I have a vacuum pump which does 20" by the gauge that is inline. Test block with worn guide. Cut a seat with Neway tool. Dropping in new valve will do 17", if I lapp some goes to 18", lapping more will get to 20". So lapp to 20" and call it good?
Bear in mind that things like throttle opening, valve lash with solid lifters, and ignition timing will also affect manifold vacuum at idle.
If the guide isnt sloppy, lap with the finer grit til it feels smooth and there is an even line on both valve and seat. If you're chasing it, then the guide may be a bit too sloppy. Then recheck vacuum with a valvespring installed.
Oooops! I just realized that you're doing your testing with the head on the bench, not on the engine. I didn't read enough into your original post. Never mind . . . !
To double check vacuum pump I replaced gauge with a new new one; with thumb stall it will do 25". So in back tracking I checked the cutter inserts with a 10x mag I could see a mark in the center of the .060 wide seat area on on all 5 inserts which left a tiny high spot in middle of seat; marking a new valve and checking in seat showed a scratch mark on valve, 20' vacuum. Spring and o ring to seal stem gets 22".Lapping a valve some will get to 24" close to .060 wide with marking. Putting new valve on lapped seat will do 24" and shows about .060 wide. So cutting seat and lapping getting 24" should be good?.
Normally lapping valve seats is only to check seat contact. Proper valve jobs really don't need lapping. Hopefully you are using fine compound (600 Grit or finer). Over doing it with course compound can wear a groove in the 45 deg. seat on the valve itself. You should have 360 deg. contact very quickly. If not, you might have a seat concentricity issue or a slightly bend valve. This is all ***uming the angles are ground correctly. I usually check the valves for run out first. I would think pulling 20in/hg should be more than good enough. Some will close off the va***m pump and watch the leak up rate on the gauge. Hope this helps.
tomcat11 : Lapping compound [clover] I have is 600 and it doesn't take much to wear the tiny high spot on the seat [shows as a scratch on valve full dia.] or to wear the compound for that matter. I am checking the valves for concentricity, in checking so far they are under .0005. I will check leak down just to see what happens. I have confidence to continue, Nervous norvis in me will be to get all the seats to same depth.. Thanks to all for help.
Do you have a picture of the set up you are using to draw the vacuum? I'm just curious as to how you are mechanically connecting things. Getting a consistent depth for all valves should be done with the cutter as the lapping process should remove very little material and produce minor depth change.
Set up in mill so I have digital read out to maintain depth and bump stop under cutter to take up any play in the universal connection between spindle and cutter. Have fixture with shot bolt that locates valve guide, spindle never moves from center of shot bolt. I move block to the shot bolt. I will work pics .
Im guessing you are just dropping the valve in with no spring? A hundred or so pounds of spring pressure will likely increase the seal.
How worn is guide? Have you checked valve stem to guide clearance? If too much clearance how do you expect cutter arbor to pick up exact center and make a concentric seat cut with respects to guide center line?
As long as arbor can get tight before it bottoms out then center is found [from what I've read]. When I put valve in to check I put a small spring on the valve stem up against underside of head of valve and then an o ring up against spring, When valve seats spring pushes o ring against guide to help seal it, good for 2" or so.
Yes and agree about spring pressure adding to seal but to check [like shops do on U tube] before ***embly as the valve seat machines all seem to have vacuum for checking.
Haven't seen valve grinding compound in professional shops this century; just isn't used any more. If the seats and valves aren't cutting true, there's something wrong with the equipment or the operator. jack vines
In 30 years of building engines I have never got a new valve where stem to tullip was dead concentric. They all go on my grinder and when corrected they need less than 1 minute of lapping to achieve a proper seal. The valve will not need a spring to pull it straight, it seats and seals on its own because of perfect concentricity.
Vacuum pump Cut down test block Seat cutter checking on lathe Valve with spring and o ring for guide seal Arbor and bounce spring Shot bolt bushing and lock Shot bolt
Seb, If you can get 24" of vac with your pump and setup that's excellent. It'll be a fine valve job. Well done.!
Looks like you have put a lot of thought and work into your process. Where there is a will there is a way.
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