Big valve heads - Steve Maney or Mick Hemmings? (2016)

My road bike, mk3 850 has stage 2 with big inlet & exhaust valves with reworked special guides etc, Maney barrels and pistons PW3 cam and 35mm Kehien FCR flatslides. married to a Quaife 5 speed in strengthened shell. I bloody love it! docile and tractable around town, with surprisingly plenty of torque but on open road it revs with a real snarl and goes like the clappers. Now I have Madass's front brake and Landsdowne fork dampers, I am pretty much done modifying... but it has taken a lot of hit and miss, as well as a large amount of cash and hours of work over 32 years ownership...I am not an engineer, and have gone up a few blind alleys in the process, but if I was starting from scratch I would probably go down the Fullauto route...

Standard FA head, even with standard 850 parts it made my gearbox very lacking and out of date. I think the head in standard trim will take some tuning of other parts before itself needs modifying.

I have an FA head and megacycle stage one cam standard compression and 23 sprocket on a road 850....plenty enough power for daily use and a kick in the pants after 4000 rpm a noticeable increase over standard, also running Jim's breather and head steady along with Lansdowne forks, Konis and Brembos, very happy with it all......surprises people that have ridden it!

Madnorton said:

Standard FA head, ................ I think the head in standard trim will take some tuning of other parts before itself needs modifying.

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My race bike, 750 Short Stroke, has a standard Fullauto head, apart from a few thou taken off for compression, could not agree more.

Perhaps a lot depends on how you intend to make your motor faster ? If you increase the valve size, you increase it's mass. So when you raise the operating rev range to get more power, valve float becomes more likely, unless you use a cam with a slower lift rate and less lift. Some of motors show an improvement in power when the valve seats are properly shaped, without even going to a larger size valve.

acotrel said:

..............Some of motors show an improvement in power when the valve seats are properly shaped, without even going to a larger size valve.

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Made sure that was the case with the Fullauto.

And going back to my first race tuned Norton motor, which was built with John Hudson's tuning notes in my hands in the winter of '75/'76 he rejected the idea of big valves but rather maximising flow around a standard valve....bit too late for tha me as I had big valves in hand at the time, but this time around with the Fullauto I followed the advice!

Treasure the speed of that inertia; helps atomization too.

My 500cc short (63mm) stroke Triumph motor used to rev to 10,000 RPM without tangling valves with pistons. However the cams were extremely long duration with slow lift rate and a lower maximum lift than normal. The best it ever went was when the valve seats were re-cut with the three cutters. I don't know if a Commando motor would be faster with bigger valves, but how would you ever know ? When you fit bigger valves you would usually re-cut the valve seats to the right shape.