I've had questions in the back of my head I didn't know how to ask so thanks everyone for posting the answers, the questions, valve size info plus the pictures.

I raced my already well used F1 750 with CRRC thats now CMRA in Texas. It came with GiaCaMoto cams same profile as NCR 7. All I was told was it revs way up there but if you like to ride it that way to keep changing the valve keeper rings so they don't shear in half. The vertical head has been repaired once and I'd love to have JPrecision fix them up.

I got a second motor built by Ferracci when new for BOTT. It has 42.5 and 37.5 valves and NCR 7 cams. I ran it until the seats were too deep and it got parked. I know that valve seats have limited life with the big cams, I now know to start with 1mm running clearance and hold my breath a little. I am a machinist so I know well what it all means but I had never thought to measure between valves seated as a second referance so to speak relative to valve size. I know folks spread seats apart for more clearance and heard of it done to desmos. I assume its a small enough amount geometrically that it doesn't mess with running valve clearance.

I'll get it on a bench and start gathering fresh parts.

Luke

Measuring when they are sitting static in their seats is just a “rule of thumb”, a good start, and what to expect using different cams. Then you can decide how to proceed. High lift cams usually require less static space (about 4.5mm, but deeper valve pockets or a taller chamber); long duration cams requires the most space (5.8 to 6.0mm). Bigger valves, yes or no? Sink the valves, yes or no? Often some compromise is needed. And you can feel like a dog chasing its tail. The proof in the pudding (as they used to say) is still what happens once they are assembled.

And there is some disagreement on what is a “safe” opening space - the typical quote of .060’ - or maybe .040” - but could you use something even less? One engine of mine survived .022” for over a year. And on another it survived .017”. Are you a risk taker?

If you still have some otherwise-worthy heads (FBF) can’t you replace the seats (and guides), or have a shop (if you can find a good one) do that for you?

MikeV

Thanks for the encouragement Mike. I took the seats out a long time ago and stopped to make a plan. The plan was make some dummy seats that are a slip fit so I can find how high they can be and avoid clash. Then fit and cut new seats with a good idea where finished is.

My next questions will revolve around valve length, groove location and base circle diameter of low and high lift cams. I have parts in the used box to show and measure, help me make the right choices

Luke

Methinks you are planning too much. Don’t over-think it. Round up the parts. Just start it. The answers will come as you do it. And they will change no matter how much you thought and calculated.

I know what base circles are, but sure don’t let them decide much. And can’t comprehend putting in temporary seats for measurements. Valve length is one of the last decisions (starting with long enough), and that determined by the finished seat/valve combination once you have your desired static or seated clearance, and how much stem is sticking out of the rocker cover - when viewed from the side you should see just a small triangle.

I will try to post a few useful photos on a new thread since we are no longer talking about cam bearing seals. And measure the typical distance from the top of the valve to the closing shim keeper rings.

MikeV