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Tractor Pulling Discussion Forum

Pistons and Wrist Pins

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BUDD

10-22-2003 16:05:30




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Can someone please explain to me how changing pistons so the location of the wrist pin is in a different location helps. I have tried to understand this, but it just baffles my brain. Please when you do explain this make it REAL simple as I have a simple mind.

THANKS {'';'') BUDD




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vjohnyboy

10-22-2003 17:22:35




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 Re: Pistons and Wrist Pins in reply to BUDD, 10-22-2003 16:05:30  
by moving the wrist pin you can either increase compression or decrease this allowing piston closer to head or farther away second on a stroker if you go from 6 inch to 8 inch this the throw of the crank will increase 1 in each way if you dont shorten the rod or move the wristpin the piston would likely be to close to the head if not hit it this you move the wrist pin or cut rod and shorten it sometimes you do both depending on application i sell pistons that the wrist pin can be bored at a variety of locations about 3 in area

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TimC

10-22-2003 17:06:17




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 Re: Pistons and Wrist Pins in reply to BUDD, 10-22-2003 16:05:30  
Where the pin is located in the piston determines the compression distance. If you stoke a crank .500 and use the same rods then the compression distance would have to be .500 less on the new pistons.

Compression distance is measured from the center of the pin to the top of the piston excluding the dome. Measure to the flat surface that is the quench pad.

If you have .100 deck clearance where the piston is down the bore .100 from the top of the block then a piston with .100 more compression distance will make a smaller combustion area and you can use what ever thickness head gasket is required to meet proper piston to head clearance. This would be a little improvement.

Don't know all the details around what you have heard so this may not be a lot of help. Wrist pin location is critical after stroking. Hope this helps. The only other thing that comes to mind is offset wrist pins used to shift the thrust away from the major thrust side in the block to get free horsepower from less side wall loading and friction.

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TimC

10-22-2003 17:22:06




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 Re: Re: Pistons and Wrist Pins in reply to TimC, 10-22-2003 17:06:17  
Correction, compression distance would need to be .250 less.



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BUDD

10-22-2003 17:15:17




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 Re: Re: Pistons and Wrist Pins in reply to TimC, 10-22-2003 17:06:17  
TimC;

If I stroke a motor .5 I need to move the wrist pin up on the piston the same amout, right? What have I ganed? The piston is still topping out at the same point given this info. And the way I see this it will bottom out .5 less than before. I have always been a gear head, but this is realy stumping me. Even if I go exstream on a stroke of lets say from 6" to 8", if I do not move the crank I do not see where I am gaining anything. What am I missing on this? ('';'') BUDD

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JDGnut

10-24-2003 10:53:49




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 Re: Re: Re: Pistons and Wrist Pins in reply to BUDD, 10-22-2003 17:15:17  
I think U got the stroke thing down... You can also lighten the rotating mass with a lighter pistons... and get a better angle between the throw of the crank and the piston... a lighter rotating mass can be easier on the crank.. (the angle too.) lots of pistons out there for the A... Someone mentiong using the same rod.. this is OK.. if you are going to be building a small stroker.. but one of any size you will run into clearance problems early with the rods.. with the rod bolts on top and bottem like on a stock JD rod the quickly hit the top and the cam... with a angled rod (at the crank journal end.. like a 4020 rod or others. ) you will have more clearance to work with... Good luck..
JDGnut

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dctom

10-22-2003 17:57:06




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 Re: Re: Re: Pistons and Wrist Pins in reply to BUDD, 10-22-2003 17:15:17  
BUDD; IF YOU STROKE IT .500 PISTON WILL MOVE UP .250 FARTHER THERE FORE YOU MAY HAVE TO MOVE WRIST PIN DOWN .250 BUT PISTON HAS MOVED .500 FURTHER IN THE BORE DC



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TimC

10-22-2003 17:29:34




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 Re: Re: Re: Pistons and Wrist Pins in reply to BUDD, 10-22-2003 17:15:17  
Lets look at it like this. If you stroke a crank 1 inch then the piston is going to come up out of the block .500 inch on top and .500 futher down the hole on the bottom. So if you use a half inch shorter rod everything comes out ok. If you use a .250 shorter rod you would need .250 less compression distance so the piston comes back up to the very top of the block just like before but does not come out of the block. So you are coming up .500 inch futher and going .500 in futher down but the piston still stops at the very top just like before. Do this make more sense?

maybe i didn't make any miztakes dis tine..... ....

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dctom

10-22-2003 17:50:54




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 Re: Re: Re: Re: Pistons and Wrist Pins in reply to TimC, 10-22-2003 17:29:34  
COME ON BUDD YOU ARE SMARTER THAN THAT EVEN IF YOU DO RUN A DEERE LOL DCTOM



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angleiorn

10-23-2003 17:40:34




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 Re: Re: Re: Re: Re: Pistons and Wrist Pins in reply to dctom, 10-22-2003 17:50:54  
I DONT KNOW TOM HE IS GREAT BUDDYS WITH GRADEY



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deadeye

10-23-2003 05:32:49




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 Re: Re: Re: Re: Re: Pistons and Wrist Pins in reply to dctom, 10-22-2003 17:50:54  
Just remember ---half the distance of the stroke....deadeye



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