850 MK II Combustion Chamber Volume?

What is the combustion chamber volume of a stock 850 Commando engine with RH10 heads, stock pistons and standard thickness head gasket. What would the stock head gasket thickness be?

I need these specs to calculate static compression ratios when the compression height is reduced.

Copper gasket is 40 thou, composition gasket was 28 thou when NOC notes were written.

Data from NOC for RH10 showing 8.5 as standard (no mention of what gasket or if barrel base gasket was fitted or not)

If the book figure of 8.5:1 is correct then the CCV (when installed) is going to be around 55cc.

If my maths is correct, you’ll reduce that by approx 4.6cc per mm. So a 1mm reduction would get you close to 9.5:1 I think.

BUT… there are lots of variables, many head gaskets are thicker than they were, some people, fit base gaskets which they didn’t have as standard, and re- cut seats will recess the valves, all reducing CR. My bike measured at less than 8:1 as bought.

If you look at the 850 combustion chamber you’ll see a large step. This looks like it’s designed for a squish band, but as stock the piston comes no where near it, rendering it useless. JS Motorsports high comp pistons sit proud of the barrel, with care you can set these up with a tight squish and that will get you 10.5:1. I rebuilt my 850 like this and it ran fabulously, but you’ll need to ensure you can get premium unleaded fuel. You can also order them as specials with a dish in the crown that will reduce the CR a little (I mean a little) whilst still giving a good working squish.

As a hot rodder, I’m sure you‘re aware of the evidence of how a squish cools the burn, in many cases a higher CR with squish will run without knock, when a lower CR without squish will knock. I‘ve never had any issues even with a 920cc and 11:1 CR using pump fuel.

kommando said:

Copper gasket is 40 thou, composition gasket was 28 thou when NOC notes were written.

Data from NOC for RH10 showing 8.5 as standard (no mention of what gasket or if barrel base gasket was fitted or not)

​

RH10

065062

850cc

8.5 : 1​

30mm

1974

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If my algebra is correct the net combustion chamber volume at 8.5:1 would be 43.58cc.

Net volume for 10.0:1 would be 37.67cc a 5.91cc reduction in volume.

a reduction in compression height of 0.50" would be necessary to raise the CR to 10:1.

I am sure there is a table around that gives even more info but can't find it currently.

If you want some 850 hot rodding inspiration …

Fast Eddie said:

As a hot rodder, I’m sure you‘re aware of the evidence of how a squish cools the burn, in many cases a higher CR with squish will run without knock, when a lower CR without squish will knock. I‘ve never had any issues even with a 920cc and 11:1 CR using pump fuel.

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I was running .035" quench height (squish) on my '06 Charger. While that was adequate (barely) for an automatic transmission with stock steel rods .045" would be minimum for the Norton.

Even though I had a cam that developed a LOT of cylinder pressure off idle. I was still able to dial in maximum ignition advance due to the significant quench advantage. Thus the engine developed 460 ft lbs of RW torque. I think 10.5:1static CR with a cam that develops a bit less cylinder pressure off idle would make for easy kick starting and less octane dependence. I could probably run 87 octane in a pinch if I was careful not to lug the engine ir push it too hard.

850 "Combat" with taller gears to avoid over revving? 22t CS sprocket maybe?

Dan1950 said:

If my algebra is correct the net combustion chamber volume at 8.5:1 would be 43.58cc.

Net volume for 10.0:1 would be 37.67cc a 5.91cc reduction in volume.

a reduction in compression height of 0.50" would be necessary to raise the CR to 10:1.

Click to expand...

One of us has got this wrong then, my understanding is:

(414+43.58) / 43.58 = 10.5:1

whereas:

(414+55) / 55 = 8.5:1

Dan1950 said:

I was running .035" quench height (squish) on my '06 Charger. While that was adequate (barely) for an automatic transmission with stock steel rods .045" would be minimum for the Norton.

Even though I had a cam that developed a LOT of cylinder pressure off idle. I was still able to dial in maximum ignition advance due to the significant quench advantage. Thus the engine developed 460 ft lbs of RW torque. I think 10.5:1static CR with a cam that develops a bit less cylinder pressure off idle would make for easy kick starting and less octane dependence. I could probably run 87 octane in a pinch if I was careful not to lug the engine ir push it too hard.

850 "Combat" with taller gears to avoid over revving? 22t CS sprocket maybe.

Click to expand...

850 Combat was my original idea, but I decided at the time that the 2S was too much ‘up top’ for my needs and instead went with the somewhat milder JS #1.

In the 920 I run a Maney race cam which is in the ball park of a 2S or 4S, but the bigger capacity has the effect of softening the cam.

If you‘re in spending mode, I am reliably informed that nickasil lined billet alloy barrels and Dullauto heads will be available soon…

Fast Eddie said:

One of us has got this wrong then, my understanding is:

(414+43.58) / 43.58 = 10.5:1

whereas:

(414+55) / 55 = 8.5

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I think you are correct. My algebras has gotten rusty.

Soooo, a chamber volume reduction of 11.4cc would be needed.

Fast Eddie said:

850 Combat was my original idea, but I decided at the time that the 2S was too much ‘up top’ for my needs and instead went with the somewhat milder JS #1.

In the 920 I run a Maney race cam which is in the ball park of a 2S or 4S, but the bigger capacity has the effect of softening the cam.

If you‘re in spending mode, I am reliably informed that nickasil lined billet alloy barrels and Dullauto heads will be available soon…

Click to expand...

Not in a "spending mode". Just looking for the most bang for the $$$.

I do like the idea for the long rod /pistons from JS.

According to the Norton Service Releases for high performance and for racing, milling the head by .075" raises the CR to 10:1, and milling it by .090" raises it to 10.5:1. But as Fast Eddie has pointed out, the actual measured CR usually turns out a bit less than that. Still, that gives you a starting point. If you go this route, you will also have to cut valve relief notches in the pistons. If you haven't seen the service releases, you might want to check them out. I think I've posted them here in the past, but not sure they've survived. If you don't have them, send me a PM with your email, and I'll send you the files.

Ken

Dan1950 said:

Not in a "spending mode". Just looking for the most bang for the $$$.

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Ok, the head and barrel would be at the high end for sure!

But if you have worn parts, it pays to think twice before buying IMHO.

In my case I needed pistons anyway, and I liked the security factor of Carrillo rods given the intended use of the bike, so suddenly the JS piston / rod kit did not seem expensive.

Similarly, I wanted a cam, and my followers and follower tunnel were badly scored, so the JS cam and lightweight follower kit suddenly made sense.

Well, at least thats how I convinced myself !!

Assuming .040" head gasket, .050" positive deck height on the stock piston and .120" recess in quench band of the cylinder head, that gives net quench height of .110".

A .050 - .055" reduction of quench height would be needed to mitigate detonation.

.055" would reduce combustion chamber volume by 6.4cc (46.6cc net) 414+46.6/46.6 = 9.9:1.

.021"copper head gasket and .035 head deck reduction would do that.

Dan1950 said:

Assuming .040" head gasket, .050" positive deck height on the stock piston and .120" recess in quench band of the cylinder head, that gives net quench height of .110".

A .050 - .055" reduction of quench height would be needed to mitigate detonation.

.055" would reduce combustion chamber volume by 6.4cc (46.6cc net) 414+46.6/46.6 = 9.9:1.

.021"copper head gasket and .035 head deck reduction would do that.

Click to expand...

I can’t recall, but I do not believe the stock set up has a .050” positive deck height…

Fast Eddie said:

I can’t recall, but I do not believe the stock set up has a .050” positive deck height…

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See post #7 for the actual measurement. (.043")

Fast Eddie said:

850 Combat was my original idea, but I decided at the time that the 2S was too much ‘up top’ for my needs and instead went with the somewhat milder JS #1.

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How does the JS#1 do at reducing cylinder pressure at lower RPMs? Is easy to kick over?

Dan1950 said:

See post #7 for the actual measurement. (.043")

Piston Deck Height

I’ve been having problems with people commenting on my pictures that things don’t look right. I think part of it’s that I’m using dramatic lighting at angles people aren’t use to seeing their bikes at. The other parts is that I’m messing up. I haven’t measured it yet but I think spec is .050”...

www.accessnorton.com

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I’m not sure, but I think that might be a 750, and / or are high comp pistons.

Post #54 from the same thread, by Jim Comstock:

“850 pistons are generally flush or a little below.
A compressed original flame ring head gasket is ~.030
The squish band is going to be anywhere from .1 to .125 deep”

Dan1950 said:

How does the JS#1 do at reducing cylinder pressure at lower RPMs? Is easy to kick over?

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Well in theory, the longer duration is definitely going to help reduce cylinder pressure at kick start speeds. But the only way to know by how much would be to try a stock cam with 10.5:1 CR, which I haven’t done.

Short answer is no, it was not easy to kick over! But it was doable. Remember though, it was 10.5:1, that’s what casued the difficulty, not the cam (obviously).

Fast Eddie said:

I’m not sure, but I think that might be a 750, and / or are high compre pistons.

Post #54 from the same thread, by Jim Comstock:

“850 pistons are generally flush or a little below.
A compressed original flame ring head gasket is ~.030
The squish band is going to be anywhere from .1 to .125 deep”

Click to expand...

Yes I saw that upon further reading.