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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