Big Bore kits
Nothing replaces displacement but even more displacement
But bear in mind that it's the cam that is the key to an engine's performance characteristics. And, in the quest for power, it's the overall combination of all factors that result in the power output.
|Bore (mm)||Displacement (cc)|
|61.25 (1st oversize)||742|
|61.50 (2nd oversize)||749|
|61.75 (3rd oversize)||755|
|62.00 (4th oversize)||761|
K series vs. late F series
The F2/F3 not only has intake valves that are 2mm bigger in diameter than the rest of the CB 750 models, they also have a bigger (approx. 5cc) larger combustion chamber. That means that, with a given piston kit, you get less compression than advertised in a F2/F3 engine. Some manufacturers (Wiseco) listed different kits for K and F models. Dynoman has 836cc kits especially for the F2 head. The F1 head has (almost) the same C/R like the K models.
Rene van Maanen has photographs on his K2 web site that show the differences between a K7/F1 piston and a F2 piston.
Uses stock cylinder block which can be bored up to 65.5mm/850cc. Necessary modifications include boring the block, new pistons/pins/clips and head gasket. Most popular 836 kit today comes from Wiseco, at 10.25 C/R and 12 C/R. 10.25 is highly recommended for milder cams and/or low octane (low 90s) gas. Arias still has a 836 kit, too. Dynoman is listing 836cc pistons especially made for the F2 head. Cyclexchange now has a 65.5mm kit for 850cc. Yoshimura had kits for 812cc and 823cc. Some manufacturers used modified Honda CB 350 twin pistons. CB 900 DOHC pistons have 64.5mm and can make fit for 823cc but there is not much left for deep valve pockets, unfortunately. CBX six cylinder pistons have 64.5mm for 823cc, and a nice compression height.
895cc kits used to be very popular back in the '70. Main supplier was Henry Abe, their pistons are marked HAINST. Wiseco used to make at least a 915 kit, but it has been discontinued, as well as Arias. Installing requires resleeving the cylinder block, which means removing the old sleeves, bore the cylinder block, install the new sleeves, bore them, plus new pistons/pins/clips and head/foot gasket. There is a potential risk of oil leaks at the cylinder foot if the cylinder block is not bored exactly. Former manufacturers included Henry Abe, MTC and Russ Collins.
970 kits are even trickier. The cylinder block needs to be bored so much that there are holes in the air gaps between cylinders 1+2 and 3+4. These need to be welded shut, the block re-bored and the new sleeves installed. You can't do that at home with your electric drill!. This requires a well skilled bore shop, or you will end up with oil leaks. I've seen engines which had the inter cylinder gap welded plus sealed with silicone. Honda VFR750 pistons can make fit after heavy machining together with CB 1100 rods with larger wrist pins (17mm i/o 15mm). Japauto had a 950SS kit based on the standard block but gave that up for the VX1000 with a special cylinder block in late 1973 (Despite the different figures, both kits had 70mm bore). Roland Eckert of Germany was a manufacturer of reliable 970cc kits, based on the stock block. I have one of those in an Egli.
1060cc (73mm) and above:
All of the above, plus even after resleeving, the inter-cylinder gap is VERY narrow. Need to machine the crankcase to make room for the sleeves. Arias listed 73mm pistons with a 12.0 C/R, Powroll even had some with 73.5mm, and MTC had 72mm and 73.75mm.
The necessary or recommended piston clearance depends on the type of piston, cast or forged, on the piston construction, the material and the expected application, e.g. street or race use. In general, forged pistons require more clearance. For racing applications, you also often add a bit more clearance than for street use. As a rule of thumb, clearance should be between .016" and .028" for broken in cylinders and pistons. Pistons shrink during the break-in phase, depending on the alloy used. Stock pistons are pre-shrunk (heat treated) to faciliate the break-in period and avoid possible seizures.
Less clearance means less blow-by, hence more power and less heat stress on the piston. Also less noise. But small clearance means a longer break-in period, and a higher risk of piston seizure.
Here is a list of piston clearances as recommended by various piston manufacturers for their CB 750 SOHC/4 big bore kits:
|Arias||836cc forged||0.0010" (Street) 0.0020" (Race)|
|Henry Abe||888cc cast||0.0030" - 0.0045"|
The above values are recommended installation values for new pistons. After break-in, the clearances will be larger in some cases as the pistons shrink. Henry Abe's recommendations are in fact on the loose side, Arias has different alloys for race and street pistons.
Stroking the 750
CB 900 F DOHC crank
For the ultimate power hungry: The Honda Bol d'Or / CB 900 F DOHC crank can made fit into the CB 750 case to deliver 69mm stroke (63mm stock). Combined with stock or big bore:
|Bore (mm)||Displacement (cc)|
Bearings fit, so much for the good news. Rotor and ignition are swapped on the Bol d'Or crank, so you'd best swap them, too. (Need to put an oil seal on the left side). For racing, you may leave out the rotor and use a total loss ignition or a magneto. Primary chain is a Hy-Vo chain now, which is good as you would need a stronger primary train anyway. A drive sprocket from a Gold Wing can be fitted on the stock primary wheel. Need to take care of a chain tensioner. Cam chain also is Hy-Vo, probably machine a Bol d'Or or late CB 250/450 N cam sprocket to fit the SOHC/4 cam. Longer stroke means either shorter rods or a spacer under the cylinder foot, which is what most people do. Find a cam chain that fits, build a chain tensioner, and you're done. :-) Well, almost...
Stroking the stock crank
Another way is to have the stock crank stroked. Pascal Regen from France is building a monster with an Automatic crank which has 69mm stroke instead of stock 63mm.
To make up for the longer stroke, Pascal uses con rods which are 3mm shorter than stock. The clutch has a GL 1000 Gold Wing sprocket, the primary chain was made out of two chains of a CB 900 DOHC.
Crank work and shorter and stronger rods were done by Falicon