Any thoughts on valve lapping. I have new heads, new intake valves, an am usung the old exhaust valves. The heads came with the seats ground but I am considering lapping the valves just to touch them up. I certainly don't want to foul anything up but am trying to save a few $s by not haveing a valve job performed on new heads..
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Re: Valve lapping
I just finished lapping the old valves in on my Cosworth Vega after having the valve guides replaced. Everyone said it wouldn't work, but they lapped right in.
There is no harm in trying. If you can't get a good seat you've lost nothing but time and the cost of the lapping compound.
I bought a lapping set at Kragen that had tubs of #120 and #280 grit compound. I also like to dykem blue the seats and valves, and if they lap in with a few seconds of turning, you're probably okay. Start with the 120 and once you've got a good seat contact pattern, finish them up with the 280.
After lapping carfully inspect both the valve seat surface and head seat surface. Especially on the exhaust side, there might be little dark specs. These are areas of erosion. If they extend all the way across the seat the valve will likely leak and burn, but if they are just randomly scattered and don't occupy a large percentage of total seat area the valves will probably last while.
You can "grind down" some of these areas of erosion by with more lapping, but they likely will not completely go away. If you want to save time lap with a drill using a piece of 5/16" fuel hose clamped to the valve stem and a suitable mandrel in the drill chuck. It's best to use a reversible drill and lap both directions.
As a check I made somme aluminum plates with vacuum hose nipples to cover the ports. Then I pump them down to about 12" vacuum and look at the bleed rate from 10" to 5". I figure five seconds or more is okay.
My CV project started out as just a valve guide seal replacement, but once I got the cam carrier off I realized the guides were severely worn, so off with the head. Being as how I plan to do a complete blueprint/rebuild in the next 10 to 20K miles, (which will be ten years from now). I was just interested in getting the engine together and reducing the oil consumption, using as many existing parts as possible as virtually everything will be replaced when I do the major rebuild.
Duke -
Re: Valve lapping
Robert:
You may do as Duke suggested and generally all will be fine. I tend to err on the side of conservation and would take the exhaust valves to a machine shop and have them faced before lapping. That way you are 100% sure of the val/seat mating surface when you lap them and will get a nice seat. In my experience, not all seats wear the same and this affects valve wear as well. Pitting is one thing but you also have to consider the wear on the valve itself. I have a Kwik-way Valve re-facer in my garage and could do this for you if you are interested. Email me if you want to talk more about this.
Regards -
PatComment
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Re: Valve lapping
Pat's idea is good. If the exhaust valves have never been ground there should be enough margin to grind them, and it shouldn't be more that a couple of bucks apiece. Then you can lap in a good seat and they should last a long time.
The reason I elected to just lap the CV valves to a seat is that they already have been ground once and don't have enough margin for another grind, so I just lapped them in, and if I get 10K to 20K out of it, I'll be happy.
When the engine comes down for that major blueprint/rebuild in ten years (200 HP SAE gross @7500 should be the outcome), it will get all new valves on both sides.
DukeComment
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