Valve Seat Issues?

[Durwood]

I added leak down and sump test to the M8 prerequisite list before disassembly, I did a stage 2 on one today and leakage was almost nothing, just like my test bike, less than 1%.

[HD/Wrench]

Sounds good but its to the point that we are checking for all this on these bikes so now its CCP test, Fuel psi test, Leak down and a sump test.   Cost will go up for sure.. Then add the break in fee .. 

[BVHOG]

BV, one of the big reasons for me is that you are lapping a valve that is in a guide that most certainly has clearance (and therefore wobble) and if it is in a guide that has miles on it it is usually bell mouthed and out of round from the rocker valve stem travel on top of that. The cup and stick is being rocked back and forth and the grinding compound is not effective in creating a concentric pattern. If the stick is not rotated a full 360* you can change the seat to the worse. As you know, the valve rotates continuously in operation.

If you have a multi angle seat the contact area will be thin and wear a groove in the opposite seat surface. Any preset interference angle between the seats ground to ensure narrow seating (and therefore a better seal) will be destroyed (especially if done on a fresh grind). The proper seating area on the valve face can me moved to a higher or lower position on the valve face when lapped and a proper narrow seat width on the valve face can become wider.

I have used fine compound when machinist's blue was not available, but just to put a track on the seat and valve face to get witness marks for checking the seat(s) for pitting, damage or uneven contact. Those things show up when you frost up the surface lightly with lapping compound buff marks.I have also lapped pitted seats etc. to get another week or 2 out of an engine while waiting for parts and the machine was needed. I just don't agree that lapping is a repair / build procedure. That's me, anyone else is welcome to take another stance.

Thanks for the detailed answer without all the other BS, makes sense.

[Gordon61]

...Unless of course this is wrong..

https://www.youtube.com/watch?v=FGg3ojioeiM

go to 7:30 in the video..

This makes more sense than your sense.. T man is right on as far as I can tell.

...they also mentioned leak testing after the seats are cut ??

[HD/Wrench]

BV, one of the big reasons for me is that you are lapping a valve that is in a guide that most certainly has clearance (and therefore wobble) and if it is in a guide that has miles on it it is usually bell mouthed and out of round from the rocker valve stem travel on top of that. The cup and stick is being rocked back and forth and the grinding compound is not effective in creating a concentric pattern. If the stick is not rotated a full 360* you can change the seat to the worse. As you know, the valve rotates continuously in operation.

If you have a multi angle seat the contact area will be thin and wear a groove in the opposite seat surface. Any preset interference angle between the seats ground to ensure narrow seating (and therefore a better seal) will be destroyed (especially if done on a fresh grind). The proper seating area on the valve face can me moved to a higher or lower position on the valve face when lapped and a proper narrow seat width on the valve face can become wider.

I have used fine compound when machinist's blue was not available, but just to put a track on the seat and valve face to get witness marks for checking the seat(s) for pitting, damage or uneven contact. Those things show up when you frost up the surface lightly with lapping compound buff marks.I have also lapped pitted seats etc. to get another week or 2 out of an engine while waiting for parts and the machine was needed. I just don't agree that lapping is a repair / build procedure. That's me, anyone else is welcome to take another stance.

Thanks for the detailed answer without all the other BS, makes sense.

Like trying to cut a seat with a pilot that is not tight in the guide .. Same thing in reverse  A full radius type cut on the seat + lapping will show up very fast and in  most cases create more damage than it will ever solve.  I posted that up years ago now. A customer decided to lap the fresh heads for what ever reason. Then filled the ports with fluid and they leaked.  He was complaining about it on here  .. Took them to zippers , they looked at it and did a new valve job.. Told him to stop doing that .. " paraphrasing on my part "