I volume with a burette, so use one hole and the air gets out of the same hole. Two holes increase your chances of error, but as long as you get a repeatable process then the error margin will not matter so much.
A good tip is to not volume with water, it's viscosity is a little low and annoying, use diesel or paraffin if you have it.... wash tank fluid works good also, if it's clean.... many wash tanks use diesel or paraffin anyway...
Rule of thumb for skimming heads is around 6-6½ thou (0.15-0.16mm) per cc. And certainly a good idea to measure before and after. Then you can carefully open up the smaller chambers to match the lagest one.
Aim for +/-0.2cc, but if you have plenty of time and a slighty tricky process/repeatability, then aim to get them exactly the same.
I am based in sweden, so you probably wont want to send the heads here for machining. but most reputable engine reconditioners will have three angled valve cutting available. Ask if they have a MIRA cutter or Serdi, they are similar and the only system I would trust.
If they start talking about neway and/or using three separate cutters, walk away.... you want a combined, all three angles in one hit cutting tool.
Make sure you have valves ground on the seat. Shows up and distortion or bent valves too.
Ask if they have a vacuum tester for checking the sealing. With new tight guides and no stem seals you will want around -700 to -800mb of negative pressure. Any less and the seats will need lapping with fine paste.
With the stem seals fitted it should be -950mb or a max sealing of -1000mb. Obviously this is all compered to zeroing the tester on a flat surface before starting. Zero = -1000mb (full seal.)
Use 1-1.5mm inlet and 1.5-2mm exhaust seat widths. And point out to them that they should set the valve seat edge on the OUTSIDE edge of the valve, not the middle or centre.. this increases your effective minimum valve seat diameter on the valve.
For example. 30mm valve with 1mm seat width in the head, gives an effective valve seat diameter of 28mm. If you run a 2mm seat width, this drops to 26mm. If they then machine the seat to land on the middle of the ground seat land of the valve itself, then you could easliy lose another 2-3mm of diameter.....
If you want me to do it I'll be happy to help.... but if you can find a good head guy close to you then it's better to keep it local!
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