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Originally posted by gharper
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There are three things going on with seat angles:
1) Gas seal
2) Gas flow
3) Ability to self clean
Folks will tell you there is even more - but for 95% of the work we'll likely see as vintage motorcyclists; these three things cover what we need to address.
For the gas seal -- pretty much any angle works. The two most common angles are 30 and 45 degrees; though motors have been built with everything from 20 to 55 degree seats. Neway has long argued for cutting at 46 degrees . . .and leaving the valve at 45. It's supposed to offer a better cut seal . . .but I've always cut at 45 anyways even when custom setting the angle on stones. (there's a reason that a degree scale is on the side of the diamond dresser for stones).
For the gas flow -- think of air the same as liquid -- it has to "flow" and it has to turn corners. Air velocity remains relatively stable if the turn is no more than about 15 degrees at a time; hence there is often a 15 degree separation in the "three angle" valve job (60 degree, 45 seat, 30 degree top cut). Too much of a turn and there is a risk of fuel dropping out of suspension. That equals lean and most people over compensate for it and create carbon in their motor. Probably the biggest contributor there is plug readings on too cold a plug for the compression ratio.
Ability to self clean is more complicated. In essence, all valves carbon and all valves rotate in service. Our seat pressure (springs and installed height) coupled with the angle allow this carbon to be "crushed" for lack of a better term and for the heat of the valve itself to burn the powdered carbon clean off the seating surfaces. You can see this when you pull down many old valves -- the valve will be crusty, the chamber crusty, the port, eh but the valve seat will shine out at you. And, it's amazingly skinny to boot!
In relatively short order it was discovered 30 degree seats flow a lot of air but don't self clean as well as 45 degree seats. This doesn't mean 30 degree seats are a great choice and 45s poor or vice versa. It's simply a data point and much is dependent on other choices affecting how the motor breathes. If your bike came with 30 -- stick with it. If it came with 45 -- well, that's most motors.
Most prewar motors do not have more than one angle (30 or 45). The reason isn't because our forebearers didn't have the technology to do multiple angles. They sure did -- and plenty of "valve reams" from before the war come in sets with throating cutters (60, 65 or 70 degrees) and topping cutters -- usually 15 or 20 degrees. The real reason is metalurgy at the time meant valve life was a challenge no matter what. Most manufacturers ran BIG seats to deal with heat -- especially in the exhaust valve. As you sank the valve with subsequent valve jobs; "topping" cuts of 15 or 20 degrees were used to "relieve" the top of the valve area. This also helped keep the carbon build up down. Throating cuts were generally for when you went to oversize valves and suddenly needed to bring the seat back up a bit.
During WW2 -- the sheer number of motors in service meant a LOT of rapid understanding of valve life and extreme conditions came into general practice. In fact, through Allied aircraft maintenance, the importance of guide to guide bore heat transfer became very clear and some of the general service proceedures I outlined in the first post come from that understanding. A lot of aircraft motors had the living hell beat out of them and dropped valves was more of an issue than many people realize. You have to remember we're talking about boosted motors running flat out. Add in the fact compression ratios started sky rocketing with better petrol and our understanding of valves and valve seats kept building throughout the war.
As part of that, the width and placement of valve seats started to change. The concept of a "three angle" had come fully into vogue -- but most people don't really understand what that is.
Here's the skinny -- when we cut seats they are almost always too wide. You CAN run a motor with wide seats. The challenge to doing so is that the very end of the valve face will carbon and either glow like the sun (auto ignition/detonation) or insulate so well the end of the valve starts distorting. The second thing is that on motors which see more than about 5000 rpm regularly -- the extra contact with the seat can cause a valve to "stick" right before it floats or to stick on the over run from the heat load -- all made worse if your motor is an oil burner. It makes a hell of a clanging! This won't happen right away and I've done more than one motor with only two cuts that is still purring along with 5 digits on the speedo with proper tuning and an attentive owner. Again, many prewar motors didn't come with topping cuts from the factory . . .just the seat. They run fine if you understand what it is and respect what the motor can do with ease and where it needs more frequent attention.
And, it's not unusual for flatheads to show port shift during casting, which is another use of throating cuts to correct (to some extent). Here's an example where you can see the roughly 1/16th shelf under the seat area from the pattern shifting just a smidge during casting (seriously the pattern makers and foundry men were very skilled craftsman to even make these cylinders!) Note, these are original 13 fin ULH cylinders and factory relieved . . . but note you're seeing only one angle for the seat.And those pits in the seat aren't rust -- those are carbon pits from it not being fully crushed and burnt due the relatively low spring pressure and low rotation of the valve.
DSCI3620.JPGDSCI3618.JPG
So, when the seat is too wide or simply in the wrong place, we need to reduce it's width. We determine which end needs reducing by checking the contact pattern on the valve face. A general rule is that you'd like the initial contact to be approximately 1/3rd of the way from the end of the valve face (towards the chamber). Sometimes, this means cutting the base of the seat (60 or 70 degree throating cut) to move the contact up and/or top cutting to move the seat down the valve face (30 or 20 degree cut). This is where artistry comes in and a bit of experience helps. Normally, I can figure out whether I need two or three turns of the Neway simply by looking at my contact pattern and port . . .and sometimes, well, you need a lot of throating cuts.
Today, a lot of automotive shops do "five angle" which results in an additional top and bottom cut for flow -- but also to expose the valve overhang (the bit on the very end that isn't in contact with the seat). On modern fuels and fuel injection -- this extra overhang ensures the carbon burns clean off which is one of the reasons modern motors can go 200-250K miles without valve attention.
In almost all cases, such "modern" valve jobs are challenging on vintage machinery. It's double a challenge with truly hemispherical cylinder heads as the topping cuts tend to be hog out a lot seat. This illustration shows why: TOP CUT PROB.pdf
Basically, as you can see in staring at it -- if you cut the 30 degree it coincides with the hemisphere and makes for a cut that is deeper on one side of the head than the other. Exposing the valve overhang on old valves may also lead to premature burning of the margin -- with modern nitrided SS valves much less an issue.
Finally, as I stated in the first post -- lapping is a waste of time. If you don't have a solid mechanical gas seal from properly cutting/grinding seats and refacing or using new valves -- you have a problem. The moment the motor fires -- the valves start growing in diameter and length. As a result, no seat lapped at room temperature will be in the right place when you are at full temperature. However, if you cut the seat properly -- the geometry is maintained throughout the range of expansion. About now, the light bulb goes on why we started the valve seat 1/3rd of the way down the valve face. As the valve heats up -- this moves to about the 1/2 spot and stays there. If we started too high or too low -- we run the risk of excessive carbon and or sticking. Hence why so many lapped motors show poor power when hot as the dickens and are carboned to the moon when you tear them back down.
Again, this is an 80,000 foot overview -- there's a ton of technical stuff behind each of these things, but it's really more than we "need" to know.
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