Seven Fifty cafe-racer. Big bore kit.

I suppose many Honda CBX750 RC17 and CB750 Seven Fifty RC42 owners have searched information about big bore piston kits for their bikes without success.  So did I, and I found no such kit available too.  But this small fact didn’t prevent me from further researches.

CBX750 and Seven Fifty have 67x53mm of bore x stroke. Cylinder sleeves are thick enough to be bored up for another 3mm.  That means 70mm pistons may be used, which in case of success will give us 815ccm instead of initial 747ccm.

But there are three bottlenecks in custom piston selection:

  1. Piston compression height. If it’s smaller than original piston compression height we’ll have problem of low compression ratio, if it’s bigger, the valves may hit the piston.
  2. Piston pin diameter that should be exactly the same as diameter of original one.
  3. The piston dome that should have relief for 4 valves.

I found and ordered 70m pistons with suitable domes which piston pin had 17mm diameter, same as Seven Fifty and CBX piston has, but their compression heights were smaller than I needed.

Comparing to Seven Fifty piston, pin to pin:

Pistons from left to right: CBX750, CB750 Seven Fifty, my 70mm piston.

Cylinders were bored and honed.

Assembling will be a bit harder than with stock pistons.

Available 2mm of difference in piston compression height is quite a problem. You couldn’t grind cylinder block or cylinder head surfaces for this height without having problems with timing, cam chain tensioner and so on.  But I had no intention to give up, so I began new research, this time I had to find longer connecting rods, compatible with Honda CB750 crankshaft and 17mm piston pin.

That was quite meticulous search and I was forced to rely on indirect information, but after some time I found out solution which looked like considerable risk, so I ordered connecting rods on ebay.  I received parts and turned out that I hit the bull’s eye. These connecting rods had dimensions exactly that I needed.

In addition, they even looked like new.

Thus, I found solution of 815ccm big bore kit for Honda CB750/CBX750.

P.S. I’ll share all information about what pistons and what connecting rods I used, but only after  this my project is finished. And anyway, everything must be tested before I could recommend this engine mode.

 

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6 Comments

  1. Matt

    I am in the middle of building an engine for my RC42. Using the gearbox and head from the RC17.

    Are you willing to share the information about which rods and pistons you used for the big bore kit? As I would like to do the same.

    Thanks alot
    Matt

    Reply
    1. gazzz (Post author)

      Hi Matt,

      It’s simple to find 70mm pistons with piston pin suitable for CBX750/CB750. I used pistons from Honda VFR750 RC36.

      The story with rods is more interesting. In 1980th Honda made 700ccm version of every 750ccm bike specially for USA market. I suppose it was made to decrease some taxes or pass limits. Such modification Honda made in next way: piston stroke was decreased, but barrels were same as for 750ccm model, thus difference was in crankshafts, therefore with same height of barrel longer connecting rods were used to bring the piston on the same level as it was on 750ccm model. Moreover, for 700ccm models Honda made some kind of high compression pistons, so I suppose there were no difference in HP and Nm between 700ccm and 750ccm models.

      The 700ccm “brother” of CBX750 was CB700 Nighthawk 1984-1986. New Old Stock CB700 connecting rods are rare now, but there are still plenty of used rods on ebay.

      Kind regards,
      Nazar

      Reply
  2. Matt

    Hi Nazar,

    Thanks a lot for your help, that’s great information!

    Great build by the way

    Reply
  3. Matt

    Just out of interest did you get a custom headgasket made up for the larger bore?

    Reply
    1. gazzz (Post author)

      There is no need in custom gasket, OEM gasket will do. As you may see on this photo, it fits perfectly:
      http://www.gazzz-garage.com/wp-content/uploads/2017/09/IMG_4235.jpg

      Reply
  4. Matt

    Ahh I see, all mine have had MLS gaskets which would overhang the bore.

    I’ll order an rc17 gasket. Cheers

    Reply

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