Cast vs Forged Pistons for Moderate Street Build

Dan1950 said:

I think I'm going to stick with the JE forged pistons with graphite coated skirts.

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Did you actually talk to a vendor that has JE pistons for a Norton in any bore size? They seem to be as scarce as hen's teeth. I didn't look that hard though.

If you have not found a vendor that carries a lineup of JE pistons for the Norton 850, you might want a plan B piston option. The parts landscape could change some by the time you get around to taking the motor apart and stop all the talking.

Lots of copper gasket options from JSM to get the CR you think you need. I would only skim the head if it is not flat enough to seal. Ask the Jim's or your reputable engine builder what amount of error constitutes the need to resurface a Norton head. I know there is some room for error particularly with a composite gasket. Copper has some give to it, but not a lot.

As a matter of fact I am communicating with both "Jims" about this project.

The JE pistons will have to be ordered and with the graphite coating will take a few months

As soon as I pull the cylinder to inspect the bores I can place the order.

I can always go to "plan B" if needed.

Top priority now is getting the head sent off to Jim Comstock.

Schwany said:

Did you actually talk to a vendor that has JE pistons for a Norton in any bore size? They seem to be as scarce as hen's teeth. I didn't look that hard though.

If you have not found a vendor that carries a lineup of JE pistons for the Norton 850, you might want a plan B piston option. The parts landscape could change some by the time you get around to taking the motor apart and stop all the talking.

Lots of copper gasket options from JSM to get the CR you think you need. I would only skim the head if it is not flat enough to seal. Ask the Jim's or your reputable engine builder what amount of error constitutes the need to resurface a Norton head. I know there is some room for error particularly with a composite gasket. Copper has some give to it, but not a lot.

Click to expand...

As a matter of fact I am communicating with both "Jims" about this project.

The JE pistons will have to be ordered and with the graphite coating will take a few months

As soon as I pull the cylinder to inspect the bores I can place the order.

I can always go to "plan B" if needed.

Top priority now is getting the head sent off to Jim Comstock.

Dan1950 said:

As a matter of fact I am communicating with both "Jims" about this project.

The JE pistons will have to be ordered and with the graphite coating will take a few months

As soon as I pull the cylinder to inspect the bores I can place the order.

I can always go to "plan B" if needed.

Top priority now is getting the head sent off to Jim Comstock.

Click to expand...

I applaud your patience on waiting for the pistons. I could not do it if I got the urge to build a motor. Plenty of high-quality performance parts that all work together sitting out there waiting to be purchased.

Sounds like you are on track. You'll have some fun wrenching on the motor. Not being facetious. They are interesting to work on. I've had my Norton a long time and rebuilt it 4 times. Never once because it was blown up, but just because I felt like doing it. The last build was the best so far.

I deleted 4 paragraphs here about the parts used in my semi-exotic Norton engine build. I figured you wouldn't really give a hoot, and I'd be talking to myself yet again. lol

When you split the cases, it would be interesting to know what the condition of your cam is, and if the valve timing was right. I've always figured it was a little flat based on your description of what the motor is doing in 4th WOT. Either that or some parts in the carburetors are in serious need of replacement.

Enjoy the build.

No doubt you'll have a strong runner when completed.
On tear down, I'm curious on your current cam condition.

MichaelB said:

No doubt you'll have a strong runner when completed.
On tear down, I'm curious on your current cam condition.

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I did check valve lash recently and it was at spec except for one exhaust that was .001 wide.

Schwany said:

I applaud your patience on waiting for the pistons. I could not do it if I got the urge to build a motor. Plenty of high-quality performance parts that all work together sitting out there waiting to be purchased.

Sounds like you are on track. You'll have some fun wrenching on the motor. Not being facetious. They are interesting to work on. I've had my Norton a long time and rebuilt it 4 times. Never once because it was blown up, but just because I felt like doing it. The last build was the best so far.

I deleted 4 paragraphs here about the parts used in my semi-exotic Norton engine build. I figured you wouldn't really give a hoot, and I'd be talking to myself yet again. lol

When you split the cases, it would be interesting to know what the condition of your cam is, and if the valve timing was right. I've always figured it was a little flat based on your description of what the motor is doing in 4th WOT. Either that or some parts in the carburetors are in serious need of replacement.

Enjoy the build.

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Since I expect to wait on the cylinder heads, lead time on pistons won't be an issue.

Once I have pistons in hand, things should go quickly after clearances are verified.

Dan1950 said:

I've been hot rodding for for over 50 years and I call BS on this post.

Cam grinds and static CR work hand in hand, one complements the other.

Decreasing the volume of the combustion chamber at TDC also promotes evacuation of the exhaust gases and better induction on the downstroke.

I have seen a 13% increase in HP by increasing static CR from 9.6:1 to 12.7:1 with not other changes whatsoever.

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If your carburation is too rich and you raise the compression ratio, it has the same effect as leaning-off the jetting. - You get more go.

When you fit a race cam to a motor, the increase in lift allows more mixture to get into the cylinder, but it limits the revs which can be used by causing valve bounce. When the inlet valve opens earlier, you get a bigger gulp of mixture, but it moves the revs at which the system starts to resonate upwards in the usable rev range. If the exhaust valve opens earlier, it makes the tuned exhaust work harder, but it makes the bike louder.
If you fit megaphone exhausts to a motor which as a race cam, and use it for racing, you often have the situation where you get balked in a corner, so the revs drop and you need to slip the clutch. Your cornering effort can end up in a mess. A 2 into 1 exhaust system is much better on a Commando than separate pipes. You don't need to be so brave. As long as the 2 into 1 pipe is not restrictive at the collector, the race cam will work well. My bike pulls well right up through the whole rev range, but I only use from 5,500 RPM to 7000 RPM. With the Commando engine, you need to keep them spinning high or they will not accelerate. With my motor, if I race change with the close box, it goes from 5,500 to peak revs in an instant. I go into every corner with a heap of revs, and the bike is geared very high. It is all quite safe.