Porting

HP Valve chest with t-slot cutter to form exhaust ports

HP Valve chest with t-slot cutter to form exhaust ports

Some jobs just feel like they should be done later… In this case it was cutting the ports in the cylinders and valve chests – you have spent so long getting a nice accurate bore, and now you are about to cut holes in it! The scope for disaster is all there – it reminds me of being given the job of fitting car aerials to new  Audi’s as an apprentice in the ’70s – somehow attacking a brand new top-of-the-range motor with a power drill was just a very scary process!

Port Dimensions turned to offsets from base centreline

Port Dimensions turned to offsets from base centreline

So firstly I marked out the ports on the mating faces of the castings, on a surface block to provide a check when machining (the old addage – “measure twice, cut once”) then converted all the dimensions from the drawing into coordinate dimensions from the centreline base of the cylinder, as I had decided to use the mill’s DRO in “jig boring mode”.

I can’t tell you how many times I checked these dimensions, as on the drawings they are marked from top and bottom, but we needed them all as offsets. ….and it payed off, I found a mistake or two!

The next step was to start machining, I fixed one angleplate to the mill table (accuratly across it) and then set the DRO zero here, then mounted each of the castings between this and another angle plate and set to with the slot drills – tense moments!

milling the ports in the LP cylinder

milling the ports in the LP cylinderLP cylinder and valve chest ports in place!LP cylinder ports with lower cover in place

Finally they are all done, and the castings are almost ready to fit together…. again I decided to jig-bore the holes for the jointing bolts, as opposed to making the recommended jig – I think this was the right approach, although a little fettling with the diamond burrs was required 😦
a full set of ports!

a full set of ports!

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