Chapter 5 “Can’t think of a quote”
At the beginning of this story I said how I had not been able to get a copy of any Leyland drawings. As I was working through this project, I found that I could reverse engineer about 90% of the data I needed. Another 9% could be found on the ‘net. But that last 1% was a bit of a problem. Specifically, the fit between the guides and head… So, I asked for some help on the Ausmini forum and got some feedback from “winabbey” who had a copy of the head drawing and was happy to share it with me!!! Wish I had asked for help on the Ausmini forum earlier! Ironically this did not have that one piece of info I needed but it did let me check a lot of my reverse engineering and helped to get the machining a whole lot better. (I did find the tolerance I needed in the end; more later) The first thing I did was compare the head stud, valve guide and pushrod hole patterns with the drawing. I wasn’t too far off, all within +/-0.5mm. of the Leyland drawing, but I still corrected everything to the exact spec. Ironically, I had just had a 6 mm ally plate laser cut to confirm the fit of the head to block… It did fit so I was within the tolerance of the oversized head stud holes.
For reference the “front centre” stud (between 2 & 3 spark plugs) is a sized hole, all others are over-sized. When fitted the head can move about this stud. (Do any of us take the time to get the head as straight as possible on the block!) When you think that modern heads are located with dowels etc. this is a bit surprising but made the head a whole lot easier to make as my positional tolerances could be a bit looser.
I have very little machining experience and no equipment so this was never going to be something I could do myself. Luckily, I have a colleague who does and has! And would do it at very reasonable Mates Rates. I gave him the CAD, a set of my drawings, a copy of the Leyland drawing and of course the castings.
This was his initial 11 steps to happiness:
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Couple of things that changed, firstly he didn’t need to counter bore the plug holes, and there is an oil way drilling missing. But it should give you an idea of the amount of machining required for what is a very simple head! (Imagine a twin cam like KAD’s) In reality fact he did not do all of each step at the same time.
We also discussed reaming for the guides and cutting and fitting the Exhaust seat inserts and agreed that this was something that I would give to a professional engine reconditioning company as they would have the experience and correct equipment to do the job correctly.
If you remember the casting images; we had some casting defects on the main face, as we machined the head they all disappeared except one which turned out to be a hole into the water jacket. This was a real surprise and a downer. However, a bottle of wine (or two) later we thought about it and realised it was not over the compression ring or the combustion chamber so we felt it could be brazed up later. Actually; it is where there is a brass insert on the 1275 head to cover a core venting riser, I thought I had given the sand cores enough venting routes through the water jacket holes but apparently not. If I did more castings I would add a vent there, just like BMC did… Hmmm.
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