Press fit vs. bushed rods?
07-20-2005, 11:37 AM
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Press fit vs. bushed rods?
Bear with me if I don't quite have the correct terms, but are there any major advantages in going with bushed vs. press fit rods? I can save some money if I go with pressed, but don't want to make a bad decision.
07-20-2005, 01:08 PM
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full floaters
I am asuming you mean connecting rods?
Unless your racing and take the motor apart a lot there is no major advantage to use bushings.
You need a hydraulic press to install pressed rods on the pistons.
They use pressed rods at the factory because of less parts and it is easier to install by pressing them on an assembly line. This means the wrist pin is pressed solid into the rod and the piston rocks on the pin.
HP motors will use full floating wrist pins and bushed rods.
This means there is a bushing in the rod and the wrist pins rotates freely in both the rod and piston. It ussually has an oil hole to lubricate it. The wrist pin is held in place with snap rings on each end of the piston. It will wear more evenly because it rotates around and is not in the same place all the time.
A pressed pin is always in the same position on the rod and no wear at all take place there. The only place it rotates is at the piston. Then it only rotates as far as the piston rocks, say 20 degrees or so. The wear for both styles will be at the top of the hole in the piston where it pushes down. The piston is aluminum so the steel wrist pin won't wear only the piston will.
So there is really no advantage in wear because the piston will wear out first. If you use a bushing in the rod that is just another part to wear out because the pressed one doesn't spin there.
The biggest advantage to have bushings is when you take the motor apart for inspection or to make piston changes etc. It is a lot easier to remove a snap ring and slip on another piston than it is to press them on and off using a hydraulic press.
If you have the money buy full floating forged pistons with bushed rods.
make sure they are weighed so they are all the same weight.
You will need to have the crank balanced to the new weights of the piston, pins and rods bushings rings etc. They use a special formula so you need to do it at a pro shop.
A forged piston with bushed rods needs a lot more clearance. This clearance makes a lot more noise. It will sound a little like a diesel when you start it up. After a few minutes the parts will expande and quiet down.
I use pressed pins on two valve knotch forged pistons (left and rights). The rods are stock, then shot peened, align honed and sized to the exact same length. Add SPS bolts.
Car has 10.75 compression ratio and around 400 HP
No problems holding together.
The lighter the better until you go to light
The bushings are great if you take the motor apart a lot.
Makes it easy to pull the rods off the pistons.
If you plan to run it 100k without taking it apart go with the pressed rods and Hyperutectic cast piston. They can run tight clearances.
It all matters in what you plan to do with the car.
Unless your racing and take the motor apart a lot there is no major advantage to use bushings.
You need a hydraulic press to install pressed rods on the pistons.
They use pressed rods at the factory because of less parts and it is easier to install by pressing them on an assembly line. This means the wrist pin is pressed solid into the rod and the piston rocks on the pin.
HP motors will use full floating wrist pins and bushed rods.
This means there is a bushing in the rod and the wrist pins rotates freely in both the rod and piston. It ussually has an oil hole to lubricate it. The wrist pin is held in place with snap rings on each end of the piston. It will wear more evenly because it rotates around and is not in the same place all the time.
A pressed pin is always in the same position on the rod and no wear at all take place there. The only place it rotates is at the piston. Then it only rotates as far as the piston rocks, say 20 degrees or so. The wear for both styles will be at the top of the hole in the piston where it pushes down. The piston is aluminum so the steel wrist pin won't wear only the piston will.
So there is really no advantage in wear because the piston will wear out first. If you use a bushing in the rod that is just another part to wear out because the pressed one doesn't spin there.
The biggest advantage to have bushings is when you take the motor apart for inspection or to make piston changes etc. It is a lot easier to remove a snap ring and slip on another piston than it is to press them on and off using a hydraulic press.
If you have the money buy full floating forged pistons with bushed rods.
make sure they are weighed so they are all the same weight.
You will need to have the crank balanced to the new weights of the piston, pins and rods bushings rings etc. They use a special formula so you need to do it at a pro shop.
A forged piston with bushed rods needs a lot more clearance. This clearance makes a lot more noise. It will sound a little like a diesel when you start it up. After a few minutes the parts will expande and quiet down.
I use pressed pins on two valve knotch forged pistons (left and rights). The rods are stock, then shot peened, align honed and sized to the exact same length. Add SPS bolts.
Car has 10.75 compression ratio and around 400 HP
No problems holding together.
The lighter the better until you go to light
The bushings are great if you take the motor apart a lot.
Makes it easy to pull the rods off the pistons.
If you plan to run it 100k without taking it apart go with the pressed rods and Hyperutectic cast piston. They can run tight clearances.
It all matters in what you plan to do with the car.
07-21-2005, 02:39 PM
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Try this page
carcraft.com/howto/1004/
this is an extreme build using new parts
but you can get 400HP to 425 HP out of mostly stock parts if you know what parts to use.
The idea is to use flat top pistons as high as possible in the block with the smallest chamber heads. Make sure every head chamber is the same size. You can do this yourself with a CCing tool. All you need is a piece of plastic with a hole in it and you fill it with water and measure how much it takes in the chamber. Then take the smallest chamber and grind it out to that size until they all take the same amount of water.
mortec.com/cckit.htm
A good running motor is one that every cylinder is the same compression ratio.
you can use the 6 inch connecting rods with a shorter piston to achieve the zero deck height. If you have the money do that.
This allows the rod angle to be less therefore reducing the stress on the crank. Plus the spark timing is changed a little due to an earlier TDC
Compression ratio changes make the most power for the dollar.
carcraft.com/howto/1004/
this is an extreme build using new parts
but you can get 400HP to 425 HP out of mostly stock parts if you know what parts to use.
The idea is to use flat top pistons as high as possible in the block with the smallest chamber heads. Make sure every head chamber is the same size. You can do this yourself with a CCing tool. All you need is a piece of plastic with a hole in it and you fill it with water and measure how much it takes in the chamber. Then take the smallest chamber and grind it out to that size until they all take the same amount of water.
mortec.com/cckit.htm
A good running motor is one that every cylinder is the same compression ratio.
you can use the 6 inch connecting rods with a shorter piston to achieve the zero deck height. If you have the money do that.
This allows the rod angle to be less therefore reducing the stress on the crank. Plus the spark timing is changed a little due to an earlier TDC
Compression ratio changes make the most power for the dollar.
07-21-2005, 06:04 PM
#5
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St. Jude Donor '04-'05-'06-'07
Go with the floating pins if you can afford it. A little less resistance and the pin bushings don't wear to any significant degree. Most of the engines I work on for a living have them along with extremely high compression ratios and the bushings usually don't even need changed at overhaul time.
