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Old 05-23-2010, 09:24 PM
Country Bumpkin Country Bumpkin is offline
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Default 74 to 80 inch differences

Ok.............. tore down my 78 FXS shovel today. It is a 74" that has .70 over pistons installed which would put it a little over 80 inches. I am not liking the shape the cylinders are in. Nothing major but you can barely feel a little scoring on one cylinder. Better to take care of that right now while I am rebuilding the bike. Also the wrist pins feel like they are a little sloppy in the pistons.

What I am looking to do is buy new cylinders and pistons and just make it an 80 inch.

My question is this:

Is the flywheel the same in the 74 and 80 inch motors? From looking at different catalogs I have, it shows the connecting rods and outer cases to be the same. Can I just buy 80 inch cylinders and pistons and be good to go?

Thanks in advance for your answers.
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Old 05-28-2010, 12:50 PM
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AlCherry AlCherry is offline
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The 74 has a 4" stroke and the 80 has a 4 1/4 " stroke with .030 more bore than the 74. Your 74 with the .070 overbore is not a 80. To go to 80" you need new wheels, and you will need new cylinders and pistons, since you are past the max the cylinders will go for stock 80 + a .030 overbore. And while you are there, replace the rods, crankpin, and all of the bearings. S$S sells excellent stroker products. There is no "Big Bore" cylinder kit that will make a 74 a 80.
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Old 05-28-2010, 01:00 PM
jaqmes jaqmes is offline
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A78 bored 70? are you sure that's not a 80 ci already
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Old 05-28-2010, 01:36 PM
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If my memory serves me correct, the 74 will go .090 over, and the 80 will go .030 over. Someone flame me if I'm wrong.
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Old 05-28-2010, 07:59 PM
Country Bumpkin Country Bumpkin is offline
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Thanks for the reply Al.

I definitely am planning on replacing the cylinders and pistons. I was thinking of using stock 80 inch cylinders and pistons if the bottom end was the same on the 74 and 80 inch motors. So you are telling that I have to have new flywheels in order to do this? When I looked up rods in the catalogs, it showed the same rod for both motors.
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Old 05-29-2010, 06:31 AM
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The Rods are the same. It's the flywheels that make the difference. The crankpin offset is different. The pistons are also different, they are larger and there is a 1/4" difference in the wristpin offset in the piston.
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Old 05-29-2010, 01:50 PM
Country Bumpkin Country Bumpkin is offline
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Got you ................ sometimes I have a thick head but it makes sense now. Appreciate the info. You saved me some headaches.
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Old 05-29-2010, 01:50 PM
 
 
 
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