Valve seats

[andreas.voigt]

How can I check the valve seats (sunken or not)? (Ariel 350 NH 1958 alloy head)

[nevhunter]

If the head is removed and the carbon carefully cleaned from the combustion chamber surface you will get a bit of an idea of whether the valves are "down" into the seats. To do it properly the valves must be removed from the head. Reface them and see how much metal still remains at the outer edge, ( minimum of .015"). Some like a bit more.
Measure the stems for wear to determine if they can be re-used also.
Assessment of the seats takes a bit of experience, but essentially. the surface of the combustion chamber, being spherical, the closer the edge of the seat is to the original surface, the better. The original seats are quite thin, the exhaust being about 1 mm and the inlet can be a little more. The valve seats are about 45 degrees and as the seats wear ,they recess and get wider. The seat has to be re cut at 45 degrees and then narrowed down to the sizes stated . The new seat should be in the middle of the refaced valve seat , so metal has to be removed from the top (outside) and sometimes in the inside throat, with a cutter. This is specialised work and sometimes some aluminium is removed to get the seat thinner. This is not acceptable and it is better to replace the insert(s) if this is the case. Modern insert material is far superior to what was available in the fifties, so done properly the job will be better than new. Fitting the inserts using liquid nitrogen (cryogenics) is the go as the interference fit is more guaranteed. Long winded , but I hope this helps. Nev

[john.whiting]

Andreas, what Nev says is correct, HOWEVER most experienced engine reconditioners will not touch alloy bike heads, except to recut/face the seats. If your seats are badly sunk or have been deeply recut, usually larger valves can help.To replace the insert, first you need to know how it was put there originally,and what it is made out of.Many bike inserts were cast in using the ALFIN process which effectively bonds the iron seat to the higher expansion rate alloy.Fitting a new seat with a large interference by shrinking may cause the head to crack. Not enough interference and it will come loose.I suggest if you live in England, send it to Draganfly, or at least get their advice.