Piston Pocketing

Sorry I've upset you Greg - not my intension and please accept my apologies 🙏
It's that I don't understand why in the 'book' the pocket diameter dimension is smaller than the inlet valve - I was hoping to ask those who have experience in cutting valve pockets who could put my mind at rest - simple question I thought? But what I did get was lots of replies about my choice of cam and lots more that was irrelevent.
The 'book' says skim the head, pocket the pistons and away you go - I'd rather see if I can get away with not using a base gasket and a thinner head gasket rather than skim the head but that's another subject.
Cheers
 
marinatlas said:
Hi did you use a vernier sprocket to time the camshaft , and if not how far are you from the traditionnal ten pins between dots ?
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Yes Marinatlas I have used a vernier sprocket and rotating the motor over several times it looks like it's spot on. I have read lots of people's negative opinions on using a vernier sprocket but I believe it will be good enough for me😉
 
acotrel said:
I know I am silly, but it makes me happy. Most idiots do not have my mindset. When you lose interest in living, you are dead. I have been given a new option by my wife. I am allowed to race again, but I have to die if I crash. - I do not know what to do with that.
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Cancel your life policy!
 
Nourish said:
The 2S cam is in - and it's staying but the valves are hitting the pistons!!!
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I guess the 850 is a little different from a 750. Oops

I used Combat pistons and shaved the head only enough to make it flat for use on the 750. About half the Combat spec. I did a lot of other things as well, but those things would not be required on a Commando set of cases.

If the valves are hitting the tops of pistons that already have valve pockets cut in them, you should have plenty of compression leigh way for using a thicker head gasket. That is your easiest fix. Push rods may or may not need to be a different length depending on the gasket thickness.

Sorry things are going kind of sideways. It happens. Anything is fixable except death.

I can measure the pockets on the pistons I used, but I figured you had it all under control.
 
Schwany said:
I guess the 850 is a little different from a 750. Oops

I used Combat pistons and shaved the head only enough to make it flat for use on the 750. About half the Combat spec. I did a lot of other things as well, but those things would not be required on a Commando set of cases.

If the valves are hitting the tops of pistons that already have valve pockets cut in them, you should have plenty of compression leigh way for using a thicker head gasket. That is your easiest fix. Push rods may or may not need to be a different length depending on the gasket thickness.

Sorry things are going kind of sideways. It happens. Anything is fixable except death.
Click to expand...
No my STD 850 pistons do not have valve pockets - if there were I could just machine them to the required depth - also if I did have access to a 750 piston I could measure the diameter of the pockets but sadly I don't☹️
 
The width of the cutout doesn't need to be the full width of the valve, as due to valve angle/timing and the amount of lift, probably less than 40% of the valve diameter will be getting near the pocket

Think of it as just a slice of a circle, not a full half

The depth of the pocket would be more important, and as specified in the tuning notes would be more than enough, as it was taking into account higher lift cams than a 2S

As you have not skimmed your head, nor are you running 10:1 compression as a result you probably won't need to take much out at all. Try 2 head gaskets together and see if it still touches, maybe start with a 40 thou clearance on the valve and gradually tighten it . That will give you an idea of how deep you might/might not need to go
 
Nourish said:
Cancel your life policy!
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Over age 70, many people cannot get life insurance. But in motorcycle racing you are always insured - that is the reason it is too expensive. If I race again, I might have the wrong mindset. I never used to worry about crashing, because I was never going to do it. I learned to race by crashing - you develop immunity, but the immunity might not last forever. Riding my Seeley 850 is actually a pretty brainless exercise, but there might be a safe minimum.
 
Nourish said:
Cancel your life policy!
Click to expand...
Over age 70, many people cannot get life insurance. But in motorcycle racing you are always insured - that is the reason it is too expensive. If I race again, I might have the wrong mindset. I never used to worry about crashing, because I was never going to do it. I learned to race by crashing - you develop immunity, but the immunity might not last forever. Riding my Seeley 850 is actually a pretty brainless exercise, but there might be a safe minimum.
 
In answer to the original question, I do not know why Norton chose the .718" radius cutout dimension. I've wondered about that a time or two myself. As Seeley920 said, it probably has to do with the geometry as the piston and valves are moving both towards and away from each other during the cycle in a very non-symmetrical path. Wish I could find a nice animated video showing the actual path piston and valve take through a cycle, showing how the distance between them varies in a very non-linear way. With most race cams, the valves are closest to the pistons somewhere around 5 to 10 degrees after TDC for intakes and before TDC for exhausts. Norton says it's 3 to 5 degrees for the 2S and 4S cams. It makes my head hurt just trying to visualize it. I've machined a lot of Commando pistons for valve clearance , in a variety of bore and valve sizes, as well as different valve angles and cam choices. There's no one size fits all, unless you stick to stock, non-modified engines. I tailor each one I do to it's own requirements, and there's usually a couple of rounds of checking the clearances with clay and machining a bit more off. Norton recommends 030" - .040" of valve-to-piston I shoot for .040".

My basic procedure is to assemble the engine, without valves, turn it over to 10 degrees before/after TDC, and use a valve with the head cut off, and ground to a point, as a center punch through the guides, to mark the center of the cutouts. Then I put the pistons in a fixture on the mill and machine the cutouts at the same angle as the valve guides (28 degrees for the intake and 30 for the exhaust on standard heads, 26 and 30 for some big valve heads, and even less angle for more extreme heads with both intake and exhaust guides re-angled). Then I reassemble the engine and use clay on the pistons to check clearance. If it's not enough, at least I know how much more to machine to get it right. Sometimes takes a couple of tries.

I realize that the way I do it is not quite what Norton specified in their tuning notes, but it works for me, even if it is a bit laborious.

This is the setup I use to machine the cutouts.

Piston Pocketing


Ken
 
Nourish said:
No my STD 850 pistons do not have valve pockets - if there were I could just machine them to the required depth - also if I did have access to a 750 piston I could measure the diameter of the pockets but sadly I don't☹️
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Nice Ken!


Nourish, I pulled a standard 750 piston out of my bin.... I figured I would post the image and you could mark up the measurements you want and I'll do my best to give you some numbers...

Piston Pocketing
 
I would never shave a cylinder head to gain more compression. If I wanted more, I would buy slightly domed pistons with provision for the squish band. In any case comp.ratio, ignition advance, jetting and fuel octane ratio form a balance. If you change one, the others need to be adjusted and we often end up in the same place. Fuel calorific value is more important than compression ratio. A fuel of low calorific value cannot provide as much power as a fuel of high calorific value, unless you use much more of it. When you use domed pistions, the progress of gas across the crowns at TDC is impeded , so very high compression probably often does not work as well as expected. Bigger bore size is a better way of getting more compression.
 
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lcrken said:
In answer to the original question, I do not know why Norton chose the .718" radius cutout dimension. I've wondered about that a time or two myself. As Seeley920 said, it probably has to do with the geometry as the piston and valves are moving both towards and away from each other during the cycle in a very non-symmetrical path. Wish I could find a nice animated video showing the actual path piston and valve take through a cycle, showing how the distance between them varies in a very non-linear way. With most race cams, the valves are closest to the pistons somewhere around 5 to 10 degrees after TDC for intakes and before TDC for exhausts. Norton says it's 3 to 5 degrees for the 2S and 4S cams. It makes my head hurt just trying to visualize it. I've machined a lot of Commando pistons for valve clearance , in a variety of bore and valve sizes, as well as different valve angles and cam choices. There's no one size fits all, unless you stick to stock, non-modified engines. I tailor each one I do to it's own requirements, and there's usually a couple of rounds of checking the clearances with clay and machining a bit more off. Norton recommends 030" - .040" of valve-to-piston I shoot for .040".

My basic procedure is to assemble the engine, without valves, turn it over to 10 degrees before/after TDC, and use a valve with the head cut off, and ground to a point, as a center punch through the guides, to mark the center of the cutouts. Then I put the pistons in a fixture on the mill and machine the cutouts at the same angle as the valve guides (28 degrees for the intake and 30 for the exhaust on standard heads, 26 and 30 for some big valve heads, and even less angle for more extreme heads with both intake and exhaust guides re-angled). Then I reassemble the engine and use clay on the pistons to check clearance. If it's not enough, at least I know how much more to machine to get it right. Sometimes takes a couple of tries.

I realize that the way I do it is not quite what Norton specified in their tuning notes, but it works for me, even if it is a bit laborious.

This is the setup I use to machine the cutouts.

View attachment 112985

Ken
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I appreciate what Seeley says - I'd be machining only a radius and not a full diameter and wonder why Norton had chosen the radius of 0.718". And why if the exhaust and inlet valves are at differnt angles Norton state 30 degs for both?
I did think of perhaps marking the pistons via the valve guides with a 'center punch' . I have another bare head I could use for this - I imagine they'd both be similar dimensionally. On my milling machine I can tilt the head although I do have a tilting table as you have there it might be too small for my round table!
Would you recommend setting the cutter at a bigger radius than the valve to allow for any valve lateral movement? - Thanks
 
dobba99 said:
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.
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Like Churchill said … “neva”
 
acotrel said:
When I built my Seeley 850, it did not matter to me what the cam was. I had it reground by a guy who had ground good cams for bikes over many years. I am fairly certain the grind is 650ss. My cam sprochet has two more keyways ground into it at random, so I get finer adjustment on the cam timing. Getting the best out of a cam by adjusting the timing, depends on the inlet tract and the exhaust system. I use 12 degrees advance ahead of normal with a two into one exhaust system, Consequentlymy exhaust valve opening point is about 92 degrees BBDC, and my inlet valve opens a 65 degrees BTDC. The power band is between 5000 and 7000 RPM, and I get smooth power delivery everywhere.
It is common practice after adjusting valve timing - to place a small piece of kid's play-dough in the valve cutaways and rotate the motor. When you section the play-dough with a razor blade, you can see how much clearance you have.
I normally set the valve timing from experience about the exhaust system I use, and adjust everything else to suit it.
Where the power band ends-up and your intended use for the bike - determines gearing, and also carb jetting.
You can make your bike go faster with high revs, or pull harder with low revs, wherever you might choose. It is difficult to get both power characteristics, without making changes to the set-up.
If I race, I never worry about the time it takes me to reach full thottle. I feed the throttle on slowly and race-change the gears. The bike is much quicker when the carb needles are in use, than when it is suddenly exposed to full throttle. Suddenly richening the mixture makes the motor slower.
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Tell us how using part throttle while racing makes you go faster.
Pace lap?
 
acotrel said:
I know I am silly, but it makes me happy. Most idiots do not have my mindset. When you lose interest in living, you are dead. I have been given a new option by my wife. I am allowed to race again, but I have to die if I crash. - I do not know what to do with that.
Click to expand...
It means she doesn't ALLOW you to live.
 
Schwany said:
Yeah that slip in the post was kind of funny.

Greg told me I have no purpose for being here and posting as much as I have. So I joined up VIP looking for a purpose. That didn't work either. I'll just stick with being a douchebag when my membership needs renewal. :)
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Well, in the Steve Martin movie, he learned about his special purpose. 🤣
 
o0norton0o said:
Nice Ken!


Nourish, I pulled a standard 750 piston out of my bin.... I figured I would post the image and you could mark up the measurements you want and I'll do my best to give you some numbers...

View attachment 112986
Click to expand...
That's very nice of you. I don't know if you'd be able to measure the radius of the cut outs - not something you could do with your vernier calipers. Not to worry if you can't. I'm unable to post you a picture as I'm not a VIP member but that's another story! 👍
 
Nourish said:
No my STD 850 pistons do not have valve pockets - if there were I could just machine them to the required depth - also if I did have access to a 750 piston I could measure the diameter of the pockets but sadly I don't☹️
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Missed that entirely. Excuse me for getting involved at all.

I never would have tried using those flat tops and would have said so earlier. For some reason I thought you were using all new rods, pistons, and so on for your rebuild. I think I got your post confused with another one. Help posts are all starting to blend together.
 
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