The DOHC crank drops straight in no problem. I've had this machined though so it'll take a CB350 K4 rotor on the left hand end and an ignition rotor on the right hand end. That will hopefully allow me to have a self generating ignition some day soon (theory again
). The CB350 K4 stator then sits in a replica magnesium generator cover which cuts engine width down by a good few inches as well.
The crank is a lightened balanced standard DOHC crank - done by Basset Down Engineering in the UK who have many years of dynamic balancing experience. The conversion work for the rotors was done by a chap called Pete Rhodes again in the UK and possibly not unfamiliar in CR750 circles. The crank just drops in with standard bearing shells.
The hy-vo primary could be made from some other models' chains - both GL1000 or even a yamaha I believe but this one is custom made by Morris in the US sourced from M3.
Gearbox is close ratio Nova - not the same ratios as Daytona though as they're not so useful for standard racing. Daytona demands something a little bit special right?! This is the third CR Nova gearbox I've run in different racers and I can recommend them to anyone - superb engineers and awesome transmissions. I've just put a 6-speed box and dry clutch from them on my main racer the CB450 Bomber.
Primary drive is a cut and shut job with a GL1000 - drill out the rivets on both the CB750 SOHC and the GL1000 drive sprocket and re-assemble with the GL1000 outer ring. You can re-rivet the whole plot together but as it doesn't receive stress in that plane it's OK to tap and thread the alloy central boss and countersink the side plates to fit 16mm countersunk bolts, locktited in of course. Before doing that, I added some heavy duty cush drive rubbers to the internals - swap out the original softer rubbers for some very hard plastic equivalents, mine came from M3. They were a beggar to fit due to the lack of give in them so it'll be interesting so see what sort of cushioning effect they actually leave me with.
Underneath the primary chain what you can't see is the DOHC chain tensioner grafted onto a plate welded into the lower case to tension the primary chain. This uses the same DOHC sprung tensioner arm fed by oil pressure off the oil pump below it. That had to have a side arm welded on to allow the feed from pump to tensioner. The top crank case is just drilled to bolt in the top blade of the DOHC tensioner.
The shift drum, at this stage in the picture anyway, was an original CB750. It got changed for the custom mirror drum just before I closed the cases. The rods are indeed custom - to maintain the same squish pattern means new rods because there is a 1mm difference in stroke and 1.5mm difference in rod length between the SOHC and DOHC cranks. I worked on the principle that 1mm stroke difference is 0.5mm at top and bottom of the crank revolution so the crank big end journals must be 0.5mm further away from the centre on an SOHC crank. To cut a very long story quite long (
that means the new rods had to be 1.5 + 0.5mm longer than stock SOHC. Enter the experts of British conrod design in the shape of "Arrow Precision engineering", who make rods for many top UK classic Honda racers and they delivered a set of what I can only describe as mechanical porn
All I did was bolt the suckers in place.
The only other minor bit is the tensioner spring was a race kit part now obsolete so I preloaded the tensioner standard spring with a washer. It was tough getting the cases to join initially as the tensioner force exerted on the very strong hy-vo primary would lift the layshaft bearing out of its seat by half a millimetre but I got around that by holding the tensioner back with a piece of flat steel through the oil pump opening until the cases were joined and then everything sits nicely in tension....at least at standstill.
And that's it for the cases....picture shows original GL1000 primary drive on left and the hybrid CB750/GL1000 finished one on the right for comparison of shafts etc.
