Mick's 70 Lowlight

Did a bit more work on the heads this weekend. Yesterday I modified the jig I had made to allow me to counterbore the holes for the spark plugs and had a go at machining the test head. Unfortunately there is a LOT of vibration when milling through the fins and the jig didn't last past the first hole. In fact the jig came apart and the hole became elongated.

So today I decided that a more substantial jig was needed. I knocked up a new jig using 6mm plate and drilled it to accept the head in the two orientations required for machining the inside and outside holes. The head simply bolts to the plate using the rocker shaft bolts or through the mounting holes - depending on which side you are drilling from. I also allowed some clearance from the side to make it easier to clamp to the table. The initial wooden version could only be clamped from the rear which was not too great.

I tested it by boring the second hole on the test head, the result was much better. No movement, vibration massively reduced and a much cleaner cut. With the machining completed for the spark plugs the only thing I have left to do it tap the threads. I'll need to get an M10x1mm tap for this as I only have regular M10x1.25's.

I also spent a bit more time evening up the chambers and un-shrouding the valves. I'm still waiting for new valve guides to arrive so cannot cut the valve seats yet which is a bit annoying as I really want to cc the heads to find out where I am. I have a feeling that I still need to remove a bit more material to get the compression ratio where I want it but will hold off until the valves are fitted and the heads cc'd. (it's easy to remove material but not so easy to add it if I go too far.) I also added a second set of Singh grooves pointing towards the second plug.

So the heads are really not too far off from being finished. I'm (mostly) happy with the porting (maybe a bit more from the exhaust floor), the chambers are reasonable considering that it's a stock casting with no welding and the machining for the second set of plugs was a success.

I think that I'm just going to fit these heads back on to the stock motor and run them. I've ordered a bunch of stuff to build up a 1776 which leaves me with a 1600 long block doing nothing so I might as well put these heads back on it and do some more dyno testing. The new cam will go into an AS41 case that I have along with the counterweighted crank, a new set of 80.5 P&C's, a new set of 4140 H-beam rods and a new pair of heads. If the dyno testing shows some reasonable figures I'll buy some new stock heads and port them myself, else I might invest in a set of Panchitos.