Don Terrill’s Speed-Talk

Often what happens with a torqued down and hot cylinder head is different to what happens on the bench.
At least starting with a near perfect valve seat allows you to asses what may be going on in a running scenario after some run time and an inspection of the seats and valves. Not always practical unfortunately.

joespanova wrote: ↑Sun Jun 10, 2018 8:35 am In freshening up the 496 in my 69 Chevelle, I'm down to the heads ( last hurdle ). After "cleaning" and reassembly is attempted , I notice the seats look consistent , at least as far as a "visual". The valves after cleaning look "normal" also. These are Brodix Race Rites with 2.250 /1.88 and ovals.
So my usual M.O. is to lap the valves to see how the seats and faces look. One seat ( intake) was bad enough that it wouldn't lap in over about 30% of the seat. On 2 seats ( intakes ) the top angle has a "chipped out" area just above the 45 ( not more than the top angle ) . On those 2 seats the 45 is intact. I'm going to assume the seat distortion on the one seat and the "chipped " areas on the 30 , on the 2 others , was a bad tune up on the previous owners part. Read that detonation. He had a TON of timing in this 10-1 engine and used pump premium. Bracket raced often. Heavy street car.
On all the other seats , they "lapped' in nicely BUT , they are NOT light tight at all. My lapping compound is not super fine so it can cover up a bad seal , I guess. If I apply pressure by finger I can ALMOST get them light tight. Jon Kaase suggested valve jobs don't even have to completely seal to be effective....in fact , he suggested even if I do get a complete light tight seal , the improvement probably wont net any thing. He stated he's seen this over and over again..........BUT , that being said , by nature we STILL want the perfect " static" valve job.
IF, and this is where it gets "dicey" ,if...........they are light tight with the springs you'd think it'd be fine..............BUT?
On my race stuff the valves ALWAYS seal light tight by hand.
ANY THOUGHTS?

Keith Morganstein wrote: ↑Sun Jun 10, 2018 11:40 am Non-concentric seats also lead to valve fatigue and failure.

Walter R. Malik wrote: ↑Sun Jun 10, 2018 12:41 pm Light Tight is great but, concentricity is essential.
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X2!! A light-tight seal on the bench is not essential, as the valve will contact the seat at different locations, each time the valve closes, as the engine is running. Concentricity of the valve face to the seat will ensure the the best possible seal. It is also typical to grind the valve with 1 degree difference between the valve and the seat, which helps promote valve seal during operation.

joespanova wrote: ↑Sun Jun 10, 2018 8:35 amMy lapping compound is not super fine so it can cover up a bad seal ,

DrillDawg wrote: ↑Sun Jun 10, 2018 11:24 am See if you can get a vacuum pulled on them with no springs. If you can call it good. Light oil on the seat and hold down the valve with your thumb\finger to get the seal started.

pamotorman wrote: ↑Sun Jun 10, 2018 5:17 pm the 1 degree difference in seat angle is the answer to a good seal. GM does this on production engines. check the SAE spec sheets

Schurkey wrote: ↑Sun Jun 10, 2018 6:46 pm

joespanova wrote: ↑Sun Jun 10, 2018 8:35 amMy lapping compound is not super fine so it can cover up a bad seal ,

The Permatex crap sold at parts stores is so coarse you might as well just grab a handful of sand from the beach, and mix it with some wheel bearing grease. I bought 400 grit, and I probably should have gone even more fine.

Some folks are against lapping at all. They say the grit embeds in the pores of the metal, can't be fully removed, and causes seat/face wear. I don't know what to think about that.

DrillDawg wrote: ↑Sun Jun 10, 2018 11:24 am See if you can get a vacuum pulled on them with no springs. If you can call it good. Light oil on the seat and hold down the valve with your thumb\finger to get the seal started.

I use a vacuum tester. The guy I bought it from (used) said that anything over 50% (15" vacuum) was fine. First set of heads, I was getting 87% (~26 inches) (valve seals installed.) Completely blocked-off, my tester will only generate 89% vacuum.

pamotorman wrote: ↑Sun Jun 10, 2018 5:17 pm the 1 degree difference in seat angle is the answer to a good seal. GM does this on production engines. check the SAE spec sheets

I know that this is common, but is it "right"? I thought it was a "cheat" to make the numbers look good at valve-job time, but promoted faster wear.

I do very, very few valve jobs...but I don't cut an interference angle.

modok wrote: ↑Sun Jun 10, 2018 7:27 pm so what if you have a customer that goes in the bathroom with your head bare, and turns out the lights? don't panic. Maybe this is ok?
Get prussian blue and use it to read the impression of the seat.
Adjust the angle of your seat or valve so you have even contact across. it does not really matter if it is 44 or 46

You CAN use light to test valve seats, but most will agree it is not the most practical method. There is no direct correlation between this and how it seals or runs. The easiest way to pass the light test is make the angles match exactly, however, as discussed they do not need to match perfectly.