HONDA CB750 FOUR PART 5 – the engine

836cc engine ready for installation

The engine I have now completed and ready for installation. The crank cases were firstly striped of components and modified. The starter motor was not going to be needed so I ground most of the housing for it away. This also meant I had to tidy the generator cover and gear selector cover. The single phase alternator kit from ‘Cycle Exchange’ in the USA is much lighter than the standard item and I decided it would be left partly exposed. Have a look at the ‘Cycle Exchange’ web site, which is excellent. I have ordered parts from them and they always been very helpful. As that end of the crankshaft is in oil, the bottom case had to be welded up. A retainer for a seal was made to use the same seal as right hand seal.  In the clean up process I had the move the engine number which is mounted normally on a small raised area of the case, just behind the starter motor housing. This was all removed and I remarked the casing.

Top crank case modified to remove starter motor

Single phase alternator is left exposed

After a few days of grinding every casting mark and seem off the cases I aqua blasted the two halves. The cases were then painted with PJ1 silver engine enamel. This paint I think is no longer available, but Simoniz do one which is also very good. I used two tins of different silver PJ1 (0ne was plain silver, the other Yamaha silver engine paint). The barrels and head are a slightly different silver to the crank cases which makes a subtle contrast. 

The engine barrels were re-bored here by Terry and a ‘Wiseco’ 836cc piston kit was fitted. The rods were changed for ‘super rods’  from ‘Cycle Exchange’. New primary chains were fitted from ‘David Silver’ spares (probably the best source of standard parts for old Honda’s). A racing ‘Tsubaki’ cam chain with a ‘KA performance’ tensioner were also fitted. The cylinder head was fitted with ‘Kimblewhite’ 5mm stem valves and guides by Terry. These are a racing valves that  are lighter and designed to give more flow through the head. You can see a picture of the head on our gallery page. The cam shaft I choose is a road racing cam from ‘Web Cam’. An adjustable cam sprocket was used the dial in the cam to the desired overlap.

Ready for the barrels - heavy duty 'Ape' studs fitted

Various other performance modifications can be done, like lightening the rocker arms, under cutting the gearbox and re-inforcing the rocker box, but as this is going to be a cafe racer and not a full race bike I have decided to stay with a milder state of tune.

Most of the suppliers I have listed here are in the USA. Unfortunately it is quite hard to obtain performance parts in this country. All the dealings I had with ordering parts from the US have been pretty good, however if you do order stuff remember the import duty, the Taxman will want his cut. If you do want something that you think no one makes please ask us as about it, we can point you in the right direction or we can make most things if you can’t find what you are looking for.

by Colin Jones