Norton ES2 - Engine Development

When my 1951 Norton ES2 first went back on the road in 2014, I was very disappointed with its performance, so I soon set about improving it. It's been through several iterations since and I've been down more than one blind alley. This article covers the engine modifications and tuning I've carried out, including compression ratio, carburation, valve timing and ignition timing, to get it to the stage it is at today.

Crank Assembly and Crankcases

Let’s start at the bottom and work our way up. I fully acknowledge that Norton’s cast iron flywheels and standard conrods have their weaknesses. But I was not looking to build a race engine here so I was happy to stick with the standard components.

For this bike I chose not to lighten the flywheels but I did rebalance the flywheels to suit the piston I was using.

Drilling holes get the required balance factor. This picture is actually of a Norton International flywheel

I have balanced several long-stroke Nortons over the years and have found a factor of between 65-70% yields good results. Phil Irving recommends 66% as a good starting point for most engines in Tuning for Speed so this ties in nicely. With this engine I ended up with a figure of 68% and the vibration has been perfectly acceptable throughout the rev range.

Once balanced I assembled the crank with a new big end and installed it in the crankcases with new main bearings and the recommended 0.005-0.008” of end float.

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Barrel and Piston

The first task here was to raise the compression ratio slightly. A standard ES2 is about 6.5:1, assuming everything is in good order. I used a 16H piston to raise this to 8.5:1. I’m happy running this on any fuel I am likely to find in the UK or Europe.

In order to fit the 16H piston I had to modify it. It needed shortening to clear the flywheels, with valve pockets machined into the crown to provide clearance for the valves.

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Cylinder Head​

When I first put the bike on the road in 2014 I fitted the correct head gasket which lasted less than 500 miles before it blew out to the stud holes. After this I went without it and machined the barrel and cylinder head to fit together without one.

I did this by copying Norton's own pre-war design, making the spigot on the barrel 0.001-0.002” taller than the recess in the head. The idea is that this joint can then be lapped together with some fine grinding paste. I have however found that if machined carefully, no further ‘grinding in’ is required.

What the pre-war bikes lack is the oil feed to the cylinder head and therefore the drain down holes which run through the head and barrel. In order to seal these I counterbored the barrel face and fitted an O-ring. The O-ring goes hard with the heat but maintains its seal until disturbed – I’ve had the head off a couple of times in the last 15,000 miles and have simply replaced the O-ring each time.

A nice dome on the piston and the O-rings fitted to seal the drain holes

The head was finished with new valves, guides and seats. I ‘cleaned up’ the inlet port to remove some sharp ridges left in the original casting but didn’t do any further porting. Again, this is not meant to be a race engine.

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Carburettor

I'll group the carburettor in with the engine development, although there isn't a great deal to report. After experimenting with different inlet lengths and a larger AMAL 289, I settled back on the standard AMAL 276 and inlet stub. I found no marked improvement with a bigger carb, if anything I lost some of the low-rev torque that makes this ES2 such a good tourer. I like the look of the standard setup and I don’t want to completely sacrifice the aesthetics of what is, in my opinion, a beautiful classic motorcycle.

The larger carb and longer inlet didn't last long

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Valve Timing

This is where the real performance improvements come from on a pushrod Norton. I’ve now run three different sets of cams in this engine under different setups and the difference each change made is sizeable.

Standard ES2 Cams

First, I tried the standard cams set up on the timing dots. The performance was hopeless. When I later put a degree disc on the engine, I found the inlet cam was a full 18° retarded from where I expected it to be. More on this later.

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Race Cams

My next attempt at valve timing came in the form of a set of race cams. These come with three woodruff key slots in the shaft, by using a different keyway I could set the timing in 6° increments rather than the 18° you get by moving a whole tooth on the gearwheel.

I ran these for several thousand miles and tried various settings, but to very little advantage. I found them too ‘hot’ for a long-stroke roadgoing ES2. The bike flew once the revs were up but it wouldn’t plod along, not a particularly useful characteristic for a road bike.

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I’ve kept these cams and might use them when I build the short-stroke race engine I have planned for another project.

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Experimenting with Standard ES2 Cams

After calling time on the race cams, I went back to the standard ones but set the timing somewhat differently. Following advice from some very helpful and experienced owners who have been building and tuning pushrod Nortons for decades, I set the valve timing using a degree disc. The target was to set them up with the lobe centres as follows:

Inlet: 98-102° ATDC

Exhaust: 106-110° BTDC

This turned out to be simpler than I expected. Each tooth on the gears that drive the cams accounts for 18° of change. That’s a lot and finer increments are not always easy. However, I found in the case of my engine all I had to do was advance the inlet cam one tooth and I was in the target range. I ended up with inlet and exhaust figures of 99° and 108° respectively.

Here you can see that the inlet cam is one tooth advanced from the timing dot, marked with green pen

This setup transformed the bike. I finally had something that would plod along in traffic, accelerate comfortably up to 60+ mph and had no flat spots. I ran the bike like this for a couple of years before I started wondering what else it had to offer.

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16H Cams

The final iteration, and the setup which I use today, uses a set of 16H cams. The OHV Nortons have a ratio between the cam lift and the valve lift, introduced by the rocker arms. This means that the valve opens approximately 1.26 times more than the cam lifts the follower.

The side-valve Nortons have no such mechanical advantage, so the cams themselves lift slightly further. They also hold the valve open slightly longer. Once these cams are fitted to an OHV engine you get timing figures very close to that of a long-stroke Norton International!

The table below shows some comparisons between the standard ES2 Cam, the 16H cam fitted to an ES2 and the 500 long-stroke International cams.

Cam	Max Valve Lift	Duration (°)
Standard ES2	Inlet: 8.4mm Exhaust: 8.6mm	Inlet: 253° Exhaust: 270°
16H in an OHV Engine	Inlet: 10.3mm Exhaust: 10.3mm	Inlet: 305° Exhaust: 294°
Long-stroke International	Inlet: 10.1mm Exhaust: 10.0mm	Inlet: 298° Exhaust: 308°

The issue with these cams was that setting them in 18° increments wasn’t getting me close enough to my target figures. To get where I wanted to be, I had a set of Norton gear wheels spark-eroded to give me three keyways, I got the idea from the race cams I had tried previously and, although in the gear rather than on the shaft, I got the same effect. I could now set the timing in 6° increments. This allowed me to get satisfactory valve timing and they are now set as follows:

Inlet: 99° ATDC

Exhaust: 105° BTDC

This setup gave another significant improvement, once the carburation had been set up to match it, and is still in the engine now. The bike will still thump away at low revs before accelerating all the way through to 70+ mph. I have no doubt it has more to give but don't like to overdo it with the standard flywheels and conrod still fitted.

To help compare some of the cams we have experimented with we have created a technical page on OHV Norton Cams which includes a table showing a number of useful comparisons. 

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Ignition Timing

Sparks are still provided by the MO1 magneto but I have experimented with ignition timing using one of our Magneto Vernier Sprockets which allows me to alter the timing very quickly in 2.4° increments and, where necessary, return to the previous setting.

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I used 5/8" (42 degrees) BTDC as a starting point but have moved it about since then. I found that if I ran it retarded, even by a couple of degrees, the engine ran a lot hotter. Getting the timing right has proven to be critical on this engine. I suspect you could set your engine to exactly the same and get different results, so experimenting to get it right is the way to go. Maybe one day I’ll run the bike on the dyno for the final refinements.

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The Result So Far

This combination of relatively simple modifications has transformed the bike without losing the character that makes an ES2 such an enjoyable road bike. It will happily plod through traffic, cruise comfortably at 60–65 mph on long journeys and accelerate cleanly beyond that when required. Most importantly, it has retained the low-speed flexibility I wanted from a touring bike rather than becoming an engine that only works well at higher revs.

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What Would You Like to Do Next?

• Read more about my 1951 Norton ES2

• Read about the Norton ES2 Chassis Development

• Compare OHV Norton Cams

• View our Norton Technical Guides

• Ask a question