I’ve embarked on building a new engine for my Bros. Like my current engine, it is based on the long stroke engine from the Africa Twin XRV750 and the main changes I’m making are to fit the 4 valve top end from the NT700-V and 89mm pistons to take capacity to nearly 900cc.
The main advantage of the 4-valve heads is that Honda deleted the second spark plug and fitted much larger intake valves, 32mm compared to the 26mm items in the 3-valve heads (see pic). There seems to be some agreement that the standard valves are one of the major limiting factors on power output from the standard engine.
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Happily used NT700V parts are cheap and plentiful in the UK - I bought a complete and sound engine for a bit more than 200 UK pounds and already had the remains of a blown up AT engine that I’d bought earlier for the heads. Other than a broken crank, the rest of it is in good order.
Perhaps the main technical challenge presented by the 4 valve conversion is that one of the 10mm cylinder studs doesn’t line up, the other 5 studs do and the cam chain tensioner is common to both models. On the AT engine, the oil way to the head also lines up - I suspect that a conversion of the 650 Hawk/Bros motors with the external oil feed would need extra work in this area but otherwise would also accept the 4 valve head if the stud problem is resolved.
The way that I did this was to find the centre of where the stud should go on the crankcase and then to bore out the case at that point to accept an aluminium insert that was then pressed in and welded. A friend of a friend kindly did the welding for me and I machined the cases for the insert by adapting my mini lathe to hold the crankcase halves. I also used this set up to drill and tap for the 10mm stud and to clean up the joint face. I finished off the joint face with some emery cloth sandwiched between the cylinder barrel and the crankcase.
With this critical stage done and the 4 valve barrel and head sitting on its studs the next thing will be to bore out the cylinders and crankcase mouths for the bigger liners. I’m expecting that I’ll have to farm this bit out to the professionals or at least the fitting of new liners and the bore and hone. I’ve got some 89mm pistons from JE which I’m hoping should do the trick. They take the same pin diameter as the AT and the other critical dimension is compatible. They are for a four valve engine although, when I get to that stage, I shall have to check carefully that there is clearance between the valves and the piston crown.
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