Rocker shafts

I've decided to replace the rocker shafts on my 750. The old ones have scrolls for the oil ( it's an Atlas head) but the new ones don't. They just have the flat section on one side. Is there any problem using these new ones or do I have to source scrolled rocker arms?

pommie john said:

I've decided to replace the rocker shafts on my 750. The old ones have scrolls for the oil ( it's an Atlas head) but the new ones don't. They just have the flat section on one side. Is there any problem using these new ones or do I have to source scrolled rocker arms?

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Presumably your "750" has the high-pressure rocker oil feed from the timing cover, in which case you should be using the flat section rocker spindles (with the flats positioned on the side away from the head)?

L.A.B. said:

Presumably your "750" has the high-pressure rocker oil feed from the timing cover, in which case you should be using the flat section rocker spindles (with the flats positioned on the side away from the head)?

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Yeah, it has the feed to the head from the timing cover. Being an Atlas head the feed goes to the top of the head, not the side like a Commando. The bottom end is also Atlas, but the barrels are Commando .

pommie john said:

The bottom end is also Atlas

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Is it '66-on type-with the double-speed oil pump?

L.A.B. said:

pommie john said:

The bottom end is also Atlas

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Is it '66-on type-with the double-speed oil pump?

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Err..!

Apparently it's a 1964 model. I've just looked at the oil pump gear and it has three starting points for the thread. Does that help?

Sorry I sound a bit thick, but the engine was built by a good friend of mine over 20 years ago and I just rebuild it when needed and put it back how I found it. Things like these rocker shafts being different causes me a bit of confusion.

pommie john said:

Apparently it's a 1964 model. I've just looked at the oil pump gear and it has three starting points for the thread.

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That's the earlier low pressure system-which used scrolled rocker spindles.

'66-on have double speed 6 start thread pump gears.

OK. maybe I'd better use the old rocker shafts. They are a bit loose in the head so I'd better dig out some high strength loctite .

Thanks for your help.

John

I'm tying to find my Dunstall tuning manual. I think he recommends the low pressure worm gear 'cos a race bike is thrashed so much it has plenty of oil pressure most of the time.

One thing leads to another . . .

Throw the six stat gears on the pump , or a Commando pump ? . Twice as much oil might be a good idea .

I've been racing it since 1984 with the current set up. It's staying that way.
I've found my Dunstall book. He recommends the 6 start worm with the early pump, modified to increase the scavenge.

I'm going to stick with what works. I will try to find some new scrolled shafts, or if I can't I'll loctite the old ones into the head.

I've been reading the Dunstall book again and wondering if I should fit a 6 start worm gear. I guess it can't do any harm.
How can I tell if my oil pump is early ( pre '63) or later with higher delivery?

Hi, to distinguish the double speed pump from the other one, you must take it of the case and have a look to the hidden face the double speed one have an elongated feed hole , which means that you have one circular the middle one always elliptical (elongated, oval....) and the third one elliptical too ( the low speed have two circular on the periphery and elliptical in the middle)........otherwise you must dismantle it and check the width of the gears which does not help very much unless you had the two different to compare as I cannot tell you the correct width of each other .

marinatlas said:

Hi, to distinguish the double speed pump from the other one, you must take it of the case and have a look to the hidden face the double speed one have an elongated feed hole , which means that you have one circular the middle one always elliptical (elongated, oval....) and the third one elliptical too ( the low speed have two circular on the periphery and elliptical in the middle)........otherwise you must dismantle it and check the width of the gears which does not help very much unless you had the two different to compare as I cannot tell you the correct width of each other .

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OK I checked, I have the early pump with two circular holes and one elliptical. ( I also found a site that says ealry pumps have 3/16" feed gears and later ones have 1/4") I have 3/16" gears.

So next question. Can I simply fit a 6 start worm gear, or does the pump need the elliptical hole to cope with the extra flow?

Hi Pommie, as far as i know, the internal oilway in the crankcase is adapted to the different oil flow, so for me it's like fitting a big water tap to a small hose or vice versa, but may be a fellow with a better knowledge could help us, but you should have the end of the story yourself as long as you had run your engine like this for long time!
about your head we may think about two different things: the oil flow due to the pump (double speed and single speed) AND the fact that your oil your head from the timing case , instead of the return, which means too stronger oil flow , thus may be the need to put rocker shafts oil holes at 180° of the rockers holes (as LAB said), if I remenber the oil scrolled shafts should have their holes the opposite way (hole facing hole), but ???? I will try to find some tech tips , may be Ben G could help??

http://motorbyte.com/norton/hybrid/G15OIL.html
hope this help, Pierre;

Thanks for that link. Sadly, I'm just more confused, the more I read !!

That article says that you should never run con rods with an oil bleed hole with the low delivery pump. Guess which con rods I've got?

So to sum up, I have a low delivery pump, the low delivery three start gear,and con rods with bleed holes that reduce the pressure. Supposedly the worst combination possible, but it's been racing for 26 years and never had a major failure!!! In fact when I stripped it this time, it was just a precaution and I'm not even going to replace the big end shells because they look so good. The only reason I am replacing the rocket shafts is because they are turning in the head, there's no evidence of wear on them.

I'm seriously thinking about leaving it as it is.

What are the possible downsides of fitting a six start gear to my current set up?

The old Norton bikes with the scrolled rocker shafts had the oil feed to them from the RETURN line to the oil tank. On start up you could open the oil cap, put your finger over the pee-hole and make it leak from around the rubber hose going from the return line to the head.

When the bikes switched to the plain rocker shafts then they needed a high-pressure oil feed like the Commando always had. If you put the scrolled shafts into a bike with a high pressure feed to them, you will have a big leak into your cylinder head, it will flood with oil and probably make the bike smoke and lower your oil pressure to your rods.

It makes no difference which oil pump gears you have in the bike, it originally had the slow pump drive ratio. I put a set of fast pump gears on a 650 once just for something to do. I figured that since I always lugged it at low rpm in high gear maybe it would help the motor with lube and cooling, but I doubt it is a necessity unless you are racing. Dunstall took an old fifties pump with the very narrow gears and plunge-milled the scavenge side to take the sixties wider gears, so he had a small feed rate and large scavenge rate.

Heinz Kegler, ex Norton experimental department and Berliner Motor's head tech, said to flip the bearing shells around to block the oil holes in the rods of any Norton twin, he said it was a bad and un-needed fix by some stupid AMC engineers.

If you use scrolled rocker shafts then the size of the return pee-hole in the oil tank is important, because the size of this orifice is what regulates the oil pressure and volume to the rocker shafts! Too big and they starve, too small and the head floods. But there was nothing wrong with the system, there are old Nortons around running with this original setup just fine fifty years after they were built, not to mention many race-wins on the early sixties Dominators with this system. A high-volume of oil to the head with the old scrolled rocker shafts may have even helped the head to stay cooler than the later plain shafts?
The early Dominators did seem to be prone to the rocker shafts coming loose though. Loctite would have to be carefully used to make sure none of it got between the shaft and the rocker on assembly. There are any number of fixes for loose rocker shafts from fast and cheap peening or locktite to an all-out weld-up and re-machining. Sometimes mix and match from the parts-bin will do the trick too.

Sounds like you should get some parts-books and service manuals for the early dommies and the Commando and find out what parts you have, how they were originally used and the right thing to do with them now.....