Can someone explain the difference between radius cutting valve seats and angle cutting? I'm familiar with angle cutting but not radius cutting. I'm finding that angle cutting is preferred, particularly on intakes, and some recommendations for radius cutting on the exhaust side. None of the comments were in reference to Y-block heads. 

My heads are being done. The machinist said that he did not think the seats would accommodate 5 angle cuts (15, 30, 45, 60, & 75). However, he said that he could do a radius cut. I'm settling for 3 angles (30, 45, & 60) plus a 30-degree backcut on the intake valves. 

That’s out of my realm of expertise as I’m only familiar with angle cuts at the seats.  There is a shop just south of me that specializes in radius cuts at the valve seats and that technology seems to be more application specific in its use.  The person doing that has a lot of experience in what radius works best for particular heads as he has a flow bench and the data to support that.  But he is a SBC and BBC guy and that’s where his expertise lies.

Lorena, Texas (South of Waco)

Sounds like I made the right choice by sticking with the three angle cuts.

I flow a lot of cylinder heads for folks as well as port heads and intake manifolds.  I have a Super Flow-600 Flow Bench that I use for testing before and after work.  I have quite a bit of experience with both angle and radius valve jobs over the years, and have found that the angle seats work best on the intake unless you are into ultra high performance racing heads like NASCAR or PRO MOD, etc.  On most street engines the intake valve job angles tend to give about 88-89% throat diameter if done properly.  The all out race engines have an intake throat diameter of ~92% with radius seat and ~50* valve seat angle.  On the exhaust side a radius works just fine for most every application since the air simply bleeds down.  On the intake side, the angles help the airflow to bend around the intake valve as it enters the combustion chambers.  A back cut on the intake valve helps with flow, whereas a straight seat cut of 45* works best on the exhaust valve.  Many high performance engines use an exhaust valve with the margin rounded over into the face of the valve.  That works if the valve is small in relation to the intake valve, but if the exhaust valve is real close to the combustion chamber wall, it seems to hurt flow.  Every head is different with respect to what works best and when in doubt, angles  work.  Here are a couple of pictures of radius exhaust seats and angle intake valve seats.   Joe-JDC

JDC

Thanks, Joe. 

Maybe I'm wrong, but I'm inclined to think that the average guy has no clue as to how complex building an engine can be. It's not uncommon to just slap a bunch of parts together and hope for the best. To some extent, at least as far as my heads are concerned, I could be guilty of that, although I've done my homework, asked questions, and I was conservative with port work, focusing primarily on the bowls. However, I won't know for certain until the engine is running. 

Not only the average guy, but the average machinist who does valve jobs for shops on the clock.  My local Ford shop has a couple of old Ford racers who are engine assemblers, do balance work, valve jobs, etc.  They are decent machinists, but the valve job guy just does not seem to understand what is necessary to increase airflow with the valve jobs.  I took a pair of FE CJ heads in for guides, cut seats for over size valves, and valve job with multi-angle on the intake seats, and radius on the exhaust.  When I went to pick the heads up, the intake seats were a radius, and the exhaust were angles.  No excuse for such a mistake, but there is no way to fix something like that when you cut the seats over size to begin.  We have a desperate shortage of machinists in this country, and with electric vehicles being pushed so hard by those in power, internal combustion engines and machine work and parts are going to be difficult to own and maintain at the current loss rate.  I get calls from folks all over the USA asking for port work because most shops only want to work with already CNC'd aluminum heads on customer builds.  Folks that want to build vintage engines are finding fewer and fewer shops who even recognize what some of these engines are much less how to properly machine them for performance.  Joe-JDC

JDC

Yep. The machinist doing my heads is 74 years old; I just celebrated my 69th. Considering his age, I thought he would be excited about doing my heads, nope. He said he has no personal interest in them and said they're "dinosaurs." Anyway, I see auto machine shops following in the footsteps of blacksmiths and farriers. With off-shore parts for popular makes being considerably more affordable, people would rather replace than repair, just as they would with TVs, appliances, and electronics.