fittment of 42 mm inlet valves

Gday All, Im currently slowly but surely building a cafe racer, 64 Slimline based with an 74 850 come RGM 920 kit to fit. Also a ED G 90 deg offset crank to help smooth out the vibes and add an interesting sound.MAP rods will hold it together. Ive also purchased 42 mm valves to fit the head and my question is: do I have to reangle them? Im not enlarging the exhaust valves nor am I installing a radical high lift cam.The new seat will nearly touch the existing exhaust seat.Looking to port the inlet out to 34 mm and have 35 mm FCR,s to fit aswell.
I call it my beer and smokes, I think it would be cheeper to be a pisspot and chronic smoker!!

rgds Foxy

You don't have to do anything you don't want too, except spend lots of money on a piece of old decaying metal :shock:.

The reangled valves job is advertised as being for better torque and power, there has been a recent thread on Britbike in the norton section where racers have praised it but no-one mentioned it as being needed to allow bigger inlet valves to be fitted.

Mick Hemmings still does the mod and from my dealings with Mick I would trust me to give the correct advise of if it was needed for 42mm inlets and potential cracks in the head.

http://www.mhm.demon.co.uk/

If you're only doing the intakes, the valves don't have to be reangled to avoid clashing with any moderate lift, duration cam. PW3, 2S and similar are all fine.

Is it my failing memory, or did Dunstall change the intake stem angle by 1.5°?

Many thanks for the replys!
Im sticking to my game plan by leaving the valve angle stock aswell as the exhaust. I want to keep the exhaust gas speed up as I read elswhere, (where bigger is not always better) exhaust gas velocity creates a vacum for the intake. I intend not to exceed 6500 rpm as being 920 cc it will develop alot of torque in the lower rev range and will be looking for another cog after 6000 rpm.

Thers only two things you cant have big enough, and your wallet is one of them!
Rgds Joe

Oh you mean your girl friends brasserie right?

Not to disagree with any of the other list members, but I'd be nervous about 42 mm valves without re-angling the intake. That's pretty much the same size (or slightly larager) that the factory used in the big valve heads, both the AMA homologated head for the standard 750 and the nead for the short stroke 750. In both cases, they changed the intake valve angle from 28 degrees to 26.5 degrees to get more clearance between the valves. All the other Norton head builders I know of, including Dunstall, Mick Hemmings, and Steve Maney, re-angle the intake to use that large a valve. C.R. Axtell used to fit 1/16" larger intakes with stock size exhausts without changing the valve angles. He said that anything larger ran the risk of valve interference with his cams (which were pretty mild, pretty much the same as the PW3), especially when he ground them with close lobe separation (102 degree). He liked the closer lobe separation for the race engines he built, but sold most of his customer cams with 105 degrees lobe separation, mostly because he was worried that the buyers wouldn't be careful enough to check the valve-to-valve clearance, particularly with larger valves.

If you do choose to go with the larger valves, I believe you'll need to replace the valve seats with larger diameter ones. As I recall, the stock seats won't accomodate the 42 mm valves.

If you choose to go ahead with the 42 mm valves without re-angling, be sure and check that you're not close to valve hook-up during the overlap. I like to keep .060" safety margin between the valves at closest approach. If you're really close, you can pick up a little more clearance by sinking the valve seats, but I dislike doing that.

Hope this is info is of some help.

Ken