liybzd

I was trying to lap the valve before assembling the head. After lapping and installing all valves I found that 3 exhaust valves were leaking. So I took them down and lapped again. I tried extra three times of lapping on the first leaking valve, then I noticed that the valve sinks into the seat a little bit compared to other exhaust valves and it's still leaking. I'm not sure it's caused by too much lapping because I didn't notice it in the first place. Any suggestions what I should do next? Thanks!

chris_barry

Are the valves in the same ports they came out of?

Any pitting or spots on the seat/valve. When you lap them is the lapped area on the valve and seat a continuous grey frosted circle? Look closely at the lapped area to make sure the valve isn't bent ( only touching half way around the valve) or its badly pitted.

Only real solution is to firstly check the valve guides are not worn resulting in poor valve location. If the guides are good then recutting the seats concentric with the guides and refacing the valves will have them sealing properly. If you do this just use some bearing blue to confirm that the contact area is good ( use a lapping action to spread a tiny amount of blue on the seats, wipe the valve clean then push the valve on/off the seat gently. If a consistent coating of blue transfers from the seat you are good to use. If its done right there is no need to lap. Lapping changes the shape of the seat and can imbed abrasive particles into the seat.

I have a Mira VG-91 with carbide pilots that I use for the seats.

liybzd

Thanks Chris. Initially I doubted if it's the problem of the valve, so I swapped it with the one adjacent and relapped both of them. Still, same one was leaking, while another one was not. The grey circle looks nice and smooth. I tested leaking by pouring water into the port and water seeped out at three spots around the valve.

I tested the guide by inserting the valve half way and checked the clearance by shaking it. It looks quite uniform between all exhaust valve, so I assumed they are all good. I'm gonna try the bearing blue, but I think the result should be quite good since the contacting area between both valve face and seat are smooth.

I'm not sure if I should continue lapping until not leaking, or too much lapping will cause excessive worn on both seat and valve so that I have to replace them. Replace seat can be troublesome.

SpitfireS

You can "measure" the valve stem-to-guide clearance with the "wobble method"
I'm not making this up, it is IN the service manual.
Slide the valve out of its guide (seat) about 10mm and measure the radial wobble with a dial indicator.
For exhaust valves it is 0.10-0.16mm (new) with a service limit of 0.22mm.

What grid(s) did you use for the lapping?

liybzd

I checked the manual, since I don't have dial gauge, I can't tell if it's in the spec. But all valve guides are close, and I checked several valve guide test video on youtube, the clearance seems normal.

I used Permatex 80037 valve grinding compound.