Unless you can do this yourself, setting up a Trident head with new valve seats and getting all of the seats cut, so you have the proper valve stem and fitted valve spring height correct, is a lot of work! Putting in the new seats is only half the job. Then you have to cut the seats so that you get the proper fitted spring height and rocker arm geometry.
Then there is the problems with these cast in iron seats where the original seats are not centered. Then when you cut the head for the new seat, you will have the new seat, half in the original cast iron seat and half in the aluminum head. This isn't going to be something you want. This means you will have to machine out what's left of the seat and weld up the head and cut a new hole for the new seat. $$$$$$$
As I said above, and it is good advice, unless your rolling in money, it would be cheaper, and you will get a better head in the end, to buy a good used head.
Call Keith Martin at Big D. He has heads 214 339-2285.
I have all the kit, including a dedicated fixture to mount the head in a Bridgeport and cut out the old seats to put new seats in Triumphs here from the days when we did service work. I have done hundreds of these heads over the past 40 years. I have all the kit to remove the old seats, and weld the head if required. I have all the dimensions required to cut the seats so the the valve stem tip is properly located so the rocker arm geometry and fitted spring length is correct. But if I could find a used Trident head that only needed a valve job, I would be hard pressed to refurbish a head that needed new seats. Even doing the job for myself it would be cheaper to find a donor head.
Unless, of course, you want larger valves and/or hardened seats. Finding a 40 year old head that has not been hacked by someone is not always easy. There is one for $385 on eBay with a dinged up chamber from a dropped valve, another for $320 but cannot see the seats and another for $230 that is nicely corroded with original copper based valve guides that you would probably want to replace.
A lot of the time your problem can be cured by cutting the existing seat to take an oversize valve. If possible, this is the least expensive course of action. Plus .060" valves are readily available for this head. If you are using stock pistons and camshafts there should be no clearance problems, but one should always check.
I did a little searching around and the going price for seat replacement seems to be $75.00 a seat. It was more for people who have experience with these types of heads. This didn't include replacing guides or additional welding or head work required. After all of the heating involved you will want to check and have the head straighten necessary. You don't want to just fly cut the head gasket surface to flatten it.
So with shipping you are approaching $500. Add labor to assemble, new valves, guides, springs and seals and you are getting close to $900.
Thanks again for all the info. I'm considering kibblewhite oversized valves and guides which I have used in Honda 4 stroke motocross engines with good results. The seats in the T150 heads use only two angles 45 and 60. The way the valve seats are in the head doesn't allow for a 30 degree cut. I'm thinking I can save the head after cutting a couple of the worst seats.
I have a question on this. I had the exhaust seats re-cut subsequent to a valve guide change. The re-cut caused the valves to "sink" or recess slightly deeper into the head. So the manual states the fitted length of the outer valve spring should be 1.229" however my actual outer spring (fitted) reading is 1.265". So the re-cut has added about .035" thou to the fitted spring length.... I assume that is acceptable?
Put the spring on the gauge piston and the whole thing in the vice jaws. Compress the spring to the desired force (piston is 1 sq in so the gauge reads direct) and measure the length of the spring. Make this the pack height when installing the springs.