850 mk3 cam/pushrod questions

I would like to start off saying hello to everybody! I am a long time lurker, first time poster. I recently created an account so that I could ask some questions I am having a hard time finding some info on.

The bike:
1975 mk3 850 electric start cafed out. Single seat re-upholstered by Bitchen Stitchen, home made rear sets, dropped bars etc.
RH4 Head (Just rebuilt w/ std valves, and shaved .045 to bump compression up to about the 9.5:1 range at altitude. Figure Denver CO altitudes)
Originally it had a 3s cam. This had a lob go soft so it needs to be replaced.
This is a daily driver/ canyon carver bike. I don't go out to the track racing but now that High Planes Race Way is here, I wouldn't mind a track day. Think spirited riding most of the time.
Stock lifters and push rods I do believe as I haven't been able to measure the pushrods yet.
Stock 4 spd, transmission.
Home made 2 into one exhaust. Equal length headers. This was done for leaning clearance, but it was built by the previous owner. I would describe it as a megaphone with a cap and a drilled tube down the center of the megaphone. I can get better details if need be.
Stock pistons, and con rods.


The questions:

One problem I am having is cam selection. I like the torque the stock bumpstick has, but this is a cafe racer type bike that I ride in a spirited fashion in the canyons of CO, so I am looking for something with just a bit more "umph" so to speak. The bike is a daily rider with a single seat so I am not riding two up much of the time.
I see on some of the threads to go with an S4 cam as it has more midrange grunt than a 2s cam. I believe that originally the S3 cam was put in a stock bike, so while it was "quick" I would not say that it was "fast". Think on par w/ a combat 750, as it would stay neck and neck in a drag race up till about the 80/100 mph mark.
I am looking for a cam that would still have good tractable power for the street, but offer a bit more than the stock cam. I was thinking the 2s, but I keep getting different info on "good" cams. I have seen everyone recommend the PW3, then say it is for track, that the 4s is a better cam than the 2s for midrange, and to just use stock as it is the best. I have to buy a new cam, but with all the different info I think I am just becoming more confused. Would anyone be able to shed some light on this and explain why?

Another question is the pushrod length/ valve train geometry. This is my first time this far into a Norton engine, and I believe that since the .045 has been shaved off the head that I will have to get some shortened push rods. Is this correct? If so how much do I need to have them shortened by? And would I have to modify the rocker at all? I was planning on going with the stock aluminum push rods, but can go w/ steel if need be. I would like this to be a reliable 20,000 mile + bike.

My last question would be the valve pockets in the piston. With the head shaved I am figuring I would have to have pockets milled into the pistons, but don't know how much. Does anyone have a general "rule of thumb" or to just use modeling clay on top of the piston and rotate the engine by hand?


I have been searching through the older posts for info, so If this needs to be moved, or you have a good thread to read please feel free to post it. I have already read these just to name a few.
cams-t12813.html
commando-pushrod-tech-t12779.html
http://www.nortonownersclub.org/support ... /camshafts

Again thanks for any info!

I would suggest a webcam #12a grind for a hot streetbike. You probably will not need to shorten the pushrods unless you are adding mushroom adjusters in combination with shaving the head. I would certainly recommend checking the piston to valve clearance but you probably will not need to cut pockets. Jim

Is there a dyno chart or any published torque specs for this cam anywhere ?
Or just a seat of-the-pants evaluation against a stock cam.
Comparing cams seems to fall into almost the complete guesswork, or suck-it-and-see school of thought.

Anyone seen a $$ value quoted ?

Cheers.

I have built and dynoed many engines using this cam. It is relatively mild but is the wildest cam I would recommend for the street.
It is softer than a 2s or 4s which is a waste on the street. It is also the best cam I have used on short tracks in my roadracers. The graph of this cam is posted. Jim

I can find the cam timing drawing, but not any dyno charts of the actual output ?

Rohan said:

I can find the cam timing drawing, but not any dyno charts of the actual output ?

Click to expand...

I have not posted any dyno charts for different camshafts. They would be very misleading. A camshaft change needs to be accompanied by a host of other changes to make the final package "in tune" with the cam. Simply installing a performance cam in a stock motor nearly always nets a loss of area "under the curve" .
When you have a wide ratio 4 speed like in a Commando the area "under the curve" is most important on the street and doubly so when riding at altitude in the mountains . Jim

It would still be nice to see dyno charts of cams compared, when they have been optimised to suit ?

We see folks here mention SS, 2S, 4S, 7S, PW3 and Webcam 12A, and others - and wonder which may be more suitable for 'me'.
With very little idea really of what to expect.
Short of trying them all....

Rohan said:

It would still be nice to see dyno charts of cams compared, when they have been optimised to suit ?

We see folks here mention SS, 2S, 4S, 7S, PW3 and Webcam 12A, and others - and wonder which may be more suitable for 'me'.
With very little idea really of what to expect.
Short of trying them all....

Click to expand...

"It would still be nice to see dyno charts of cams compared, when they have been optimised to suit ?"

Yeah, that would be nice. If you want to support me for the next few years I will see what I can do. :lol:

Realistically, unless you have a dyno handy and are willing to spend a lot of time doing development you are probably best off to stay with something pretty close to stock. I have seen far too many people with big cam hot rods get left behind by their buddy with a stock cam and a well tuned engine. Jim

Rohan said:

Is there a dyno chart or any published torque specs for this cam anywhere ?
Or just a seat of-the-pants evaluation against a stock cam.
Comparing cams seems to fall into almost the complete guesswork, or suck-it-and-see school of thought.

Anyone seen a $$ value quoted ?

Cheers.

Click to expand...

This basically hits the nail on the head. I have only ridden what I believe to be "stock" cammed bikes or so I thought until this came came apart. If the cam was good I would just see about bumping compression as there are many threads here stating that most if not all cams benefit a good deal from a compression bump.

Jim is this webcam 12a "forgiving" to not dyno tuning? The head has not been flowed or cc'd as the original idea with this rebuild is a simple reliable build, and then purchase another engine down the road when I have the time [I am in my final year of college] and build a a really tuned engine.
I am not using the mushroom adjusters as far as I can tell, just the std flat nosed adjusters that are stock.

Also do you happen to have any idea what the clearance should be between the piston and the valve? I have heard over the years that .060 to .080 vertical clearance but have not found anything yet.

Eric

The #12a likes a raise in compression ratio of around 1 to 2 points. And it needs a small diameter open exhaust. Tuning to length will help and that is best done on a dyno. Heavy duty springs are needed.

If everything else is optimised then it will have the ability to make less power below 4000, the same power from 4000 to 5000 ,and more power from around 5000 to 7000 in comparison to a stock cam. It's pretty warm for the street.

For more flexibility you could use the #12 grind. It is only slightly hotter than stock and is a drop in so it does not require heavy duty springs or tuning changes. It operates best in the original rpm ranges of the engine. It is the cam I use the most for near stock applications.

Keep in mind that the stock cam is already pretty hot for a production engine. A lot of roadraces have been won using the stock cam. Jim

+1 for the stock cam. As Jim says, it is a fairly high performance cam for its time. I raced a PR with a stock cam for a couple years, and it was quite competitive against other bikes in the '70s. I've also found the 2S cam to be pretty good for a street bike with a little more compression ratio. As Jim pointed out, the 4S is not really great with a stock engine. You can live with it on the street, but it really wants some engine work (compression, porting, carbs, etc.) to be any better than the stock cam, and you are giving up some low speed performance and ride quality. I don't know about the Webcams for Nortons. I've only used them in singles, but they do make quality cams. Jim has lots of experience with them, and I'd listen to his advice.

Ken

Thanks again guys for the help. I will probably order a #12 Webcam, along w/ all the other bits that will be needed. If everything goes well the bike will be back on the road in the spring time.

Till then, cheers!

I had a Web 12a for about ten years. When I tore down I found one lobe seriously worn down. I called Web and talked to the owner. She, yes it's a woman, said to send her the cam and she would rebuild it for $100. Also gave me a heck'va deal on lightweight collets and springs. Good company to do business with.