Exhaust valve seat in a small engine?

This is an Onan NHC 23 horse, 2 cylinder engine. A while back I posted my problem with the loss of power on here. One symptom was that blue flames come out from one of the mufflers.

I removed the exhaust valve from that problem cylinder, and the valve seat fell out. There was a gap about the size of a toothpick between the outside of the valve seat and the hole where it"s supposed to sit.

Before taking the engine apart, I ordered a new seat, valve, springs, and head gasket. The new seat is smaller in diameter than the old seat that was in the engine.

I"ve found an aftermarket supplier of valve seats. You can choose the seat you need by o.d. i.d. height and seat angle.

For simplicity sake, let"s say the diameter of the hole in the block where the seat goes in is 2.000" The supplier I found doesn"t have 2.000" seats, but they have 1.996" and 2.004".

Which one should I buy? They are only 6 bucks each, but they have a minimum purchase amount of $25. I haven"t taken the other cylinder head off, but I"ll do that next and see what the other exhaust valve seat is like.
 
You will have to use the larger diameter because you will have to take the head to a machine shop and have them cut a new hole and press the seat in and cut new angles on the seat. You can't just buy a new seat and drive it back in.
 
You'll want to order the seat that is larger than the hole it'g going into because it needs to be a press fit. Without any real numbers it's impossible to say wether your going to need to have any machine work done, but typically you'd want fairely tight press fit on something like that so your probably looking at a seat that would be about .007 larger than the hole for aluminium heads, and about .005 larger for cast iron heads.

Here's a link to a place that carries the cutters to prep the heads for an insert. The info I just gave came straight from their site.
SBI International
 
On a lot of aluminum heads they use a 'deeper' seat for more surface area to hold it in (or that's my theory).

I haven't looked at one lately but on some small motors like briggs they used a 'thinner' seat, sink it some and then peen or roll the the top of the block over the seat to help hold it.

RT (Just my 2 cents)
 

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