Differences between Atlas and G15cs engines

Elfix · Jan 10, 2024

Hi all-

I recently discovered the Walridge Motors site and in their blog they mention that Matchless made a considerable number of improvements to the Atlas motor that improved reliability.

I own a ‘68 G15cs and have begun restoring a ‘64 Atlas, and while I have noted a few external changes between the engines (I have not yet dismantled the Atlas mill) I am curious if anyone out there might have a list of the changes that Matchless made, how the changes improved the engine, and if these could be done to my Atlas engine upon its rebuild.

Matchless · Jan 10, 2024

The machine shop at Plumstead was far more modern than that at Nortons/Shellys, having been re equiped by Alfred Herberts in the late fifties, & so maybe tighter tolerances on machined parts. One change that comes to mind is the oil pump speed was doubled, & the rockers pressure fed. They changed the balance factor, but it didn't seem to improve the vibes. Also, the barrel spigot was deleted & the head machined to suit.

acotrel · Jan 10, 2024

The barrel flange is one of the major differences with the 850 Commando motor. The first Atlas which came to Melboune had it's barrel flange break off. That was the reason they did not sell. But the Atlas which Bob Rosenthal raced in A grade, did not develop that problem. I always thought that Norton twin motor development stopped at the Atlas.. It is interesting that the Matchless version might have been better. The Commando motor was changed to get smoother running - it worked. But an Atlas was a more serious motorcycle. When the Atlas arrived, there were 3 motorcycles - the 650cc Super Rocket BSA, the 650cc Triumph Bonneville and the 750cc Atlas.

norton bob · Jan 11, 2024

My experience of Nortons is that AMC pushed more oil through the motor but had difficulty in controlling it. The Quality of the head castings went down with the transfer of production to AMC..

Ron L · Jan 12, 2024

I believe AMC and Norton production was merged in 1962, the year that the first Atlas appeared. So while there are differences from year to year, I doubt there are differences between Atlas and G15 (or N15) engines of the same model year. Some one more knowledgeable may correct me.

74ok · Jan 12, 2024

primary covers not the same & might be gearbox mounting scenarios

Elfix · Jan 12, 2024

I know about the spigotted head changes, but not the oiling details. Transmissions and primarys were absolutely different, as were the drillings on the cases to mount them, and there is a case to primary spacer on the G/N15 and probably the P11.

Looking at parts books I don’t see much else to support the claim that Matchless improved the Atlas engine- but I’m eager to be proven wrong about that.

mdt-son · Jan 13, 2024

norton bob said:
My experience of Nortons is that AMC pushed more oil through the motor but had difficulty in controlling it. The Quality of the head castings went down with the transfer of production to AMC..

You mean Plumstead -- Norton had been part of AMC since 1954! However, the factories didn't produce castings themselves - they were cast by BMC, and from 1963 on, by BIRCO. How did casting quality deteriorate? The alloys used for cylinder heads at the time were standardized, and the process was gravity fed sand casting. As casting was performed by dedicated foundries, I much doubt quality suffered, rather the opposite, due to much larger production figures throughout the 60's. You would have to examine hundreds of heads and perform metallurgical testing of many heads to reach a conclusion like that.

S/N 116372 Feb 1966 6-Start Oiling System, This includes the pump, positive pressure lubrication of the head, plain spindles. (enlarged oilways in the head?), redesigned oilways in the timing cover, oilways in the engine case (enlarged), modified oil junction block, and hoses to/from the oil tank. I believe the rationale was improved lubrication and cooling due to experiences made when tuning the engines for racing.

It's sometimes claimed that the 6-start pump capacity was wasted, since only 10-15% is spent on the crankshaft and ca. 20% goes to the top end. However, 65-70% is left for heat absorption and transport - an important and sometimes underrated task in a combustion engine.

- Knut

Matchless · Jan 14, 2024

Many of the castings used by AMC were produced by Stone Foundry in Charlton, about four miles west of Plumstead. I believe they were known for quality work, but I don't think they cast heads for the Norton twins.

Matchless · Jan 16, 2024

From looking through some notes four more changes to the Atlas were:
Altered breather timing
Crankshaft bore reduced from 7/8" to 5/8"
Conrod section changed for greater strength
Rockers reduced in width to give greater support in head.

mdt-son · Jan 16, 2024

Matchless said:
From looking through some notes four more changes to the Atlas were:
Altered breather timing
Crankshaft bore reduced from 7/8" to 5/8"
Conrod section changed for greater strength
Rockers reduced in width to give greater support in head.

By crankshaft bore you mean the internal bore for the oil chamber? That's interesting.

Rockers reduced in width ... I don't follow. If AMC reduced rocker width, then obviously for another reason --- maybe Norton had made rockers unnecessarily wide, and thereby too heavy? Or was it to make room for shimming?

- Knut

Matchless · Jan 17, 2024

The rockers were reduced in width across the bearing to enable more material to be left in the outer part of the head. On my recent Atlas Domiracer build I machined a set of (1964) rockers to this later spec. to allow me to use them in a stage 1 Maney Commando head. I think I reduced them to 1.250", but can't be certain as I have a memory like a sieve.

norton bob · Jan 18, 2024

mdt-son said:
You mean Plumstead -- Norton had been part of AMC since 1954! However, the factories didn't produce castings themselves - they were cast by BMC, and from 1963 on, by BIRCO. How did casting quality deteriorate? The alloys used for cylinder heads at the time were standardized, and the process was gravity fed sand casting. As casting was performed by dedicated foundries, I much doubt quality suffered, rather the opposite, due to much larger production figures throughout the 60's. You would have to examine hundreds of heads and perform metallurgical testing of many heads to reach a conclusion like that.

S/N 116372 Feb 1966 6-Start Oiling System, This includes the pump, positive pressure lubrication of the head, plain spindles. (enlarged oilways in the head?), redesigned oilways in the timing cover, oilways in the engine case (enlarged), modified oil junction block, and hoses to/from the oil tank. I believe the rationale was improved lubrication and cooling due to experiences made when tuning the engines for racing.

It's sometimes claimed that the 6-start pump capacity was wasted, since only 10-15% is spent on the crankshaft and ca. 20% goes to the top end. However, 65-70% is left for heat absorption and transport - an important and sometimes underrated task in a combustion engine.

- Knut

I own heads fitted by Norton Bracebridge St in 1960 which are very good castings . I also own heads fitted by the AMC factory in the mid 1960's which were from a different Foundry. There is a huge difference in the finish and grain of the alloy. I had to spend a day filing off lumps and getting bead blasting done to make the later head passable . What more proof do I need other than the evidence of my own eyes ?.

Matchless · Jan 18, 2024

norton bob said:
I own heads fitted by Norton Bracebridge St in 1960 which are very good castings . I also own heads fitted by the AMC factory in the mid 1960's which were from a different Foundry. There is a huge difference in the finish and grain of the alloy. I had to spend a day filing off lumps and getting bead blasting done to make the later head passable . What more proof do I need other than the evidence of my own eyes ?.

I have to agree with this, as the head on my Birmingham 650 ss is a reasonably nice casting, where as the head on my Plumstead Commando was dreadfull until I spent a lot of time tidying it up. But as I said before, I don't think the Norton twin heads were cast in London. Matchless heads were/are always very nice castings in my experience.

Differences between Atlas and G15cs engines

Similar threads