Can you put in new valves with old valve seats?

This is regarding a Nissan ka24e OHC with an aluminum head:

My question is, if new valves can be installed while keeping the original seats?  Could the old seats be ground to match the new valve? Or, would new seats really be needed as well in order to get the proper dimensions?

I'm asking because the FSM says that you can't press the old seats out, you need to ream them out and then put new ones in.  Not sure a machine shop would be equipped for that.

Thank you so much!

Without knowing the condition of your current seats we can't say.

In general, if the seats are in good shape, haven't been cut too many times and haven't been pounded out by excessive valve spring pressures and high RPM, then yes they can be recut. It would just be a normal valve job.

Also, pretty much every automotive machine shop should be equipped to install new seats. It is a very common job.

Are you stepping up to a larger valve size? That would likely put you in the new seats category

Seats SHOULD be cut for new valves.  Being cheap, I have slammed valves in and crossed fingers with a quick smear of lapping compound just to verify the sealing surface.  Don't try to cut a valve seat with lapping compound, it may cut curved and only seal well when at room temperature.  The head of the valve grows quite a lot compared to the seat, especially the exhaust valves.

OTOH if you install new seats, which can be perilous because replacement seats are more likely to fall out, they WILL need to be cut because they probably won't be concentric with or perpendicular to the valve guide.  Replacing seats is not something one does for the hell of it, it's an involved repair.  Personally if a head needs new seats because the old ones are too damaged to cut, I'd look for another head.

This assumes stock size valves of course.  If you install larger valves, you'll probably not only need to cut the seats, but you'll also want to open the port at the throat to take actual advantage of the larger valve.  See oldskewltoy's head porting thread for better details.

Why are you installing new valves?

Soooo, grind & lap the seats-valves and assemble according to Hoyle.  Shim as necessary.

  Has the head been Zyglo or pressure tested?

 (do this before spending time or money).

Bored, or did an issue prompt this? 

Dan

In reply to 914Driver :

I would be doing it as part of a general rebuild, just because the engine has 177k miles on it and has a lot of blowby.   It'd be stock sized valves. Ideally, I'd like to rebuilt to as close to new OEM as possible.

I really appreciate that advice, everyone!

If the valves aren't so badly pitted that they can't be cut, I'd just send the head out for a valve job.

OTOH sometimes new valves are cheaper than that.

The answer to your question is yes. 
 

Regarding oversize valves on stock seats, yes you can do it, and if the cylinder head had been previously rebuilt it's strongly recommended. Grinding the seats drops the valve into the bowl which is really bad for flow. Grinding both is even worse. If you do the guides the seats will have to be ground as well. But generally speaking, if you do insist on new valves try and get .5-1mm oversize

Peabody said:

Grinding the seats drops the valve into the bowl which is really bad for flow. Grinding both is even worse. 

So basically, getting a standard valve job on the old valves and old seats will make flow worse? 

Yes