fit the BBC rod since they're wider. Cutting the crank down to 2.200 does not mean offset, just cut down but still at 3.90 stock stroke. If your block has been decked before, you need to verify how much it was decked, or you may end up with pistons out of the block. The crank work would need to be complete, block bored, then mocked up to verify how much actual deck you have left. Then the block gets decked square. Also remember, BBC rods have a .990 pin, so the small end of the rods will have to be honed to Buick 1.00".
We are offset grinding the crank to a 4.00 inch stroke and the mechanic wants to go with a 6.535 inch rod in it. What do u think about that?
What u mean by a smaller journal rod like what? I want to know because he already have the h-beam bbc rod. I am trying to reach the 6.70 in the 1/8th mile. Along with the rods and pistons I have a set of big port 430 heads fully ported and the sp1 intake and wants to go back with the 408s camshaft. Th400 trans with a manual reverse valve body with a 4.10 gear and 3400 to 4400 stahl. In a full body buick regal.
You cannot stroke the crank to 4.0" and use a 2.200" rod. There is not enough material with the stock journal being 2.25". You would have to use a 2.100" journal rod.
What kind of rod are you talking about? If we make the stroke 4.00 what would be the cubic inch on it. Or if we make the stroke 4.125 what would the cubic inch would it be?
Your not getting the picture. You need to sit down with your engine builder and have him explain. You cant do what you want to do with the parts you have. 4.350" bore + 4.0" stroke = 475.5 ci 4.350" bore + 4.125" stroke = 490.4 ci
Maybe this will help. Since you are starting out with a stock BBB crankshaft you will have 3.90 stroke and a 2.25" diameter at the rod journals (stock, unturned crank). If you offset grind the crankshaft diameter to 2.20" for the BBC rods, that is a rod journal reduction of .050" in total diameter, only half of which will be an increase in stroke, or .025" from the centerline of the crankshaft (nothing off the outside, .050 off the inside). The total amount that will be added to the stroke from that change is .050" (+.025" radius x 2), for a new stroke of 3.95". You dont gain .050" stroke radius, only .025". Where does that leave you with a stock BBB 10.57 deck height? Well, half of the new 3.95" stroke is 1.975", and the piston height is 1.98". The formula for rod length is: deck height minus (1/2 stroke + pin height) = rod length In your case, 10.57-(1.975+1.98)= 6.615" rod length if the top of the piston is flush with the deck. Now if you truly want a longer stroke, you will have to grind the rod journals to a smaller diameter, once again taking nothing from the outside, and all the material from the inside. Lets try the math again with a 2.10" rod journal (from a 68' and newer SBC). 2.25-2.10=0.15" This increases the crank stroke that same amount for a total stroke of 4.05". 10.57-(2.025+1.98)= 6.565" rod length, once again with the piston flush with the deck. You could get an exact 4" stroke by taking a little off the outside of the rod journal as well as some on the inside, and the math would look like this: 10.57-(2.00+1.98)=6.59" rod length. All this assumes a stock deck height, and if it has been milled down then you will have to shorten the rod length (or the piston pin height) by the same amount if you want to run a "zero deck" motor. You can also run the rod length shorter if you want to leave a quench space between the top of the piston and the deck, although you can accomplish the same thing by using a thicker head gasket. By the way, if you decide to use the BBC rods, have your machine shop mill the big end of the rods to match the width of the factory Buick rods, rather than trying to widen the rod journals on the BBB crankshaft. I hope this helps clear things up. Class dismissed, Randy
Talked to Rick last night and he assured me that what I am trying to do is ok and it will yield me 470 cubic inches and it will carry me where I want to go. He dicided on changing the camshaft to the 308s from the 408s so I am satisfied now.
Formula for cubic inches is: bore x bore x stroke x number of cylinders x .7854 In your case, 4.35 x 4.35 x 3.95 x 8 x .7854 = 469.63 C.I. Looks like a strong motor from the parts you described so far. Let us know how it all works out, and include some dyno numbers if you get the chance. Later, Randy
I dont think that I will b dynoing this motor. I will be just putting it together and running it. I will be putting my Barry Grant 1000 carb on the motor. I hope that it will be pretty strong. I was wondering could I put a set of BBC aluminum rods in this motor to free up some additional horsepower? The only thing that is got me worried is the torque convertor. It is a 3400 - 4400.
Hi Guys, I realize this response is a few days late looking at the dates but AM&P's 470ci stroker crank is 3.955" cut for 2.200" BBC rods. AM&P can set you up with the correct 6.800 Eagle rods and pistons if you give Mike a call! (270)729-5556. Sorry, I've been in the hospital.
Absolutely narrow the rod to fit as cutting the crank is taking off valuable material that is needed for reliability and strength.
