Piston Pocketing

Nourish

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Dec 2, 2009
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In the Norton Service Sheet N3 56 there is a dimensional drawing to modify the pistons for fitting a 2S cam. The drawing dimension for the two pistons pockets is 0.718" radius - that'll be 1.436" diameter. The Inlet valve is 1.490" diameter and so is the valve likely to clout the piston - and can the exhaust cut away be smaller?
 
You don’t mention if you are trying to fit a combat cam to an 850. For 750 the original valve cut outs work for both standard and combat cams.
 
Yes 850 - the service note is about tuning stage one and two for the 850s 1973
 
I am running a 2S cam grind in my 850 and have been since 1982, I am running 40th oversize flat top Hepalite pistons and I am running my original push rods and original valves from the factory, my head has been shaved and ported but when I built this motor I made sure the valve weren't touching the pistons when turning over, so in 43 + years running this set up and clocked up a lot of miles I haven't had any problems, my tappets are set in between stock setting and Combat settings.

Ashley
 
Unless you plan to spend most of your time between 3/4 and full throttle, I submit that the bike will be more fun with a standard cam. Most people I know with Combats use a 19T front sprocket to restore the low end lost by the 2S cam and I suspect that you'll want to come off the standard 21T on a 850 for the same reason.
 
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Why not test first to see if you need to pocket the 850 pistons, I have 11mm of lift on the valves and use standard pistons with no issues.
 
I've bought a 2S cam - so that's going in! along with all new rods, mains, valves, pistons and bores. I've also bought a nice glass Burette and before it gets broken it I'll measure the CC's. I would like to aim for a 9.5:1 CR but I believe it's way below that as it is.
At the moment there is no base gasket but a 0.040" copper head gasket, I've set the cam at 158" lift on the inlet pushrod at TDC and the valves are touching the pistons.

Now my concern is that in Norton's notes the valve cut outs are machined out to 1.438" diameter but the inlet valves are 1.490" in diameter and so in my mind the valves would hit the pistons!

Perhaps someone who has the ability to attach images here could do so for me from 'Norton 850 Commando Stage One Perf Mods' up in the 'Resources' tab at the top of this page or just go straight to https://www.accessnorton.com/resources/norton-850-commando-stage-one-perf-mods.15/
Cheers
 
marshg246 said:
Confused! Why not simply download the document?
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I have downloaded it but I can't post images to show you all to explain my quandry and was hoping someone who can could post the image on here for you all to see :oops:
 
Nourish said:
I've bought a 2S cam - so that's going in! along with all new rods, mains, valves, pistons and bores. I've also bought a nice glass Burette and before it gets broken it I'll measure the CC's. I would like to aim for a 9.5:1 CR but I believe it's way below that as it is.
At the moment there is no base gasket but a 0.040" copper head gasket, I've set the cam at 158" lift on the inlet pushrod at TDC and the valves are touching the pistons.

Now my concern is that in Norton's notes the valve cut outs are machined out to 1.438" diameter but the inlet valves are 1.490" in diameter and so in my mind the valves would hit the pistons!

Perhaps someone who has the ability to attach images here could do so for me from 'Norton 850 Commando Stage One Perf Mods' up in the 'Resources' tab at the top of this page or just go straight to https://www.accessnorton.com/resources/norton-850-commando-stage-one-perf-mods.15/
Cheers
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Sell the 2S, buy a standard cam and have a more flexible, tractable engine that will be nicer to ride. Dealers will sell you anything including performance cams along with the words like 'best road cam ever' to quote Mick Hemmings. The one thing dealers won't do is ask you how you ride and what are you going to use the bike for. Instead of 'best road cam ever' they should ask 'Are you going to ride around at WOT? if so, you need a 2S, 4S, PW3 etc cam' They will let you cope with all the fallout from using said cams and by fall out i mean all the extra work to do to the engine to get it to run safely. The higher lift cams WILL give the valve gear a harder time and be noisier. The horse power might be a little better at the top end but the payoff is the crap bottom end grunt.
Just to get the C.R. back to something like where you want AFTER you have pocketed the pistons will mean shaving the head, or deleting the head gasket or a mixture of both.
 
Nourish said:
I've bought a 2S cam - so that's going in! along with all new rods, mains, valves, pistons and bores. I've also bought a nice glass Burette and before it gets broken it I'll measure the CC's. I would like to aim for a 9.5:1 CR but I believe it's way below that as it is.
At the moment there is no base gasket but a 0.040" copper head gasket, I've set the cam at 158" lift on the inlet pushrod at TDC and the valves are touching the pistons.

Now my concern is that in Norton's notes the valve cut outs are machined out to 1.438" diameter but the inlet valves are 1.490" in diameter and so in my mind the valves would hit the pistons!

Perhaps someone who has the ability to attach images here could do so for me from 'Norton 850 Commando Stage One Perf Mods' up in the 'Resources' tab at the top of this page or just go straight to https://www.accessnorton.com/resources/norton-850-commando-stage-one-perf-mods.15/
Cheers
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that will not be the full diameter of your valves which may (will) kiss the piston ,only a fraction of that diameter ,just because the valves are angled vs the pistons , dunno if I am clear enough with my briton /english vocabulary ?? plus the 850 and the 750 valves have the same diameter
 
Nourish said:
Now my concern is that in Norton's notes the valve cut outs are machined out to 1.438" diameter but the inlet valves are 1.490" in diameter and so in my mind the valves would hit the pistons!
Click to expand...
On a 750 Combat, with the pistons in backwards, the valves lightly kiss the pistons (built by dummy - brought to me due to the weird noise). You don't need anything near the full pockets on a 750 so I doubt that you do on an 850. A Combat has .040" skimmed from the head. The diameter of the valve can be much more than the cut out is wide. Distance from the edge is more important.

I wouldn't do it at all, but since you insist, put it together, find out exactly where the valves hit, if they do, and make a small cut to accommodate, keeping in mind that if you over-rev, the valves might float, and you'll need more clearance. If both the intake and exhaust hit, you'll notice that the distance from the edge of the piston is different.

IMHO, you must get to 10:1 compression or better for 2S cams.
 
74 Cafe' said:
I went VIP from Day 1 and given the amount of time and money we spend on our Norton's, as well as the resources this forum provides, I don't understand why everyone isn't VIP......
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Are you sure?
You posts don't show the green VIP tag?

Glen
 
Nourish said:
:rolleyes:
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That is the correct response. Ads are gone now without being a VIP member. Jerry posted a note about it. https://www.accessnorton.com/NortonCommando/access-norton-ads.38448/

I joined to see if it would make me a better person. Total fail. 🤣

By the way, I'd ignore all the doom and gloom about the 2S cam and carry on. You can make it work. It's not an issue if done right and works with taller gearing. You just won't be able to ride around at 25 MPH in 4th gear and get the throttle response you want.
 
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I have no opinion on the VIP choice. As Jerry says, it's OK either way.
Maybe 74ok forgot to renew. It's easy enough to do.
It was just a chuckle, the Post and the no VIP status :)

Glen
 
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