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Tips for turning a '80 CB750F into 890cc

davewnh

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Hello all! I thought I'd have some fun with a 1980 CB750F engine I have lying around and upgrade the internals.

First stop is getting the block bored out and re-sleeved to 890cc with a Wiseco kit I recently picked up. So far that seems like the easy part. I know I want to upgrade the cams and valve components, and have the crank balanced. The question I have is if I upgrade the connecting rods.

This upsize brings the 9.0:1 compression to 10.25:1. I can take my old connecting rods and check them on a liquid MT bath for stress fractures and integrity, but this being the first time I've tinkered around with an engine I just don't know if the original rods will handle the increased force. I've been quoted a new Carrillo set for $1150.00, that's a tough pill to swallow. Do any of you know if that's the right investment to make, or are the original rods perfectly capable of handling the job?

Any helpful info or tips would be greatly appreciated. Thanks!
 
I think you will be ok with the stock rods as long as you aren't going to be racing or seeing redline frequently. At a minimum I would have them shot peened and maybe look at getting new bolt. Carrillos are the bulletproof answer and needed if you plan on running it hard. If it is going to be a fun street bike I would do a little work to the stock rods and run them. I send all of my motorcycle machine work to APE...they do the best work in the business. Your biggest cost in this build is going to be buying a set of good aftermarket carbs because the stock ones are junk...even on a stock motor.
 
Thanks for the reply! I guess it depends on how healthy they look once they're out. Shot peening sounds pretty simple. I have access to forgers who can heat treat materials, don't know if that's a viable option too.

One of the first things I did was throw out my exhaust and carbs. Once I figure out what my exhaust is going to be, likely a 4 into 2, I'll figure out what intake will work best.
 
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