Valve Seat Angles

One often hears of "three angle seats." Currently, I cut five angles and I'm debating going to seven. Skeptics argue that once the engine is warmed up the first time that the angles are flattened and lost. I will grant that the most important and enduring angle is 45 or 46 degrees.

Ideally, we should like a bell-shaped seat - a logarithmic curve like the mouth of a trumpet.

If the seats were more consistent in size and concentricity, I'd endeavor to have curved cutters for the intake and the exhaust (they are different enough where one won't do it). And yes, I am ignoring the 700 and 750. I get so very few queries about performance goods for them that I don't see the need to support them. If you built a 700/750 as much as you could, it would cost as much as hopping up a 920 or larger bike, and - at best - would slightly out-perform a well worn 920, but with a lot more shifting.

Regarding seats: before I got my cutters, when I would pull the top-end, I would either swap-out barrels or get by on a hone. For the valves, I would slather valve grinding paste on the seat; push the valve through; chuck the stem up in a reversible drill and spin away. When the seat got shiny, I'd clean off the paste and reassemble. These seats always passed the water-in-the-port test.

I highly recommend against this practice. Yamaha did NO hardening to the valve faces (or tips) and they would bell out with a crease where the face was worn. This sort of "valve job" never lasted very long. Real valve jobs at machine shops vary between $130 to $190 for two valve twins. If they "face" your valves as part of the deal, it's worth it. Better is to acquire after-market valves -- about $200 each for trick materials, and you must order them for 500 singles (they fit). Or alternately, we handle valves made on "retired" Yamaha tooling (in Taiwan) that have stellite faces and tips and hard chromed stems.

I have heard of guides wearing out of spec, but have never run across such myself. They seem to last forever. And with Viton seals, there's never any smoke from them. I've looked into aluminum/bronze/manganese guides for the race crowd. The stems are very thick - 8mm. The bore they press into is 14mm. A set of VW pancake guides (8 pieces) costs less than $50, but their OD is too small. Similarly BMW boxer guides run about $15 a piece -- again, too small of an OD. I approached Kibblewhite about valve gear including guides. In a lot of forty, they wanted $19.54, not bad, but this was contingent on my buying a lot of other custom parts with a total expenditure of over four thousand dollars. I may still get bronze guides, but I will buy the raw stock cut to length and have a local machine shop finish them. They will not be cheap.

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So - are multi-angle valve jobs worth the money? If your bike is stock or close, no. Let a shop "sardi" your seats. If you run big bore and cam, you should start shopping for carbs. Personally, I like TM40-6 pairs. Easy to fit; easy to tune. At this stage, you should open up your intake ports to match the size of the carb outlet. Polish and raise the outlet (but not too much) to your exhaust ports. Now, you want as many angles as the person doing the job can do. XV's had odd sized stems (8mm) and big (39mm and 47mm) valve faces. Not many shops can handle them. The intake port should be rough (240 to 280 grit) up to the guide. Just below the seat, it should angle more into it and sport a 320 or finer finish. The exhaust should come down straighter and exit higher - all with a finish that is as shiny as you can make it. All polishes and grits leave a waxy residue behind; acetone (be careful) will remove it. If you can't come up with a finish you like, mask off the valve and shoot a small amount of aluminum header paint. This is good to 1200 degrees and will tend to reflect the heat back into the escaping gases.