Valve-lapping for dummies

That little bit of surface pitting that you had on the valve face is pretty normal, and it may have been seating just fine. The best test is to squirt some carb cleaner into the port with the valve installed, and see if any leaks past the valve.

Having a wide contact area between valve and seat can actually cause that pitting that you saw. The exhaust valve runs VERY hot, and the hot valve hits the seat and transfers heat when in contact with it. If the contact patch is too big, it can cause the seat to get hot enough to cause micro-welds to the valve, and that is what causes the pitting.

I sometimes forget that on this forum, we are not always talking about modern engines. <G>
Modern engine design has hardened valve seats, and they are ground to three angles. The multiangle valve job allows better flow through the port because it makes a more rounded path for the gasses to flow past.

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Some motorcycle heads and valves do not tolerate regrinding, because there ain't enough meat there. I am a little more used to Harley engines. They have automotive type valves that can be reground, and the seats can be reground or replaced. The 3-angle valve job is de rigueur.

Here is what the service manual for my '81 Suzuki GS450 says about reconditioning the valve seats.

Valve Seat Reconditioning
This job is best left to your dealer or local
machine shop. They have the special equip-
ment and knowledge for this exacting job. You
can still save considerable money by removing
the cylinder head and taking just the head to
the shop. The following procedure is provided
in the event that you are not near a dealer and
the local machine shop is not familiar with the
seat reconditioning specifications.

1. With a 15° valve seat cutter, remove just
enough metal to make bottom of seat concen-
tric. See Figure 77.

2. With a 75" valve seat cutter, remove just
enough metal from top of seat to make it
concentric.

3. With a 45° valve seat cutter, cut a seat that is
1.0-1.2 mm (0.039-0.047 in.) wide.
​

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Now, a word about valve grinding compound.
The smallest traces of it in your engine is highly destructive. It doesn't just float around and eventually find its way to the oil filter or get drained out in an oil change. It embeds into engine parts and stays there to cause wear forever. After you are done lapping the valves, be totally anal/compulsive about cleaning them. Use hot water and detergent. Valve grinding compound is water borne, so don't try to clean it out with things like carb cleaner, use water and detergent. Get in there with soapy brushes and be VERY thorough.

I can't speak to the specifics of your particular engine without the service manual in front of me.That little bit of surface pitting that you had on the valve face is pretty normal, and it may have been seating just fine. The best test is to squirt some carb cleaner into the port with the valve installed, and see if any leaks past the valve.Having a wide contact area between valve and seat can actually cause that pitting that you saw. The exhaust valve runs VERY hot, and the hot valve hits the seat and transfers heat when in contact with it. If the contact patch is too big, it can cause the seat to get hot enough to cause micro-welds to the valve, and that is what causes the pitting.I sometimes forget that on this forum, we are not always talking about modern engines. Modern engine design has hardened valve seats, and they are ground to three angles. The multiangle valve job allows better flow through the port because it makes a more rounded path for the gasses to flow past.Some motorcycle heads and valves do not tolerate regrinding, because there ain't enough meat there. I am a little more used to Harley engines. They have automotive type valves that can be reground, and the seats can be reground or replaced. The 3-angle valve job is de rigueur.Here is what the service manual for my '81 Suzuki GS450 says about reconditioning the valve seats.Now, a word about valve grinding compound.The smallest traces of it in your engine is highly destructive. It doesn't just float around and eventually find its way to the oil filter or get drained out in an oil change. It embeds into engine parts and stays there to cause wear forever. After you are done lapping the valves, be totally anal/compulsive about cleaning them. Use hot water and detergent. Valve grinding compound is water borne, so don't try to clean it out with things like carb cleaner, use water and detergent. Get in there with soapy brushes and be VERY thorough.

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Valve lapping is quite a curious engineering process which of necessity I've studied in detail over the years. Prolonged lapping, especially with coarse paste, actually makes the seating surfaces of the valve and head insert concave so the two only make contact on their inner and outer edges. This is obviously very bad for heat dissipation as well as airflow. I can see the effects of heavy lapping very clearly on my valve refacing machine or head seat cutting machine as the grinding wheel or cutter makes initial contact with the concave faces and only touches them along the edges.

You can also easily see the concavity on a valve after prolonged lapping by putting a high quality straight edge across the seat and holding it up to the light. Try it on an old head some time. The mechanism at work here is that the paste on the inner and outer edges of the contact area quickly squeezes out as you start lapping leaving most of the abrasive action taking place along only the centre line of the seat. So lapping can't restore a badly cut or badly worn seat properly as they used to think in't olden days. It might have sufficed for a 30 bhp per litre truck engine from the 1940s but is not what you want for today's high performance machines generating much more heat which needs dissipating properly through surfaces in perfect contact with each other.

However a very light lap with fine paste for just 10 seconds or so to check that the valve and seat are truly concentric and with no high or low spots is a good idea and not a problem. If there isn't an even grey contact area all round both valve and head seat after that then it's probably time for remedial machining rather than further lapping.

For many years now I've used special diamond grit based paste rather than the normal carborundum grit paste you get in little tins with two lids for coarse and fine at each end from car accessory shops. It's horribly expensive but it has a completely different abrasive action which I can't really describe but it's much nicer. Being so hard and sharp, diamond grit abrades the surfaces really fast before the paste has had time to squeeze out and the grit particles don't break down into powder immediately like carborundum does so you don't get the concavity and it takes less time to check that the surfaces are making good contact. However the fine paste from those little tins is perfectly ok for general use. The coarse paste is a definite no no.

I used to have a customer in the early 90s for whom I did the CVH heads for his race car along with many other people's. They generally got a quick refurbish mid season and it took me a while to work out why every time I recut the seat on one of his valves (but no one else's) they were badly concave and only touching the grinding wheel on the inner and outer edges. After speaking to him it turned out that every time I sent a finished head back, despite my own quick lapping to check the seats were perfect he'd stand there for half an hour grinding them in further before assembling everything thinking he was contributing to the general cause and doing some good when in fact he was just buggering up my delicate machining work. After actually showing him what his tinkering had been doing to the concavity of the seats there was one of those "oh _____ what have I done?" expressions on his face and he left things well alone after that.

In OE engine production valve seats are never lapped which would be horribly time consuming to do on every engine but of course there are constant quality control checks being carried out to make sure the valve and head seat surfaces are being machined to a perfect specification. They also sometimes use a very slightly different angle on the seat in the head and the seat on the valve, maybe half a degree or so, to make the two components "hammer" into full contact after the engine is first started. Not my idea of perfection engineering really. Unfortunately you can't just assume that Joe Bloggs your general engine reconditioner is even capable of cutting proper valve seats which most aren't in my experience so checking them with a quick lap is essential. The much vaunted Serdi machine which is the popular choice these days is a ah heck for cutting non concentric seats in the head if there's even a fraction of a thou of valve guide wear. I prefer seat cutting systems with fixed rather than rotating pilots like the Sunnen system.

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